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

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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.