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.

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 25–50%.

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 2.2–4.9 mi (1–2.2 kg) active ingredient/ha, and the typical projection is for one to two treatments per forest rotation of 40–100 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 243–375 lb (110–170 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): 402–409.

——. "Controversy Over the Use of Herbicides in Forestry, With Particular Reference to Glyphosate Usage." Environmental Carcinogenesis Reviews C8 (1991): 277–286.

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 (1954–1975). 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-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 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

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 selective—that 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 well—a 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

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 benefit–cost 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>.

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 (372–287 B.C.E.) mentions killing trees by pouring olive oil over their roots. Cato (234–149 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