Skip to main content
Select Source:

Herbicides

Herbicides


Herbicides are chemicals used to destroy unwanted plants (terrestrial or aquatic) called weeds. Herbicides fall into two broad categories: inorganic (e.g., copper sulfate, sodium chlorate, and sodium arsenite) and organic (e.g., chlorophenoxy compounds, dinitrophenols, bipyridyl compounds, carbamates, and amide herbicides). Historically, inorganic compounds were the first available and the first used. There has been over a long period a continuous effort to develop herbicide compounds that are more selectivethat affect weeds, as opposed to desirable plants.

Historical Developments

The decade 1890 to 1900 saw the introduction of sprays for controlling broad-leaved weeds in cereal crops, and the first efforts by the U.S. Army Corps of Engineers, using sodium arsenite, to control aquatic plants in waterways. In 1925 sodium chlorate (directly applied to soil) was first used for killing weeds. The earliest importation (from France) of sodium nitrocresylate, as the first selective weed killer, was in 1934. The year 1945 witnessed the introduction of organic herbicides and the advent of 2,4-D growth regulator (2,5-dichlorophenoxyacetic acid), subsequently leading to development of 2,4,5-T (2,4,5-trichlorophenoxyacetic acid). During the years 1965 to 1970, U.S. military forces used 2,4,5-T (Silvex) and related materials as defoliants in Vietnam, without knowing that an inevitable by-product of the synthesis of 2,4,5-T was a toxic substance, 2,3,7,8-tetrachlorodibenzodioxin (dioxin). There is still debate over the extent of damaging effects sustained by those airmen, soldiers, and civilians who were exposed to this material. Dioxin was present at a level of about 2 ppm (mg/kg sample) in some of the samples of 2,4,5-T (called Agent Orange), but other samples contained more than 30 ppm of the by-product. Dioxin was eventually found to be highly toxic to guinea pigs (the LD50 value was 1 ppb, or 1 μ g compound/kg of sample), which led to the labeling of dioxin as "the world's most deadly poison," an impressive, if inaccurate, title (inaccurate because of a unique sensitivity of guinea pigs and because some natural toxins are known to be more potent).

The U.S. federal government's experience with 2,4,5-T demonstrates a significant principle: One must be concerned not only with the safety of the active components of commercial products, but also with the safety of byproducts that may be present in those products or that may form during natural degradation. Adherence to this principle is a major and costly challenge to those who develop herbicides, and concern for safety is partly responsible for the (at present) decreasing number of herbicides that are available for treating aquatic weeds.

It is thought that the first water hyacinths were introduced into the United States during an 1884 horticultural exposition in New Orleans, in

the course of which these plants, imported from Argentina, were given away as souvenirs. It is suspected that they were accidentally put into the St. Johns River in Florida and that they, shortly thereafter, multiplied. The plant grows (under optimal conditions) at the rate of 1.8 daughter plants per parent plant per week, and rapid growth generated dense mats that affected the navigation of boats on this river and others. In 1898 the U.S. Army Corps was given responsibility for maintaining the navigability of rivers, and aquatic plant control became its responsibility as wella responsibility that has persisted to this day.

Herbicide Toxicity

Because plants and mammals differ in organization and physiology, it might be expected that herbicides would constitute only a slight chemical hazard to mammals. Whereas some herbicides have very low toxicities in mammals, others have considerable. A number of test species are used to appraise toxicity, and their sensitivities are graded as acute (short-term) LD50 values.

LD50 refers to the amount (LD or lethal dose) that will elicit the deaths of 50 percent of the test species. It is typically expressed as the weight of herbicide per kilogram of body weight. The smaller the LD value, the greater the toxicity.

The chlorophenoxy compounds 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) may be the most familiar herbicides. They have been used in agriculture (to eradicate broadleaf weeds) and to control woody plants in ditches and along highways. They act as growth hormones in many plants, and can evoke active plant growth in areas in which abnormal, twisted, or curtailed growth occurs. Massive doses

CHARACTERISTICS OF REPRESENTATIVE HERBICIDES
Herbicide (type) Control/Purpose Acute Toxicity LD 50mg/kg
source: Weed Science Society of America (1994). Herbicide Handbook, 7th edition. Lawrence, KS: Allen Press.
2,4-D (2,4-dichlorophenoxy acetic acid) Systematic herbicide 3001,000: rats, guinea pigs, rabbits
Acetochlor Control of most annual grasses, some broadleaf weeds. Tolerant crops include corn, soybeans, peanuts, sugarcane. 2,953: rat acute oral
Amitrole (triazine) Broadleaf weeds and grasses in noncrop areas, generally low toxicity. >5,000: male rats
Arsenic acid (inorganic) Desiccation of cotton which is to be stripped 48: young rat 100: older rats
Atrazine Widely used selective herbicide for broadleaf and grassy weeds. no ill effects in rats, dogs with diet of 25 ppm
Dinosep (dinitrophenol) Control of seedlings, not established perennial weeds except with repeat treatments. Applicable to variety of crops, except cruciferous crops. 58: rats
Diquat (dipyridyl) General aquatic herbicide; preharvest top killer or desiccant. 230: rats
Diuron (carbamate) Low ratesbroadleaf and grass weeds in cotton, sugarcane etc. general weedkiller at higher rates 3,400: oral rats
Glyphosate Broad-spectrum herbicide Used in crop, noncrop, weed control (Rabbit acute dermal, >5,000 mg/kg)
Metolachlor Selective herbicide used to control annual grassweeds, yellow nutsedge, some broadleaf in corn, cotton, peanuts, and other crops 2,780: rat acute oral
Paraquat (dipyridyl) Weed control during establishment of grass seed crops 138: male rats
Propanil (aromatic amide) Grasses and broadleaf weeds in certain wheat crops (north) and rice (south) 1,870: rats

of either 2,4-D or 2,4,5-T cause ventricular fibrillation in mammals. Lower doses cause contact dermatitis and chloracne (a kind of severe dermatitis) in workers who have contact with 2,4,5-T (which, as noted, may be mixed with 2,3,7,8-tetrachlorodibenzodioxin, or dioxin).

Dinitrophenols (as alkali salts or aliphatic amine salts) have long been used in weed control. Human exposure to these compounds has led to nausea, gastric upset, rapid breathing, tachycardia (rapid heartbeat), cyanosis, and ultimately coma. Death or recovery occurs within 24 hours.

Paraquat and diquat are the best-known examples of bipyridyl compounds. These compounds appear to act via a free radical mechanism, competing for and depriving plants of an essential reducing agent . These compounds are hazardous to human beings. About 200 deaths from accidental poisoning or suicide attempt occurred in the 1960s. The fatalities showed lung, liver, and kidney damage. Paraquat tends to become concentrated in the kidney, with the accumulation of toxic amounts in the lung being secondary to kidney damage.

Propanil is one of a group of amide herbicides (made from aniline treated with organic acids), and is used extensively to control weeds in rice crops. Rice itself contains an enzyme that hydrolyzes propanil to 3,4-dichloroaniline and propionic acid, and so it is resistant to the herbicide. Weeds, lacking this enzyme, are adversely affected by it. (Mammalian liver cells also have an enzyme that causes this hydrolysis.)

The effects of trace contaminants in herbicides are a major concern. For example, the use of Silvex was canceled by the U.S. Environmental Protection Agency in 1979 because the herbicide contained dioxin, a toxic. However, the Army Corps of Engineers argued against the cancellation, noting the overall U.S. waterways navigation benefits. The sum total of benefits of Silvex-based weed control were judged to correspond to approximately $40 million, and the benefitcost ratio was about 11 to 1. Set against this must be the unknown costs of a toxic substance (dioxin), whose adverse effects are still being evaluated.

The entire world market for crop protection in 2000 was estimated to be $31 billion, and it probably will not grow significantly in the near future. Herbicides are sold as special formulations, and their use in the United States occurs only after extensive testing and governmental approval. Although new chemicals are being developed, the relatively static size of the herbicide market has resulted in a reduction in the number of agrochemical companies (through mergers and acquisitions). The number of new herbicides that will become available in the future will probably be a low one.

see also Agricultural Chemistry; Gardening; Insecticides; Pesticides.

Dean F. Martin

Barbara B. Martin

Bibliography

Baker, D. R., ed. (2002). Synthesis and Chemistry of Agrochemicals, Vol. VI. Symposium Series 800. Washington, DC: American Chemical Society.

Carson, Rachel (1962). Silent Spring. Boston: Houghton Mifflin.

Gough, M. (1986). Dioxin, Agent Orange: The Facts. New York: Plenum.

Klaassen, C. D., ed. (2001). Casarett and Doull's, 6th edition. New York: McGraw-Hill.

Weed Science Society of America (1994). Herbicide Handbook, 7th edition. Lawrence, KS: Allen Press.

Internet Resources

Weed Science Society of America. Available from <http://www.wssa.net>.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Herbicides." Chemistry: Foundations and Applications. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"Herbicides." Chemistry: Foundations and Applications. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/science/news-wires-white-papers-and-books/herbicides

"Herbicides." Chemistry: Foundations and Applications. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/news-wires-white-papers-and-books/herbicides

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Herbicides

HERBICIDES

HERBICIDES. Weeds have been deemed undesirable during much of human history for their negative influence on crop production, their unsightly appearance in the landscape, and in some cases their toxic properties and negative effects on human and animal health. Consequently, weed control is as old as the discovery of agriculture, eight to ten thousand years ago. Techniques for weed control have progressed from the employment of intensive human labor to complex systems involving mechanical, chemical, and biological methods. The earliest methods to eliminate weeds involved physical removal by grubbing or hoeing, followed by cultivation practices using first draft animals and then tractors. Since 1945, the use of chemical herbicides has become the predominant weed control technique in many parts of the world.

Chemicals have been suggested for weed control since antiquity. Theophrastus (372287 B.C.E.) mentions killing trees by pouring olive oil over their roots. Cato (234149 B.C.E.) advocated the use of amurca (the watery residue left after the oil is drained from crushed olives) for weed control. Other chemicals include sodium chloride, sulfuric acid, sodium arsenite, copper sulfate, iron sulfate, carbon bisulfate, arsenic trichloride, and petroleum oils. The first synthetic herbicide, 2-methyl-4,6-dinitrophenol (dinitro) was developed in France in 1932 for selective weed control in beans. In 1940 ammonium sulfamate was introduced for control of woody plants.

The chemical herbicide age began in 1941 when R. Pokorny first synthesized 2,4-dichlorophenoxy acetic acid (2,4-D) and reported that it had growth-regulating effects on plants. E. J. Krause of the University of Chicago later suggested that 2,4-D might be used to kill weeds, which stimulated research to test this and other newly synthesized chemicals for weed control in the field. These herbicides proved effective, and in 1945 the American Chemical Paint Company was awarded a patent for 2,4-D as a weed killer. The great potential of synthetic herbicides to control weeds and reduce human labor stimulated the birth of the herbicide chemical industry, resulting in the development of over 180 herbicides for weed control by the end of the twentieth century.

Herbicides are now primarily developed in the private sector. Chemists typically synthesize a variety of compounds, which are screened for their ability to control weeds and then modified and formulated for efficient use. Present herbicides tend to have very low mammalian toxicity because they inhibit biochemical pathways that are unique to plants.

There are a number of chemical classes of herbicides and various mechanisms by which herbicides kill plants. Herbicides generally act by inhibiting specific cellular functions, including photosynthesis, plant-specific amino acid biosynthesis, pigment formation, shoot and root growth, cell membranes, cellulose biosynthesis, lipid biosynthesis, and growth hormone activity.

Herbicides may be applied in many ways. Some herbicides are applied to the soil and absorbed by the plant root and/or shoot and move to their site of inhibition within the plant. Others are primarily applied to emerged foliage and either have an immediate contact effect on the foliage by burning or desiccation, or are translocated throughout the plant, leading to total plant death (systemics). Most soil-applied herbicides kill weed seedlings as they emerge from the soil, while foliage-applied herbicides control emerged weeds and can kill quite large plants.

Herbicide selectivity, the ability to kill weeds but not crops, can be accomplished either by directed application or through biochemical mechanisms. Placement of the herbicide to avoid contact with the crop is widely used. For example, tree crops with deep roots often do not absorb soil-applied herbicides. While it is an effective herbicide for killing most broadleaf plants (dicots), 2,4-D is ineffective on most grassy weeds (monocots). This makes it useful in monocot crops, such as grains and turf. Others selectively kill monocot grasses but not dicots, making them effective in crops such as soybean. Some crops metabolize an applied herbicide to an inactive form while the weeds cannot, so the weed is killed, but the crop is not harmed. For example, atrazine is metabolized to an inactive form by maize while weeds are killed.

In many weed and crop situations there are no good selectivity mechanisms for herbicides. With the advent of recombinant DNA technology (genetic engineering) certain crop plants, such as soybean, corn, and cotton, have been made resistant to nonselective herbicides such as glyphosate by adding genes that make the crop immune to the herbicide. This technology is expected to increase, though its rate of acceptance has been slowed by the reluctance of the food industry to utilize transgenic crops because of concerns expressed by certain consumer advocacy groups.

Modern agriculture in the United States is almost inconceivable without the use of herbicides. Herbicides reduce labor inputs for weed control and make it possible to control weeds where cultivation is infeasible. They reduce the need for mechanical cultivation that can injure crop plants and lead to soil degradation via structure loss and compaction. Herbicides allow the use of no-till crop production, which reduces the need for plowing, now considered a destructive practice. Efficient weed control improves crop growth by reducing weed competition for nutrients and water, and results in improved harvesting and crop quality.

A Source of Controversy

Despite the obvious advantages of herbicides, their use has raised concerns relating to human health and the environment. Since herbicides are toxic to plants, critics have questioned their toxicity to other organisms exposed directly or indirectly. The persistence of some herbicides in the environment has led to concerns relating to their carryover in the soil and effects on subsequent crops as well as their influences, due to drift or volatilization, on non-target plants. Furthermore, through repeated exposure to herbicides, many weeds have become resistant, which reduces the efficacy of previously effective herbicides.

Other concerns involve herbicide costs, the requirement for additional equipment for precision application, and questions relating to proper disposal of unused herbicides.

The advantages and disadvantages of herbicide use are thoroughly evaluated by the U.S. Environmental Protection Agency (EPA) prior to registration and labeling of any new compound. All new pesticides must be granted a registration, permitting their distribution, sale, and use. The EPA assesses a wide variety of potential human health and environmental effects associated with use of the product, including the particular site or crop on which it is to be used; the amount, frequency and timing of its use; and recommended storage and container disposal practices.

In evaluating a pesticide registration application, the registrant must provide data from tests done according to specific EPA guidelines conducted under recognized "Good Laboratory Practice." Results of these tests determine whether a pesticide has the potential to cause adverse effects on humans, wildlife, fish, or plants, including endangered species and non-target organisms, as well as possible contamination of surface water or groundwater from leaching, runoff, and spray drift. The potential human risks evaluated include short-term toxicity and long-term effects, such as cancer and reproductive system disorders. A pesticide will only be registered if it is determined that it can be used to perform its intended function without unreasonably adverse effects on applicators, consumers, or the environment. The EPA also must approve the specific language that appears on each pesticide label; the product can only be legally used according to label directions. The EPA continually evaluates herbicides as to their safety, and any compound that is found to cause any adverse effect is immediately removed from the market.

At the present time herbicides provide consistent, broad-spectrum, and effective weed management in an economical manner. In the future, herbicides will be required to pass even more stringent tests related to their safety. While new-generation herbicides will likely be applied at even lower doses with less environmental persistence and exceedingly low toxicity to non-target organisms, herbicides are now recognized as only one factor in efficient weed control. Weed management is an everevolving system that will continue to use an integrated approach, combining cultural, mechanical, chemical, and biological techniques. In this process, however, herbicides will remain an essential component for weed control to help insure a sustainable food production system that reduces unacceptable risks to the environment while producing an abundant and safe food supply.

See also Agricultural Research; Contaminants, Chemical; Ecology and Food; Government Agencies; Pesticides; Safety, Food; Toxins, Unnatural, and Food Safety .

BIBLIOGRAPHY

Monaco, Thomas J., Stephen C. Weller, and Floyd M. Ashton. Weed Science: Principles and Practices. 4th ed. New York: Wiley, 2002.

Zimdahl, Robert L. Fundamentals of Weed Science. 2d ed. San Diego, Calif.: Academic Press, 1999.

Stephen C. Weller

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Herbicides." Encyclopedia of Food and Culture. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"Herbicides." Encyclopedia of Food and Culture. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/food/encyclopedias-almanacs-transcripts-and-maps/herbicides

"Herbicides." Encyclopedia of Food and Culture. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/food/encyclopedias-almanacs-transcripts-and-maps/herbicides

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Transgenics

Transgenics

The term transgenics refers to the process of transferring genetic information from one organism to another. By introducing new genetic material into a cell or individual, a transgenic organism is created that has new characteristics it did not have before. The genes transferred from one organism or cell to another are called transgenes. The development of biotechnological techniques has led to the creation of transgenic bacteria , plants, and animals that have great advantages over their natural counterparts and sometimes act as living machines to create pharmaceutical therapies for the treatment of disease. Despite the advantages of transgenics, some people have great concern regarding the use of transgenic plants as food, and with the possibility of transgenic organisms escaping into the environment where they may upset ecosystem balance.

Except for retroviruses that utilize ribonucleic acid (RNA) , all of the cells of every living thing on Earth contain DNA (deoxyribonucleic acid ). DNA is a complex and long molecule composed of a sequence of smaller molecules, called nucleotides, linked together. Nucleotides are nitrogen-containing molecules, called bases, that are combined with sugar and phosphate. There are four different kinds of nucleotides in DNA. Each nucleotide has a unique base component. The sequence of nucleotides, and therefore of bases, within an organism's DNA is unique. In other words, no two organisms have exactly the same sequence of nucleotides in their DNA, even if they belong to the same species or are related. DNA holds within its nucleotide sequence information that directs the activities of the cell. Groups, or sets of nucleotide sequences that instruct a single function are called genes.

Much of the genetic material, or DNA, of organisms is coiled into compact forms called chromosomes . Chromosomes are highly organized compilations of DNA and protein that make the long molecules of DNA more manageable during cell division. In many organisms, including human beings, chromosomes are found within the nucleus of a cell. The nucleus is the central compartment of the cell that houses genetic information and acts as a control center for the cell. In other organisms, such as bacteria, DNA is not found within a nucleus. Instead, the DNA (usually in the form of a circular chromosome) chromosome is free within the cell. Additionally, many cells have extrachromosomal DNA that is not found within chromosomes. The mitochondria of cells, and the chloroplasts of plant cells have extrachromosomal DNA that help direct the activities of these organelles independent from the activities of the nucleus where the chromosomes are found. Plasmids are circular pieces of extrachromosomal DNA found in bacteria that are extensively used in transgenics.

DNA, whether in chromosomes or in extrachromosomal molecules, uses the same code to direct cell activities. The genetic code is the sequence of nucleotides in genes that is defined by sets of three nucleotides. The genetic code itself is universal, meaning it is interpreted the same way in all living things. Therefore, all cells use the same code to store information in DNA, but have different amounts and kinds of information. The entire set of DNA found within a cell (and all of the identical cells of a multicellular organism) is called the genome of that cell or organism.

The DNA of chromosomes within the cellular genome is responsible for the production of proteins. The universal genetic code simply tells cells which proteins to make. Proteins, in turn have many varied and important functions, and in fact help determine the major characteristics of cells and whole organisms. As enzymes , proteins carry out thousands of kinds of chemical reactions that make life possible. Proteins also act as cell receptors and signal molecules, which enable cells to communicate with one another, to coordinate growth and other activities important for wound healing and development. Thus, many of the vital activities and characteristics that define a cell are really the result of the proteins that are present. The proteins, in turn, are determined by the genome of the organism.

Because the genetic code with genes is the same for all known organisms, and because genes determine characteristics of organisms, the characteristics of one kind of organism can be transferred to another. If genes from an insect, for example, are placed into a plant in such a way that they are functional, the plant will gain characteristics of the insect. The insect's DNA provides information on how to make insect proteins within the plant because the genetic code is interpreted in the same way. That is, the insect genes give new characteristics to the plant. This very process has already been performed with firefly genes and tobacco plants. Firefly genes were spliced into tobacco plants, which created new tobacco plants that could glow in the dark. This amazing artificial genetic mixing, called recombinant biotechnology , is the crux of transgenics. The organisms that are created from mixing genes from different sources are transgenic. The glow-in-the-dark tobacco plants in the previous example, then, are transgenic tobacco plants.

One of the major obstacles in the creation of transgenic organisms is the problem of physically transferring DNA from one organism or cell into another. It was observed early on that bacteria resistant to antibiotics transferred the resistance characteristic to other nearby bacterial cells that were not previously resistant. It was eventually discovered that the resistant bacterial cells were actually exchanging plasmid DNA carrying resistance genes. The plasmids traveled between resistant and susceptible cells. In this way, susceptible bacterial cells were transformed into resistant cells.

The permanent modification of a genome by the external application of DNA from a cell of different genotype is called transformation . Transformed cells can pass on the new characteristics to new cells when they reproduce because copies of the foreign transgenes are replicated during cell division. Transformation can be either naturally occurring or the result of transgenics. Scientists mimic the natural uptake of plasmids by bacterial cells for use in creating transgenic cells. Certain chemicals make transgenic cells more willing to take-up genetically engineered plasmids. Electroporation is a process where cells are induced by an electric current to take up pieces of foreign DNA. Transgenes are also introduced via engineered viruses . In a procedure called transfection, viruses that infect bacterial cells are used to inject the foreign pieces of DNA. DNA can also be transferred using microinjection, which uses microscopic needles to insert DNA to the inside of cells. A new technique to introduce transgenes into cells uses liposomes. Liposomes are microscopic spheres filled with DNA that fuse to cells. When liposomes merge with host cells, they deliver the transgenes to the new cell. Liposomes are composed of lipids very similar to the lipids that make up cell membranes, which gives them the ability to fuse with cells.

With the aid of new scientific knowledge, scientists can now use transgenics to accomplish the same results as selective breeding.

By recombining genes, bacteria that metabolize petroleum products are created to clean-up the environment, antibiotics are made by transgenic bacteria on mass industrial scales, and new protein drugs are produced. By creating transgenic plants, food crops have enhanced productivity. Transgenic corn, wheat, and soy with herbicide resistance, for example, are able to grow in areas treated with herbicide that kills weeds. Transgenic tomato plants produce larger, more colorful tomatoes in greater abundance. Transgenics is also used to create influenza immunizations and other vaccines.

Despite their incredible utility, there are concerns regarding trangenics. The Human Genome Project is a large collaborative effort among scientists worldwide that announced the determination of the sequence of the entire human genome in 2000. In doing this, the creation of transgenic humans could become more of a reality, which could lead to serious ramifications. Also, transgenic plants used as genetically modified food is a topic of debate. For a variety of reasons, not all scientifically based, some people argue that transgenic food is a consumer safety issue because not all of the effects of transgenic foods have been fully explored.

See also Cell cycle (eukaryotic), genetic regulation of; Cell cycle (prokaryotic), genetic regulation of; Chromosomes, eukaryotic; Chromosomes, prokaryotic; DNA (Deoxyribonucleic acid); DNA hybridization; Molecular biology and molecular genetics

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Transgenics." World of Microbiology and Immunology. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"Transgenics." World of Microbiology and Immunology. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/transgenics

"Transgenics." World of Microbiology and Immunology. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/transgenics

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Herbicides

HERBICIDES

Herbicides are a class of pesticides that are marketed specifically for the purpose of killing or inhibiting the growth of weeds. Under the Federal Insecticide, Fungicide, and Rodenticide Act, a weed is defined as "a plant that grows where it is not wanted." The benefits of herbicide use have been many. In agriculture, herbicides control weeds that may rob water and nutrients from crop plants. Compared to other methods, like tillage, herbicides have been promoted as methods of weed control that lessen the impact of soil erosion. They have also been used to control aquatic weeds that block water intakes or invade natural ecosystems, as well as in forestry, and even in swimming pools to inhibit growth of algae. These benefits have resulted in a steady demand for pesticides in the United States, where about 550 million to 600 million pounds per year were used between 1979 and 1997.

In the United States in 1997, there were an estimated $6.8 billion in sales of herbicides and plant growth regulators. Herbicides constitute a large percentage of total pesticide use. Worldwide in 1997, there were 5.7 billion pounds of pesticides used, of which 2.2 billion were herbicides. Of the1.2 billion pounds of conventional pesticides used in the United States in 1997, a total of 568 million pounds of herbicides were used470 million pounds in agriculture, 48 million pounds in industry and government, and 49 million pounds in households. The largest quantities are associated with on crops planted to large acreages, such as soy, cotton, corn, and canola.

There are numerous classes of herbicides (see Table 1) with different modes of action for killing weeds, as well as different potentials to have an adverse effect on health and the environment. Herbicides from different classes also differ in their environmental persistence and fate.

Almost all herbicides can cause acute toxicity. Phenoxy herbicides are involved in acute symptomatic illnesses with relative frequency, accounting

Table 1

Class of Herbicide Examples
source: Sine, C. ed. (1998). Farm Chemicals Handbook.
Acetamides and analides Alachlor, acetochlor, metolochlor, propachlor, propanil
Carbamates and thiocarbamates Asulam, terbucarb, thiobencarb
Chlorphenoxy herbicides 2,4,-D, 2,4-DP, 2,4-DB, 2,4,5-T, MCPA, MCPB, MCPP, Dicamba
Dipyridyls Paraquat, diquat
Heavy metals Lead arsenate, arsenicals
Nitrophenolic and dinitrocresolic herbicides Dinitrophenol, dinitrocresol, dinoseb, dinosulfon
Pentachlorophenol Pentachlorophenol
Phosphonates Glyphosate, glyfusinate, fosamine ammonium
Triazines Atrazine, simazine, cyanazine, propazine
Urea derivatives Diuron, flumeturon, linuron, rimsulfuron, tebuthiuron

for a reported 453 illnesses in 1996. Glyphosate, a phosphonate herbicide, causes eye, skin, and upper respiratory effects in pesticide workers. Paraquat, a dipyridil pesticide, causes skin irritation and has been frequently associated with accidental death and suicide, especially in developing countries. Access to paraquat is restricted in the United States.

Herbicides are associated with a variety of chronic health risks. Most notable have been concerns about carcinogenicity. Both 2,4,5-T and pentachlorophenol are contaminated by carcinogenic dioxins and furans in manufacture. A number of the acetamide/analide and triazine pesticides are carcinogenic in animals. Studies of U.S. farmers have indicated that general exposure to herbicides is correlated with elevated rates of non-Hodgkin's lymphoma and certain other cancers; however, no specific chemicals have been pinpointed definitively. Many have been banned or severely restricted in the United States and elsewhere, including most of the chlorphenoxy herbicides, the dipyridyls, lead arsenate and arsenicals, and the nitrophenol/dinitrophenol herbicides.

Lynn R. Goldman

(see also: Farm Injuries; Pesticides; Toxic Substances Control Act; Toxicology )

Bibliography

Reigart, J. R., and Roberts, J. R. (1999). Recognition and Management of Pesticide Poisonings, 5th edition. Washington, DC: U.S. Environmental Protection Agency.

Sine, C., ed. (1998). Farm Chemicals Handbook. Willoughby, OH: Meister.

Zahm, S. H., and Blair, A. (1992). "Pesticides and Non-Hodgkin's Lymphoma." Cancer Research 52(19):5485s 5488s.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Herbicides." Encyclopedia of Public Health. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"Herbicides." Encyclopedia of Public Health. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/education/encyclopedias-almanacs-transcripts-and-maps/herbicides

"Herbicides." Encyclopedia of Public Health. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/education/encyclopedias-almanacs-transcripts-and-maps/herbicides

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

herbicide

herbicide (hr´bəsīd´), chemical compound that kills plants or inhibits their normal growth. A herbicide in a particular formulation and application can be described as selective or nonselective. In agriculture, selective herbicides are often used instead of tillage, or in combination with tillage and other agronomic practices, to control weeds without damaging crops. For these no-till or low-till systems, scientists have used biotechnology to develop crop varieties with increased tolerance for herbicides. Nonselective herbicides (e.g., paraquat) toxic to all plants, are used where complete control of plant growth is required.

Contact herbicides kill only the parts of the plant they touch; systemic herbicides are absorbed by foliage or roots and translocated to other parts of the plant. Preemergence herbicides, mixed into the soil, will kill germinating seeds and small seedlings. Postemergence herbicides either hinder photosynthesis or inhibit growth.

Early chemical herbicides were inorganic compounds. Herbicides such as ashes, common salts, and bittern have been used in agriculture since ancient times. Observation in 1896 that Bordeaux mixture, a fungicide, also provided control of certain weeds, led to the use of copper sulfate as a selective weed killer to control charlock (wild mustard) in cereals. By 1900, solutions of sulfuric acid, iron sulfate, copper nitrate, and ammonium and potassium salts were known to act as selective herbicides; soon thereafter sodium arsenite solutions became the standard herbicides, and they were used in large quantities until about 1960. Other inorganic herbicides include ammonium sulfamate, carbon bisulfide, sodium chlorate, sulfuric acid solutions, and formulations containing borate.

Organic herbicides began to be produced in earnest with dinitrophenol compounds in 1932. A breakthrough occurred in the 1940s with 2,4-D (2,4-dichlorophenoxyacetic acid), a compound similar to plant hormones, which is a highly selective systemic herbicide when used in very small quantities. 2,4-D was quickly adopted to control broad-leaved weeds in corn, sorghum, small grains, and grass pastures, as well as in lawns and other ornamental turf. The phenoxyaliphatic acids and their derivatives, another major group of organic herbicides, succeeded because of their selectivity and ease of translocation. Other groups of organic herbicides include organic arsenicals, substituted amides and ureas, nitrogen heterocyclic acids, phenol derivatives, triazines, and sulfonylureas.

In the 1960s and 1970s, a combination of 2,4-D and 2,4,5-T was widely used in Vietnam as a defoliant under the name Agent Orange. As a result of questions concerning the possible health effects of the use of Agent Orange, heightened awareness of possible ecological and health dangers attributable to herbicides has resulted in reevaluation of many compounds and has called indiscriminate use into question. Use of the dioxin-containing 2,4,5-T was prohibited in the United States in 1984. In 1975, Mexico, at the urging of the United States government, began spraying fields of marijuana with paraquat, which both eliminated the crop and raised fears of toxic side effects in marijuana users.

Glyphosate, a compound first identified as a herbicide in 1970 and sold beginning in the 1970s under the tradename Roundup, has been widely used as a broad-spectrum weedkiller because of its relatively low toxicity and tendency to degrade relatively quickly in the environment. Beginning in the 1990s, the use of crop strains that were resistant to its herbicidal effects also contributed to widespread use. In the early 21st cent., however, such use led to the development of so-called superweeds that had developed resistance to glyphosate.

See also pesticide.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"herbicide." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"herbicide." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/herbicide

"herbicide." The Columbia Encyclopedia, 6th ed.. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/herbicide

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

transgenics

transgenics is the transfer of genetic information that is not normally present into the genome of a species. To create a transgenic species the relevant gene (transgene) is introduced into stem cells, which are in turn introduced into a blastocyst (the product of the early cell division of a fertilized egg). If the resulting progeny have the transgene in their germ cells then transgenic species can be derived by breeding. The new characteristics of the resulting animals (their altered phenotype) reveal or confirm the function of the transgene. By introduction of genetic material to disrupt a gene into a species, such as mice, accurate models of human genetic diseases can be created. These so called ‘knockout’ animals are essential for formulating approaches to the treatment of human genetic disease.

Alan W. Cuthbert


See genetics, human; stem cells.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"transgenics." The Oxford Companion to the Body. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"transgenics." The Oxford Companion to the Body. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/transgenics

"transgenics." The Oxford Companion to the Body. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/transgenics

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Genetic Engineer

Genetic Engineer

Plant genetic engineers create new varieties of plants, including row crops, vegetables and berries, forest and fruit trees, and ornamentals. These new varieties contain any number of new or improved traits, such as resistance to pests and diseases, resistance to poor growing conditions, resistance to herbicides, and improved nutrition, wood quality, storage characteristics, and horticultural traits. Plant genetic engineers are also modifying plants to produce industrial enzymes , biodegradable plastics, pharmaceutical products, and edible vaccines. Plant genetic engineers collaborate closely with molecular biologists to identify and clone the necessary genes, and with plant breeders, who breed these into improved plant varieties.

Genetic engineers are drawn to the discipline because of the power that creating new plant varieties has to help preserve crop yields, produce a better, healthier product for the consumer, and help safeguard the environment. Though some of the daily tasks can be routine and repetitive, the field is advancing rapidly, and mastering the new advances continuously provides challenges and prevents research and development from becoming routine.

Plant genetic engineers must have a strong background in biology, with an emphasis in botany, biochemistry, and genetics. An understanding of agriculture or forestry can be particularly helpful, especially for the selection of the traits to be modified. Plant genetic engineers begin with an undergraduate degree in one of the agricultural plant sciences, forestry, botany, genetics, biotechnology, or biochemistry, and many obtain an M.S. and/or a Ph.D. in these fields.

Those with B.S. and M.S. degrees usually work in a laboratory and handle the necessary deoxyribonucleic acid (DNA), plant cell cultures, and analytical work. Those with a Ph.D. set research goals and determine research directions. Salary range depends strongly on educational level. In 1999, people with a B.S. or M.S. may have earned $20,000 to $30,000 for an entry-level position, while entry positions for a Ph.D. degree were in the vicinity of $50,000. Senior-level Ph.D. positions may have earned $150,000. Chief areas of employment would be research universities, biotechnology companies, forest products companies, and international research centers. The greatest amount of genetic engineering takes place in the United States and Europe. However, because plant genetic engineering is taking place in many places of the world, there may be employment opportunities throughout the world.

Very vocal groups of opponents to genetic engineering technology claim that genetic engineering will lead to genetic pollution, introduce toxins into the food supply, and damage the environment. Such opposition has led to bans on genetically engineered plants and food products in many countries, as well as an extensive patchwork of regulations. Sustained opposition to genetically engineered plants may limit employment opportunities in the future.

see also Breeder; Breeding; Genetic Engineering; Molecular Plant Genetics; Transgenic Plants.

Scott Merkle

Wayne Parrott

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Genetic Engineer." Plant Sciences. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"Genetic Engineer." Plant Sciences. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/science/news-wires-white-papers-and-books/genetic-engineer

"Genetic Engineer." Plant Sciences. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/news-wires-white-papers-and-books/genetic-engineer

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Herbicides

Herbicides

Herbicides are chemicals that kill plants. Herbicides are widely used in modern agriculture to control weeds, reduce competition, and increase productivity of crop plants. They are also used by homeowners to control lawn weeds and by turf grass managers, foresters, and other professionals. Herbicides are used not only on land, but also in lakes, rivers, and other aquatic environments to control aquatic weeds.

The modern use of herbicides began in the 1940s, with the development of 2,4-D (2, 4-dichlorophenelyacetic acid). By the end of that decade, herbicide use had grown from a few thousand acres to several million. There are now approximately four hundred different herbicides registered for use in the United States. While the rates of application vary by crop, the vast majority of commercial agricultural crop acreage receives at least one application of herbicide every year.

Herbicides may be applied directly to the soil or to the leaves of the target plant. Soil applications may be targeted at preventing seed germination, to affect root growth, or to be absorbed and to work systemically (within the whole plant body). Foliar (leaf) applications may target the leaves or be absorbed. In addition to directly killing the target weed, herbicides can, over time, reduce the number of weed seeds in the soil, decreasing the need for continued intensive applications in the future.

Herbicides kill plants by interfering with a fundamental process within their cells. 2,4-D is a synthetic auxin . It promotes cell elongation (rather than cell division), and in effective concentrations kills the target plant by causing unregulated growth. Plants treated with 2,4-D display misshapen stems, inappropriate adventitious root growth, and other aberrant effects (growing in an unusual location on the plant). The excessive growth exhausts food reserves, and the combination of effects eventually causes the death of the plant. 2,4-D is often used to kill dicot weeds growing among monocot crops, since monocots are more resistant to its effects. 2,4-D and a related compound , 2,4,5-T were combined in Agent Orange, the defoliant used in the Vietnam War. Health effects from exposure to Agent Orange are believed to be due to contamination with dioxin, and not to the herbicides themselves.

Glyphosphate (marketed as Roundup®) interferes with an enzyme involved in amino acid synthesis, thereby disrupting plant metabolism in a variety of ways. It is one of the most common herbicides and is available for homeowner use as well as for commercial operators. Glyphosphate is a non-selective herbicide, killing most plants that it contacts. However, it is fairly harmless to animals, including humans, since amino acid metabolism is very different in animals. A gene for glyphosphate resistance has now been introduced into a number of important crop plants, allowing increased use of glyphosphate to control weeds on these crops.

Atrazine interferes with photosynthesis. Atrazine is taken up by roots and transported to chloroplasts , where it binds to a protein in the Photo-system II reaction center . This prevents the normal flow of electrons during photosynthesis and causes chloroplast swelling and rupture.

Paraquat also interferes with photosynthesis, but through a different mechanism. This herbicide accepts electrons from photosystem I and then donates them to molecular oxygen. This forms highly reactive oxygen free radicals , which are immediately toxic to the surrounding tissue. Paraquat is also toxic to humans and other animals.

As with any agent that causes death in a group of organisms, herbicides cause natural selection among weed species. Evolution of herbicide resistance is a serious problem and has spurred research on new herbicide development and a deeper understanding of mechanisms of action. These concerns have joined with environmental and health concerns to promote a more integrated approach to weed management, combining tillage practices, selection for weed-tolerant varieties, better understanding of weed biology, and better timing of herbicide application. This integrated approach requires more time and attention from the farmer but can also offer significant benefits.

see also Agriculture, Modern; Dicots; Hormones; Monocots; Photosynthesis, Light Reactions and.

Richard Robinson

Bibliography

Aldrich, R. J., and R. J. Kremer. Principles in Weed Management, 2nd ed. Ames, IA: Iowa State University Press, 1997.

Devine, Malcolm D., Stephen O. Duke, and Carl Fedtke. Physiology of Herbicide Action. Englewood Cliffs, NJ: Prentice-Hall, 1993.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Herbicides." Plant Sciences. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"Herbicides." Plant Sciences. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/science/news-wires-white-papers-and-books/herbicides-0

"Herbicides." Plant Sciences. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/news-wires-white-papers-and-books/herbicides-0

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

herbicide

herbicide A chemical substance which suppresses, and is usually designed to eliminate, plant growth. It may be a non-selective weed-killer (e.g. paraquat); or selective, for example eliminating dicotyledonous (see DICOTYLEDON) plants from among monocotyledonous (see MONOCOTYLEDON) stands (e.g. phenoxyacetic acids) or vice versa (e.g. dalapan). A notable consequence of this kind of chemical control in cereal crops is the decline in typical weeds, e.g. poppy, and the increase of weed grasses, e.g. Avena fatua (wild oat) and Alopecurus myosuroides (black grass).

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"herbicide." A Dictionary of Plant Sciences. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"herbicide." A Dictionary of Plant Sciences. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/herbicide-0

"herbicide." A Dictionary of Plant Sciences. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/herbicide-0

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

herbicide

herbicide Pesticide used to kill weeds and other unwanted plants. Selective herbicides kill the weeds growing with crops, leaving the crops unharmed; non-selective herbicides, such as paraquat, kill all the vegetation. There are concerns over their toxicity to humans and their persistence in the environment.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"herbicide." World Encyclopedia. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"herbicide." World Encyclopedia. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/herbicide-0

"herbicide." World Encyclopedia. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/herbicide-0

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

herbicide

herbicide A chemical substance which suppresses, and is usually designed to eliminate, plant growth. It may be a non-selective weed-killer (e.g. paraquat); or selective, for example, eliminating dicotyledonous plants from among monocotyledonous stands (e.g. phenoxyacetic acids) or vice versa (e.g. dalapan).

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"herbicide." A Dictionary of Ecology. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"herbicide." A Dictionary of Ecology. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/herbicide

"herbicide." A Dictionary of Ecology. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/herbicide

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

herbicide

herb·i·cide / ˈ(h)ərbəˌsīd/ • n. a substance that is toxic to plants and is used to destroy unwanted vegetation.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"herbicide." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"herbicide." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/herbicide-0

"herbicide." The Oxford Pocket Dictionary of Current English. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/herbicide-0

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

herbicide

herbicide See pesticide.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"herbicide." A Dictionary of Biology. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"herbicide." A Dictionary of Biology. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/herbicide-1

"herbicide." A Dictionary of Biology. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/herbicide-1

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

herbicide

herbicidebackside, trackside •bedside • airside •Tayside, wayside •lakeside • stateside • graveside •quayside, seaside, Teesside •beachside • hillside • ringside •suicide • herbicide • regicide •fungicide • filicide • Barmecide •homicide •germicide, spermicide •tyrannicide • parricide •fratricide, matricide, patricide •uxoricide • countryside • infanticide •insecticide • pesticide • parasiticide •mountainside • Merseyside •Tyneside •dioxide, dockside, hydroxide, monoxide, oxide, peroxide •alongside •diopside, topside •broadside • downside • roadside •poolside • upside • nearside •fireside • Humberside • underside •genocide • waterside • riverside •silverside • overside •kerbside (US curbside) • Burnside

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"herbicide." Oxford Dictionary of Rhymes. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"herbicide." Oxford Dictionary of Rhymes. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/herbicide

"herbicide." Oxford Dictionary of Rhymes. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/herbicide

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Herbicide

Herbicide


Herbicides are chemical pesticides that are used to manage vegetation. Usually, herbicides are used to reduce the abundance of weedy plants, so as to release desired crop plants from competition . This is the context of most herbicide use in agriculture, forestry, and for lawn management. Sometimes herbicides are not used to protect crops, but to reduce the quantity or height of vegetation, for example along highways and transmission corridors. The reliance on herbicides to achieve these ends has increased greatly in recent decades, and the practice of chemical weed control appears to have become an entrenched component of the modern technological culture of humans, especially in agroecosystems.

The total use of pesticides in the United States in the mid-1980s was 957 million lb per year (434 million kg/year), used over 592,000 mi2 (1.5 million km2). Herbicides were most widely used, accounting for 68% of the total quantity [646 million lb per year [293 million kg/year]), and applied to 82% of the treated land [484,000 square miles per year (121 million hectares/year)]. Note that especially in agriculture, the same land area can be treated numerous times each year with various pesticides.

A wide range of chemicals is used as herbicides, including:

  • chlorophenoxy acids, especially 2,4-D and 2,4,5-T , which have an auxin-like growth-regulating property and are selective against broadleaved angiosperm plants;
  • triazines such as atrazine , simazine, and hexazinone;
  • chloroaliphatics such as dalapon and trichloroacetate;
  • the phosphonoalkyl chemical, glyphosate, and
  • inorganics such as various arsenicals, cyanates, and chlorates.

A "weed" is usually considered to be any plant that interferes with the productivity of a desired crop plant or some other human purpose, even though in other contexts weed species may have positive ecological and economic values. Weeds exert this effect by competing with the crop for light, water, and nutrients. Studies in Illinois demonstrated an average reduction of yield of corn or maize (Zea mays ) of 81% in unweeded plots, while a 51% reduction was reported in Minnesota. Weeds also reduce the yield of small grains, such as wheat (Triticum aestivum ) and barley (Hordeum vulgare ), by 2550%.

Because there are several herbicides that are toxic to dicotyledonous weeds but not grasses, herbicides are used most intensively used in grain crops of the Gramineae. For example, in North America almost all of the area of maize cultivation is treated with herbicides. In part this is due to the widespread use of no-tillage cultivation, a system that reduces erosion and saves fuel. Since an important purpose of plowing is to reduce the abundance of weeds, the notillage system would be impracticable if not accompanied by herbicide use. The most important herbicides used in maize cultivation are atrazine, propachlor, alachlor, 2,4-D, and butylate. Most of the area planted to other agricultural grasses such as wheat, rice (Oryza sativa ), and barley is also treated with herbicide, mostly with the phenoxy herbicides 2,4-D or MCPA.

The intended ecological effect of any pesticide application is to control a pest species, usually by reducing its abundance to below some economically acceptable threshold. In a few situations, this objective can be attained without important nontarget damage. For example, a judicious spot-application of a herbicide can allow a selective kill of large lawn weeds in a way that minimizes exposure to nontarget plants and animals.

Of course, most situations where herbicides are used are more complex and less well-controlled than this. Whenever a herbicide is broadcast-sprayed over a field or forest, a wide variety of on-site, nontarget organisms is affected, and sprayed herbicide also drifts from the target area. These cause ecotoxicological effects directly, through toxicity to nontarget organisms and ecosystems, and indirectly, by changing habitat or the abundance of food species of wildlife . These effects can be illustrated by the use of herbicides in forestry, with glyphosate used as an example.

The most frequent use of herbicides in forestry is for the release of small coniferous plants from the effects of competition with economically undesirable weeds. Usually the silvicultural use of herbicides occurs within the context of an intensive harvesting-and-management system, which may include clear-cutting , scarification, planting seedlings of a single desired species, spacing, and other practices.

Glyphosate is a commonly used herbicide in forestry and agriculture. The typical spray rate in silviculture is about 2.24.9 mi (12.2 kg) active ingredient/ha, and the typical projection is for one to two treatments per forest rotation of 40100 years.

Immediately after an aerial application in forestry, glyphosate residues are about six times higher than litter on the forest floor, which is physically shielded from spray by overtopping foliage. The persistence of glyphosate residues is relatively short, with typical half-lives of two to four weeks in foliage and the forest floor, and up to eight weeks in soil . The disappearance of residues from foliage is mostly due to translocation and wash-off, but in the forest floor and soil glyphosate is immobile (and unavailable for root uptake or leaching ) because of binding to organic matter and clay, and residue disappearance is due to microbial oxidation. Residues in oversprayed waterbodies tend to be small and short-lived. For example, two hours after a deliberate overspray on Vancouver Island, Canada, residues of glyphosate in stream water rose to high levels, then rapidly dissipated through flushing to only trace amounts 94 hours later.

Because glyphosate is soluble in water, there is no propensity for bioaccumulation in organisms in preference to the inorganic environment , or to occur in larger concentrations at higher levels of the food chain/web . This is in marked contrast to some other pesticides such as DDT, which is soluble in organic solvents but not in water, so it has a strong tendency to bioaccumulate into the fatty tissues of organisms.

As a plant poison, glyphosate acts by inhibiting the pathway by which four essential amino acids are synthesized. Only plants and some microorganisms have this metabolic pathway; animals obtain these amino acids from food. Consequently, glyphosate has a relatively small acute toxicity to animals, and there are large margins of toxicological safety in comparison with environmental exposures that are realistically expected during operational silvicultural sprays.

Acute toxicity of chemicals to mammals is often indexed by the oral dose required to kill 50% of a test population, usually of rats (i.e., rat LD50). The LD50 value for pure glyphosate is 5,600 mg/kg, and its silvicultural formulation has a value of 5,400 mg/kg. Compare these to LD50s for some chemicals which many humans ingest voluntarily: nicotine 50 mg/kg, caffeine 366, acetylsalicylic acid (ASA) 1,700, sodium chloride 3,750, and ethanol 13,000. The documented risks of longer-term, chronic exposures of mammals to glyphosate are also small, especially considering the doses that might be received during an operational treatment in forestry.

Considering the relatively small acute and chronic toxicities of glyphosate to animals, it is unlikely that wildlife inhabiting sprayed clearcuts would be directly affected by a silvicultural application. However, glyphosate causes large habitat changes through species-specific effects on plant productivity, and by changing habitat structure. Therefore, wildlife such as birds and mammals could be secondarily affected through changes in vegetation and the abundance of their arthropod foods. These indirect effects of herbicide spraying are within the context of ecotoxicology . Indirect effects can affect the abundance and reproductive success of terrestrial and aquatic wild life on a sprayed site, irrespective of a lack of direct, toxic effects.

Studies of the effects of habitat changes caused by glyphosate spraying have found relatively small effects on the abundance and species composition of wildlife. Much larger effects on wildlife are associated with other forestry practices, such as clear-cutting and the broadcast spraying of insecticides. For example, in a study of clearcuts sprayed with glyphosate in Nova Scotia, Canada, only small changes in avian abundance and species composition could be attributed to the herbicide treatment. However, such studies of bird abundance are conducted by enumerating territories, and the results cannot be interpreted in terms of reproductive success. Regrettably, there are not yet any studies of the reproductive success of birds breeding on clearcuts recently treated with a herbicide. This is an important deficiency in terms of understanding the ecological effects of herbicide spraying in forestry.

An important controversy related to herbicides focused on the military use of herbicides during the Viet Nam war. During this conflict, the United States Air Force broadcast-sprayed herbicides to deprive their enemy of food production and forest cover. More than 5,600 mi2 (14,503 km2) were sprayed at least once, about 1/7 the area of South Viet Nam. More than 55 million lb (25 million kg) of 2,4-D, 43 million lb (21 million kg) of 2,4,5-T, and 3.3 million lb (1.5 million kg) of picloram were used in this military program. The most frequently used herbicide was a 50:50 formulation of 2,4,5-T and 2,4-D known as Agent Orange . The rate of application was relatively large, averaging about 10 times the application rate for silvicultural purposes. About 86% of spray missions were targeted against forests, and the remainder against cropland.

As was the military intention, these spray missions caused great ecological damage. Opponents of the practice labelled it "ecocide," i.e., the intentional use of anti-environmental actions as a military tactic. The broader ecological effects included severe damage to mangrove and tropical forests, and a great loss of wildlife habitat.

In addition, the Agent Orange used in Viet Nam was contaminated by the dioxin isomer known as TCDD, an incidental by-product of the manufacturing process of 2,4,5-T. Using post-Vietnam manufacturing technology, the contamination by TCDD in 2,4,5-T solutions can be kept to a concentration well below the maximum of 0.1 parts per million (ppm) set by the United States Environmental Protection Agency (EPA). However, the 2,4,5-T used in Viet Nam was grossly contaminated with TCDD, with a concentration as large as 45 ppm occurring in Agent Orange, and an average of about 2.0 ppm. Perhaps 243375 lb (110170 kg) of TCDD was sprayed with herbicides onto Vietnam. TCDD is well known as being extremely toxic, and it can cause birth defects and miscarriage in laboratory mammals, although as is often the case, toxicity to humans is less well understood. There has been great controversy about the effects on soldiers and civilians exposed to TCDD in Vietnam, but epidemiological studies have been equivocal about the damages. It seems likely that the effects of TCDD added little to human mortality or to the direct ecological effects of the herbicides that were sprayed in Vietnam.

A preferable approach to pesticide use is integrated pest management (IPM). In the context of IPM, pest control is achieved by employing an array of complementary approaches, including:

  • use of natural predators, parasites , and other biological controls;
  • use of pest-resistant varieties of crops;
  • environmental modifications to reduce optimality of pest habitat;
  • careful monitoring of pest abundance; and
  • a judicious use of pesticides, when necessary as a component of the IPM strategy.

A successful IPM program can greatly reduce, but not necessarily eliminate, the reliance on pesticides.

With specific relevance to herbicides, more research into organic systems and into procedures that are pest-specific are required for the development of IPM systems. Examples of pest-specific practices are the biological control of certain introduced weeds, for example:

  • St. John's wort (Hypericum perforatum ) is a serious weed of pastures of the United States Southwest because it is toxic to cattle, but it was controlled by the introduction in 1943 of two herbivorous leaf beetles;
  • the prickly pear cactus (Opuntia spp.) became a serious weed of Australian rangelands after it was introduced as an ornamental plant, but it has been controlled by release of the moth Cactoblastis cactorum, whose larvae feed on the cactus.

Unfortunately, effective IPM systems have not yet been developed for most weed problems for which herbicides are now used. Until there are alternative, pest-specific methods to achieve an economically acceptable degree of control of weeds in agriculture and forestry, herbicides will continue to be used for that purpose.

See also Agricultural chemicals

[Bill Freedman Ph.D. ]


RESOURCES

BOOKS


Freedman, B. Environmental Ecology. 2nd Edition. San Diego, CA: Academic Press, 1995.

McEwen, F. L., and G. R. Stephenson. The Use and Significance of Pesticides in the Environment. New York: Wiley, 1979.


PERIODICALS

Pimentel, D., et al. "Environmental and Economic Effects of Reducing Pesticide Use." Bioscience 41 (1991): 402409.

. "Controversy Over the Use of Herbicides in Forestry, With Particular Reference to Glyphosate Usage." Environmental Carcinogenesis Reviews C8 (1991): 277286.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Herbicide." Environmental Encyclopedia. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"Herbicide." Environmental Encyclopedia. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/herbicide

"Herbicide." Environmental Encyclopedia. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/herbicide

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Herbicides

Herbicides

Resources

Herbicides are chemicals that kill plants considered to be pests; usually weeds. Some herbicides act against a wide variety of weeds; they are described as having a broad spectrum of activity. Other herbicides having a narrow spectrum of activity, and act against only one or several weeds.

The reason for the use of herbicides is that weeds compete with desired crop plants for light, water, nutrients, and space. This ecological interaction may decrease the productivity and yield of crop plants, resulting in economic damage. Plants may also be judged to be weeds if they interfere with some desired aesthetic effect, as is the case of weeds in lawns.

To reduce the intensity of the negative effects of weeds on the productivity of desired agricultural or forestry crops, fields may be sprayed with a herbicide that is toxic to the weeds, but not to the crop species. The commonly used herbicide 2,4-dichlorophenoxy-acetic acid (2,4-D), for example, is toxic to many broad-leaved (that is, dicotyledonous) weeds, but not to wheat, maize or corn, barley, or rice, all of which are members of the grass family (Poaceae), and therefore monocotyledonous. Consequently, the pest plants are selectively eliminated, while maintaining the growth of the desired plant species.

Modern, intensively managed agricultural systems rely on the use of herbicides and other pesticides (compounds that kill pests). Some high-yield varieties of crop species are not very tolerant of competition from weeds. Therefore, if those crops are to be successfully grown, herbicides must be used. However, the use of biocontrol in the form of insects and the use of herbicides that are less toxic is growing in popularity.

The most important chemical groups of herbicides are chlorophenoxy acids such as 2,4-D and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), triazines such as atrazine, hexazinone, and simazine; organic phosphorus chemicals such as glyphosate; amides such as alachlor and metolachlor; thiocarbamates such as butylate; dinitroanilines such as trifuralin; chloroaliphatics such as dalapon and trichloroacetate; and inorganic chemicals such as various arsenicals, cyanates, and chlorates. The first three of these groups are described in more detail below.

Chlorophenoxy acid herbicides are toxic to plants because they mimic the plants natural hormonelike auxins, and so cause lethal growth abnormalities. These herbicides are selective for broad-leaved or angiosperm plants, and are tolerated by monocots and conifers at the spray rates normally used. These chemicals are moderately persistent in the environment, with a half-life in soil typically measured in weeks, and a persistence of a year or so. The most commonly used compounds are 2,4-D; 2,4,5-T; 2-Methyl-4-chlorophenoxyacetic acid (MCPA); and 2-(2,4,5-Trichlorophenoxy)-propionic acid (silvex).

Triazine herbicides are mostly used in corn agriculture, and sometimes as soil sterilants. These chemicals are not very persistent in surface soils, but they are mobile and can cause a contamination of groundwater. Important examples of this class of chemicals are: atrazine [2-Chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine]; cynazine [2-(4-Chloro-6-ethylamino-5-triazin-2-ylamino)-2-methylpropionitrile]; hexazinone [3-Cyclohexyl-6-(dimethyl-amino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione]; metribuzin [4-Amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one]; and simazine [2-chloro-4,6-bis-(ethyl-amino)-s-triazine].

Only a few organic phosphorus herbicides are in use; they include the commonly used chemical, glyphosate (N-phosphonomethyl-glycine). Glyphosate has a wide range of agricultural uses, and it is also an important herbicide in forestry. To kill plants, glyphosate must be taken up and transported to perennating tissues, such as roots and rhizomes, where it interferes with the synthesis of certain amino acids. Because glyphosate can potentially damage many crop species, its effective use requires an understanding of seasonal changes in the vulnerability of both weeds and crop species to the herbicide. Glyphosate is not mobile in soils, has a moderate persistence, and is not very toxic to animals. Recently, varieties of certain crops, notably the oilseed canola, have been modified through genetic engineering (transgenics) to be tolerant of glyphosate herbicide. Previously, there were no effective herbicides that could be applied to canola crops to reduce weed populations, but now glyphosate can be used for this purpose. However, this has become controversial because many consumers do not want to eat foods made from transgenic crops.

In terms of quantities applied, by far the major usage of herbicides is in agriculture. Intensive systems of cultivation of most major species of annual crops requires the use of herbicides. This is especially true of crops in the grass family. For example, the great majority of the North American acreage of corn involves treatment with herbicides. The amounts of herbicide used has been declining as more acreage is planted with genetically engineered, herbicide resistant corn.

Herbicides are also widely used in landscaping, mostly to achieve grassy lawns that are relatively free of broad-leaved weeds, which many people find unattractive. Herbicides are commonly used in this way by individual landowners managing the lawns around their home, and by authorities responsible for maintaining lawns around public buildings, along roadways, and in parks. Some municipalities have banned the use of certain herbicides, and require environmentally friendly alternatives, if such lawn care is used at all. Golf courses rely heavily on intensive use of herbicides, although increasingly the use of more environmentally friendly herbicide formulations is occurring.

Forestry also uses herbicides. Usually, silvicultural herbicide use is intended to achieve a greater productivity of the desired conifer trees, by reducing the abundance of unwanted weeds. However, in most regions forestry usage of herbicides is much smaller than agriculture and lawn uses, typically less than 5% of the total use.

Herbicides were used extensively by the U.S. military during the Vietnam War (19541975). Large quantities of these chemicals were sprayed in Vietnam as a military tactic intended to deprive enemy forces and their supporters of agricultural production and forest cover. So-called Agent Orange, a 1:1 mixture of 2,4,5-T and 2,4-D, was the most commonly used herbicide. Because the intention was to destroy forests and agricultural productivity, herbicides were used at about ten times the rate typically used in forestry for management purposes. This tactical strategy of war was labeled ecocide by its opponents, because of the severe damage that was caused to natural and agricultural ecosystems, and possibly to people (there is ongoing debate about whether scientific evidence actually demonstrates the latter damage). For these reasons, and also because Agent Orange was significantly contaminated by a very toxic chemical in the dioxin family, called TCDD, the military use of herbicides in Vietnam was extremely controversial.

As exemplified in Vietnam, if herbicides are not used properly, damage may be caused to crop plants, especially if too large a dose is used, or if spraying occurs during a time when the crop species is sensitive to the herbicide. Unintended but economically important damage to crop plants is sometimes a consequence of the inappropriate use of herbicides.

In addition, some important environmental effects are associated with the use of herbicides. These include unintended damage occurring both on the sprayed site, and offsite. For example, by changing the vegetation of treated sites, herbicide use also changes the habitat of animals such as mammals and birds. This is especially true of herbicides use in forestry, because biodiverse, semi-natural habitats are involved. This is an indirect effect of herbicide use, because it does not involve toxicity caused to the animal by the herbicide. Nevertheless, the effects can be severe for some species. In addition, not all of the herbicide sprayed by a tractor or aircraft deposits onto the intended spray area. Often there is drift of herbicide beyond the intended spray site, and unintended, offsite damages may be caused to vegetation. There are also concerns about the toxicity of some herbicides, which may affect people using these chemicals during the course of their occupation (i.e., when spraying pesticides), people indirectly exposed through drift or residues on food, and wildlife. For these and other reasons, there are many negative opinions about the broadcast spraying of herbicides and other pesticides, and this practice is highly controversial.

The intention of any herbicide treatment is to reduce the abundance of weeds to below some economically acceptable threshold, judged on the basis of the amount of damage that can be tolerated to crops. Sometimes, this objective can be attained without causing significant damage to non-target plants. For example, some herbicides can be applied using spot applicators or injectors, which minimize the exposure to non-pest plants and animals. Usually, however, the typical method of herbicide application is some sort of broadcast application, in which a large area is treated all at once, generally by an aircraft or a tractor-drawn apparatus.

An important problem with broadcast applications is that they are non-selectivethey affect many plants and animals that are not weedsthe intended target of the treatment. This is especially true of herbicides, because they are toxic to a wide variety of plant species, and not just the weeds. Therefore, the broadcast spraying of herbicides results in broad exposures of non-pest species, which can cause an unintended but substantial mortality of non-target plants. For example, only a few species of plants in any agricultural field or forestry plantation are abundant enough to significantly interfere with the productivity of crop plants. Only these competitive plants are weeds, and these are the only target of a herbicide application. However, there are many other, non-pest species of plants in the field or plantation that do not interfere with the growth of the crop plants, and these are also affected by the herbicide, but not to any benefit in terms of vegetation management. In fact, especially in forestry, the non-target plants may be beneficial, by providing food and habitat for animals, and helping to prevent erosion and leaching of nutrients.

This common non-target effect of broadcast sprays of herbicides and other pesticides is an unfortunate consequence of the use of this non-selective technology to deal with pest problems. So far, effective alternatives to the broadcast use of herbicides have not been discovered for the great majority of weed management problems. However, there are a few examples that demonstrate how research could discover pest-specific methods of controlling weeds that cause little non-target damage. These mostly involve weeds introduced from foreign countries, and that became economically important pests in their new habitats. Several weed species have been successfully controlled biologically, by introducing native herbivores of invasive weeds. For example, the klamath weed (Hypericum perforatum) is a European plant that became a serious pasture weed in North America, but it was specifically controlled by the introduction of two species of herbivorous leaf beetles from its native range. In another case, the prickly pear cactus (Opuntia spp.) became an important weed in Australia after it was introduced there from North America, but it has been successfully controlled by the introduction of a moth whose larvae feed on the cactus. Unfortunately, few weed problems can now be dealt with in these specific ways, and until better methods of control are discovered, herbicides will continue to be used in agriculture, forestry, and for other reasons.

Most herbicides are specifically plant poisons, and are not very toxic to animals. (There are exceptions, however, as is the case with the herbicide paraquat.) However, by inducing large changes in vegetation, herbicides can indirectly affect populations of birds, mammals, insects, and other animals through changes in the nature of their habitat.

For example, studies in Britain suggest that since the 1950s, there have been large changes in the populations of some birds that breed on agricultural land. These changes may be partly caused by the extensive use of herbicides, a practice that has changed the species and abundance of non-crop plants in agroeco-systems. This affects the structure of habitats, the availability of nest sites, the food available to granivorous birds, which mostly eat weed seeds, and the food available for birds that eat arthropods, which rely mainly on non-crop plants for nourishment and habitat. During the time that herbicide use was increasing in Britain, there were also other changes in agricultural practices. These include the elimination of hedgerows from many landscapes, changes in cultivation methodologies, new crop species, increases in the use of insecticides and fungicides, and improved methods of seed cleaning, resulting in fewer weed seeds being sown with crop seed. Still, a common opinion of ecologists studying the large declines of birds, such as the gray partridge (Perdix perdix), is that herbicide use has

KEY TERMS

Active ingredient The particular chemical within a pesticidal formulation that causes toxicity to the pest. Pesticide formulations also contain non-pesticidal chemicals, known as inert ingredients. These may be used to dilute the active ingredient, to improve its spread or adherence on foliar surfaces, or to otherwise increase the efficacy of the formulation.

Agroecosystem Any agricultural ecosystem, comprised of crop species, non-crop plants and animals, and their environment.

Drift Movement of sprayed pesticide by wind beyond the intended place of treatment.

Half-life The time required for disappearance of one-half of an initial quantity of a pesticide.

Non-target effects Effects on organisms other than the intended pest target of pesticide spraying.

Persistence The length of time that a pesticide occurs in some component of the environment (e.g., in soil). Persistence is influenced by the rate of chemical breakdown and by mass-transport processes such as volatilization, erosion of pesticide-containing particles, and the flushing of water in streams or ponds.

Weed Any plant that is growing abundantly in a place where humans do not want it to be.

played a central but indirect role by causing habitat changes, especially by decreasing the abundance of weed seeds and arthropods available as food for the birds.

Resources

BOOKS

Committee on the Assessment of Wartime Exposure to Herbicindes in Vietnam. Characterizing Exposure of Veterans to Agent Orange and Other Herbicides Used in Vietnam: Interim Findings and Recommendations. Washington, DC: National Academies Press, 2003.

Vietnam Courier. Herbicides and Defoliants in War: The Long-Term Effects on Man and Nature. Vietnam Courier, 2003.

Weed Science Society of America. Herbicide Handbook. 8th ed. Lawrence, KS: Weed Science Society of America, 2002.

Bill Freedman

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Herbicides." The Gale Encyclopedia of Science. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"Herbicides." The Gale Encyclopedia of Science. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/herbicides

"Herbicides." The Gale Encyclopedia of Science. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/herbicides

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Herbicides

Herbicides

Introduction

Weeds can be considered as plants growing in the wrong places and often having few, if any, desirable properties. They compete with crops and garden plants for light and nutrients from the soil. Traditionally, weeds are removed by hand, but they can also be dealt with by applying chemicals called herbicides that will kill them. Herbicides are widely used today in agriculture, horticulture, and in recreational gardening.

A successful herbicide is specific, killing only the weed and not the crop. It should also not be toxic to humans or other animals, and should break down rapidly so that it does not contaminate soil or groundwater. The modern trend is toward using genetic modification to create crops that are resistant to herbicides. Such crops are protected against the impact of herbicides used on nearby weeds. However, the general public has been reluctant to accept such developments.

Historical Background and Scientific Foundations

Pesticides are chemicals that kill off any animal or plant that threatens a crop. Herbicides are a type of pesticide that kills weeds, which are plants that compete with crops for resources. Although some organisms produce their own pesticide molecules, modern pesticides tend to be synthetic organic chemicals produced by the chemical industry, usually from raw materials derived from petroleum. The first herbicides were produced in the 1940s and formed part of the so-called Green Revolution, where pesticides and fertilizers began to be widely used in agriculture in an attempt to boost crop productivity.

Today, chemists put as much effort into creating a new herbicide as they would a new drug. There are several different classes of herbicide and hundreds of different products. Some inhibit photosynthesis, so the weed turns yellow and dies, others are so-called root mitotic inhibitors, which stop root cells from dividing so the root withers away. One of the oldest herbicides is 2,4-dichlorophenoxyacetic acid (2,4-D), which is a hormonal agent that makes weeds grow extremely rapidly and then disintegrate. One of the most widely used herbicides today is glyphosphate, known as Roundup, which inhibits a specific plant enzyme. It is a broad-spectrum herbicide, which kills most weeds and is often used to clear ground.

The most important property of a herbicide is selectivity, which is the ability to kill the weed, but not the crop or any other plant or animal. Modern herbicides must not harm the environment by persisting in the soil or in groundwater. They are potentially toxic to humans if deliberately or accidentally ingested, so people using them on a large scale need to be properly trained and wear protective clothing. Most herbicides are meant for either foliar application, in which case the leaves of the weeds are sprayed, or application to the ground. They are usually applied in agriculture from a low-flying plane or using a tractor.

Impacts and Issues

In the past, there have been problems with a herbicide called 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), a popular herbicide for broad-leaved weeds. The manufacturing process contaminated the product with dioxins, which are very toxic to humans. Accordingly, 2,4,5-T was withdrawn from use in the United States in 1983. The herbicide 2,4,5-T was also a component of Agent Orange, used as a defoliant by the U.S. military during the Vietnam war, so-called because it was stored in orange barrels. Its use has been linked to ill health in both Vietnam veterans and the Vietnamese people.

Today, the science of herbicides has advanced to include plants genetically modified to be resistant to glyphosphate or glufosinate. These plants are not affected if they are sprayed with the relevant herbicide because they carry a gene for resistance to it. This means that crop residue can be left on land after spraying, which has the benefit of reducing soil erosion. However, there is concern that the resistance genes could be transferred in the wild to weeds, creating a breed of superweed, which would not be sensitive to the herbicide. There is

WORDS TO KNOW

FOLIAR: Pertaining to the leaf.

HERBICIDE SELECTIVITY: The ability to kill weeds while leaving a crop unharmed.

also public concern over genetically modified food crops, so herbicide resistance has tended to be confined to crops like alfalfa or cotton that are not meant for human consumption.

See Also Agricultural Practice Impacts; Groundwater Quality

BIBLIOGRAPHY

Books

Hanson, J. R. Chemistry in the Garden. CambridgeRoyal Society of Chemistry, 2007.

Web Sites

Division of Crop Sciences, University of Illinois. “How Herbicides Work.” http://web.aces.uiuc.edu/vista/pdf_pubs/HERBWORK.PDF (accessed March 25, 2008).

Mount Shasta Herald. “Growers Hear About New Roundup Ready Alfalfa.” February 16, 2005. http://www.monsanto.co.uk/news/ukshowlib.phtml?uid=8599 (accessed March 25, 2008).

U.S. Department of Agriculture. “Agricultural Chemical Usage.” http://www.nass.usda.gov/Statistics_by_State/Pennsylvania/Publications/Annual_Statistical_Bulletin/2006_2007/nur_chemuse.pdf (accessed March 25, 2008).

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Herbicides." Environmental Science: In Context. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"Herbicides." Environmental Science: In Context. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/environment/energy-government-and-defense-magazines/herbicides

"Herbicides." Environmental Science: In Context. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/environment/energy-government-and-defense-magazines/herbicides

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Herbicides

Herbicides

A herbicide is a chemical used to kill or otherwise manage certain species of plants considered to be pests . Plant pests, or weeds, compete with desired crop plants for light , water , nutrients , and space. This ecological interaction may decrease the productivity and yield of crop plants, thereby resulting in economic damage. Plants may also be judged to be weeds if they interfere with some desired aesthetic effect, as is the case of weeds in lawns.

Clearly, the designation of plants as weeds involves a human judgment. However, in other times and places weeds may be judged to have positive values. For example, in large parts of North America , the red raspberry (Rubus strigosus) is widely considered to be one of the most important weeds in forestry . However, this species also has positive attributes. Its fruits are gathered and eaten by people and wildlife . This vigorously growing plant also provides useful ecological services. For example, it binds soil and helps prevent erosion , and takes up nutrients from the soil, which might otherwise be leached away by rainwater because there are so few plants after disturbance of the site by clear-cutting or wildfire . These ecological services help to maintain site fertility.

Still, it is undeniable that in certain situations weeds exert a significant interference with human purposes. To reduce the intensity of the negative effects of weeds on the productivity of desired agricultural or forestry crops , fields may be sprayed with a herbicide that is toxic to the weeds, but not to the crop species. The commonly used herbicide 2,4-D, for example, is toxic to many broad-leaved (that is, dicotyledonous) weeds, but not to wheat , maize or corn, barley , or rice , all of which are members of the grass family (Poaceae), and therefore monocotyledonous. Consequently, the pest plants are selectively eliminated, while maintaining the growth of the desired plant species.

Modern, intensively managed agricultural systems have an intrinsic reliance on the use of herbicides and other pesticides . Some high-yield varieties of crop species are not very tolerant of competition from weeds. Therefore, if those crops are to be successfully grown, herbicides must be used. Many studies have indicated the shorter-term benefits of herbicide use. For example, studies of the cultivation of maize in Illinois have demonstrated that the average reduction of yield was 81% in unweeded plots, while a 51% reduction was reported in Minnesota. Yields of wheat and barley can be reduced by 25-50% as a result of competition from weeds. To reduce these important, negative influences of weeds on agricultural productivity, herbicides are commonly applied to agricultural fields. As noted above, the herbicide must be toxic to the weeds, but not to the crop species.

Types of herbicides

The most important chemical groups of herbicides are chlorophenoxy acids such as 2,4-D and 2,4,5-T; triazines such as atrazine, hexazinone, and simazine; organic phosphorus chemicals such as glyphosate; amides such as alachlor and metolachlor; thiocarbamates such as butylate; dinitroanilines such as trifuralin; chloroaliphatics such as dalapon and trichloroacetate; and inorganic chemicals such as various arsenicals, cyanates, and chlorates. The first three of these groups are described in more detail below.


Chlorophenoxy acid herbicides

Chlorophenoxy acid herbicides cause toxicity to plants by mimicking their natural hormone-like auxins, and thereby causing lethal growth abnormalities. These herbicides are selective for broad-leaved or angiosperm plants, and are tolerated by monocots and conifers at the spray rates normally used. These chemicals are moderately persistent in the environment, with a half-life in soil typically measured in weeks, and a persistence of a year or so. The most commonly used compounds are 2,4-D (2,4-Dichlorophenoxyacetic acid); 2,4,5-T (2,4,5-Trichlorophenoxyacetic acid); MCPA (2-Methyl-4-chlorophenoxyacetic acid); and silvex [2-(2,4,5-Trichlorophenoxy)-propionic acid].


Triazine herbicides

Triazine herbicides are mostly used in corn agriculture, and sometimes as soil sterilants. These chemicals are not very persistent in surface soils, but they are mobile and can cause a contamination of groundwater . Important examples of this class of chemicals are: atrazine [2-Chloro-4-(ethylamino)-6-(isopropylamino)s-triazine]; cynazine [2-(4-Chloro-6-ethylamino -5-triazin-2-ylamino)-2-methylpropionitrile]; hexazinone [3-Cyclohexyl-6-(dimethyl-amino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione]; metribuzin [4-Amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one]; and simazine [2-chloro-4,6-bis-(ethyl-amino)-s-triazine].


Organic phosphorus herbicides

Organic phosphorus herbicides are few, but they include the commonly used chemical, glyphosate (N-phosphonomethyl-glycine). Glyphosate has a wide range of agricultural uses, and it is also an important herbicide in forestry. To kill plants, glyphosate must be taken up and transported to perennating tissues, such as roots and rhizomes, where it interferes with the synthesis of certain amino acids. Because glyphosate can potentially damage many crop species, its effective use requires an understanding of seasonal changes in the vulnerability of both weeds and crop species to the herbicide. Glyphosate is not mobile in soils, has a moderate persistence, and is not very toxic to animals. Recently, varieties of certain crops, notably the oilseed canola, have been modified through genetic engineering (transgenics ) to be tolerant of glyphosate herbicide. Previously, there were no effective herbicides that could be applied to canola crops to reduce weed populations, but now glyphosate can be used for this purpose. However, this has become controversial because many consumers do not want to eat foods made from transgenic crops.

Use of herbicides

In 1990, a total of about 290 herbicidal chemicals were available for use. Many of these chemicals are used in various types of formulations, each of which is a specific combination of the herbicidal active ingredient, a solvent such as water or kerosene, and various chemicals intended to enhance the efficacy of the herbicide, for example, by increasing the ability of the spray to adhere to foliage, or to spread freely on leaf surfaces. In addition, different companies often manufacture and sell the same formulations under different names, so the number of commercial products is larger than the number of actual formulations.

The United States accounts for about one-third of the global use of pesticides, much more than any other country. In 1989, herbicides accounted for about 61% of the 1,100 million lb (500 million kg) of pesticides used domestically in the United States. During recent years, eight of the ten most commonly used pesticides in the United States have been herbicides. Listed in order of decreasing quantities used, these herbicides are: alachlor (100 million lb [45 million kg] used per year), atrazine (100 million lb [45 million kg]), 2,4-D (53 million lb [24 million kg]), butylate (44 million lb [20 million kg]), metolachlor (44 million lb [20 million kg]), trifluralin (31 million lb [14 million kg]), cynazine (20 million lb [9 million kg]), and metribuzin (13 million lb [6 million kg]). During the mid-1980s, amide herbicides accounted for 30% of the herbicides used in the United States, triazines 22%, carbamates 13%, N-anilines 11%, and phenoxys 5%. These data reflect a large decrease in the usage of phenoxy herbicides, which were used much more commonly prior to the 1980s. For example, during the mid-1970s, about 50-80% of the small-grain acreage in North America was treated with phenoxy herbicides, mostly with 2,4-D. Since the mid 1990s, the use of glyphosate has increased tremendously.

In terms of quantities applied, by far the major usage of herbicides is in agriculture. Intensive systems of cultivation of most major species of annual crops requires the use of herbicides. This is especially true of crops in the grass family. For example, at least 83% of the North American acreage of maize (or corn) cultivation involves treatment with herbicides. In part, herbicide use is important in maize cultivation because of the common use of zero tillage systems. Zero tillage involves direct seeding into unploughed soil, a system that has great benefits by reducing erosion and saving fuel, because tractors are used much less. However, one of the most important agricultural benefits of ploughing is the reduction of weeds that results. Consequently, zero tillage systems would be not be practical if they were not accompanied by the use of herbicides. This is only one example—most of the areas of grain crops cultivated in North America and other industrialized countries receive herbicide treatments.

Herbicides are also widely used in landscaping, mostly to achieve grassy lawns that are relatively free of broad-leaved weeds, which many people find unattractive. Herbicides are commonly used in this way by individual landowners managing the lawns around their home, and by authorities responsible for maintaining lawns around public buildings, along roadways, and in parks. Golf courses rely heavily on intensive use of herbicides. This is particularly true of putting greens, where it is important to have a very consistent lawn. In fact, the intensity of pesticide use on golf-course putting greens is greater than in almost any other usage in agriculture.

Forestry also uses herbicides. Usually, silvicultural herbicide use is intended to achieve a greater productivity of the desired conifer trees, by reducing the abundance of unwanted weeds. However, in most regions forestry usage of herbicides is much smaller than agriculture and lawn uses, typically less than 5% of the total use.

Herbicides were used extensively by the U.S. military during the Vietnam War. Large quantities of these chemicals were sprayed in Vietnam as a military tactic intended to deprive enemy forces and their supporters of agricultural production and forest cover. So-called "Agent Orange," a 1:1 mixture of 2,4,5-T and 2,4-D, was the most commonly used herbicide. Because the intention was to destroy forests and agricultural productivity, herbicides were used at about ten times the rate typically used in forestry for management purposes. This tactical strategy of war was labeled "ecocide" by its opponents, because of the severe damage that was caused to natural and agricultural ecosystems, and possibly to people (there is ongoing debate about whether scientific evidence actually demonstrates the latter damage). For these reasons, and also because Agent Orange was significantly contaminated by a very toxic chemical in the dioxin family, called TCDD, the military use of herbicides in Vietnam was extremely controversial.

Environmental effects of herbicide use

As has been suggested above, some substantial benefits can be gained through the use of herbicides to manage unwanted vegetation. Compared with alternative means of weed control, such as mechanically weeding by hand or machine, herbicides are less expensive, often safer (especially in forestry), faster, and sometimes more selective.

However, if herbicides are not used properly, damage may be caused to crop plants, especially if too large a dose is used, or if spraying occurs during a time when the crop species is sensitive to the herbicide. Unintended but economically important damage to crop plants is sometimes a consequence of the inappropriate use of herbicides.

In addition, some important environmental effects are associated with the use of herbicides. These include unintended damage occurring both on the sprayed site, and offsite. For example, by changing the vegetation of treated sites, herbicide use also changes the habitat of animals such as mammals and birds . This is especially true of herbicides use in forestry, because biodiverse, semi-natural habitats are involved. This is an indirect effect of herbicide use, because it does not involve toxicity caused to the animal by the herbicide. Nevertheless, the effects can be severe for some species. In addition, not all of the herbicide sprayed by a tractor or aircraft deposits onto the intended spray area. Often there is drift of herbicide beyond the intended spray site, and unintended, offsite damages may be caused to vegetation. There are also concerns about the toxicity of some herbicides, which may affect people using these chemicals during the course of their occupation (i.e., when spraying pesticides), people indirectly exposed through drift or residues on food, and wildlife. For these and other reasons, there are many negative opinions about the broadcast spraying of herbicides and other pesticides, and this practice is highly controversial.

The intention of any herbicide treatment is to reduce the abundance of weeds to below some economically acceptable threshold, judged on the basis of the amount of damage that can be tolerated to crops. Sometimes, this objective can be attained without causing significant damage to non-target plants. For example, some herbicides can be applied using spot applicators or injectors, which minimize the exposure to non-pest plants and animals. Usually, however, the typical method of herbicide application is some sort of broadcast application, in which a large area is treated all at once, generally by an aircraft or a tractor-drawn apparatus.

An important problem with broadcast applications is that they are non-selective—they affect many plants and animals that are not weeds—the intended target of the treatment. This is especially true of herbicides, because they are toxic to a wide variety of plant species, and not just the weeds. Therefore, the broadcast spraying of herbicides results in broad exposures of non-pest species, which can cause an unintended but substantial mortality of non-target plants. For example, only a few species of plants in any agricultural field or forestry plantation are abundant enough to significantly interfere with the productivity of crop plants. Only these competitive plants are weeds, and these are the only target of a herbicide application. However, there are many other, non-pest species of plants in the field or plantation that do not interfere with the growth of the crop plants, and these are also affected by the herbicide, but not to any benefit in terms of vegetation management. In fact, especially in forestry, the non-target plants may be beneficial, by providing food and habitat for animals, and helping to prevent erosion and leaching of nutrients.

This common non-target effect of broadcast sprays of herbicides and other pesticides is an unfortunate consequence of the use of this non-selective technology to deal with pest problems. So far, effective alternatives to the broadcast use of herbicides have not been discovered for the great majority of weed management problems. However, there are a few examples that demonstrate how research could discover pest-specific methods of controlling weeds that cause little non-target damage. These mostly involve weeds introduced from foreign countries, and that became economically important pests in their new habitats. Several weed species have been successfully controlled biologically, by introducing native herbivores of invasive weeds. For example, the klamath weed (Hypericum perforatum) is a European plant that became a serious pasture weed in North America, but it was specifically controlled by the introduction of two species of herbivorous leaf beetles from its native range. In another case, the prickly pear cactus (Opuntia spp.) became an important weed in Australia after it was introduced there from North America, but it has been successfully controlled by the introduction of a moth whose larvae feed on the cactus. Unfortunately, few weed problems can now be dealt with in these specific ways, and until better methods of control are discovered, herbicides will continue to be used in agriculture, forestry, and for other reasons.

Most herbicides are specifically plant poisons, and are not very toxic to animals. (There are exceptions, however, as is the case with the herbicide paraquat.) However, by inducing large changes in vegetation, herbicides can indirectly affect populations of birds, mammals, insects , and other animals through changes in the nature of their habitat.

For example, studies in Britain suggest that since the 1950s, there have been large changes in the populations of some birds that breed on agricultural land. These changes may be partly caused by the extensive use of herbicides, a practice that has changed the species and abundance of non-crop plants in agroecosystems. This affects the structure of habitats, the availability of nest sites, the food available to granivorous birds, which mostly eat weed seeds , and the food available for birds that eat arthropods , which rely mainly on non-crop plants for nourishment and habitat. During the time that herbicide use was increasing in Britain, there were also other changes in agricultural practices. These include the elimination of hedgerows from many landscapes, changes in cultivation methodologies, new crop species, increases in the use of insecticides and fungicides, and improved methods of seed cleaning, resulting in fewer weed seeds being sown with crop seed. Still, a common opinion of ecologists studying the large declines of birds, such as the gray partridge (Perdix perdix), is that herbicide use has played a central but indirect role by causing habitat changes, especially by decreasing the abundance of weed seeds and arthropods available as food for the birds.

Similarly, the herbicides most commonly used in forestry are not particularly toxic to animals. Their use does however, cause large changes in the habitat available on clear-cuts and plantations, and these might be expected to diminish the suitability of sprayed sites for the many species of song birds , mammals, and other animals that utilize those habitats.

Modern, intensively managed agricultural and forestry systems have an intrinsic reliance on the use of herbicides and other pesticides. Unfortunately, the use of herbicides and other pesticides carries risks to humans through exposure to these potentially toxic chemicals, and to ecosystems through direct toxicity caused to non-target species, and through changes in habitat. Nevertheless, until newer and more pest-specific solutions to weed-management problems are developed, there will be a continued reliance on herbicides in agriculture, forestry, and for other purposes, such as lawn care.


Resources

books

Briggs, S.A. Basic Guide to Pesticides: Their Characteristics and Hazards. Washington, DC: Taylor & Francis, 1992.

Freedman, B. Environmental Ecology. 2nd ed. San Diego: Academic Press, 1995.

Hayes, W.J., and E.R. Laws, eds. Handbook of Pesticide Toxicology. San Diego: Academic Press, 1991.

periodicals

Pimentel, D., et al. "Environmental and Economic Costs of Pesticide Use." Bioscience 41 (1992): 402-409.


Bill Freedman

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Active ingredient

—The particular chemical within a pesticidal formulation that causes toxicity to the pest. Pesticide formulations also contain non-pesticidal chemicals, known as inert ingredients. These may be used to dilute the active ingredient, to improve its spread or adherence on foliar surfaces, or to otherwise increase the efficacy of the formulation.

Agroecosystem

—Any agricultural ecosystem, comprised of crop species, non-crop plants and animals, and their environment.

Drift

—Movement of sprayed pesticide by wind beyond the intended place of treatment.

Half-life

—The time required for disappearance of one-half of an initial quantity of a pesticide.

Non-target effects

—Effects on organisms other than the intended pest target of pesticide spraying.

Persistence

—The length of time that a pesticide occurs in some component of the environment (e.g., in soil). Persistence is influenced by the rate of chemical breakdown and by mass-transport processes such as volatilization, erosion of pesticide-containing particles, and the flushing of water in streams or ponds.

Weed

—Any plant that is growing abundantly in a place where humans do not want it to be.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Herbicides." The Gale Encyclopedia of Science. . Encyclopedia.com. 17 Dec. 2018 <https://www.encyclopedia.com>.

"Herbicides." The Gale Encyclopedia of Science. . Encyclopedia.com. (December 17, 2018). https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/herbicides-0

"Herbicides." The Gale Encyclopedia of Science. . Retrieved December 17, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/herbicides-0

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.