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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>.

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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

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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

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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.

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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.

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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.

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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

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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.

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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).

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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.

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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).

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herbicide

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

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herbicide

herbicide See pesticide.

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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

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