Background (Encyclopedia of Global Resources)
Herbicides are used for the control of grasses, weeds, and other plant pests. These chemical compounds kill plants or inhibit their normal growth. In general, herbicides work by interfering with photosynthesis, so that a plant dies from lack of energy, or by a combination of defoliation (leaf removal) and systemic herbicidal action.
Herbicides are used to clear rights-of-way beneath power lines and along railways and roads. In agriculture and forest management, they are used to control weeds or to remove the leaves from some crop plants to facilitate harvesting. While herbicides may be employed in lieu of tillage, their use is more often in conjunction with tillage and other agronomic practices. During wartime, defoliants and other herbicides have been used to destroy plants that an enemy uses for cover during battle or for food.
(The entire section is 134 words.)
Types of Herbicides (Encyclopedia of Global Resources)
Herbicides may be selective or nonselective. Selective herbicides, such as amitrole, atrazine, monuron, pyridine, 2,4-dichlorophenoxyacetic acid (2,4-D), and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), target a particular plant pest and will kill or stunt weeds among crop plants without injuring the crop. For example, 2,4-D targets soft-stemmed plants, while 2,4,5-T is effective against woody plants. Cereals are crops particularly suited for treatment with 2,4-D, since the compound does not harm narrow-leafed plants but kills broad-leaved weeds. Selective toxicity minimizes the environmental impact of an herbicide. Nonselective herbicides (also called broad-spectrum or general-usage herbicides) are toxic to all plants. Examples include dinoseb, diquat, paraquat, and arsenic trioxide. Nonselective compounds are best suited for areas where all plant growth is to be suppressed, such as along railroad rights-of-way.
Some compounds, known as contact herbicides, kill only those plant parts to which they are directly applied. Others, called systemic herbicides, are absorbed through the plant’s foliage or roots and carried to other parts of the plant. When mixed with the soil, some herbicides kill germinating seeds and small seedlings.
Popular inorganic herbicides include ammonium sulfate, sodium chlorate, sulfuric acid solutions, and borate formulations. Among the organic herbicides are the organic arsenicals,...
(The entire section is 231 words.)
History (Encyclopedia of Global Resources)
Agricultural societies have used simple chemical herbicides such as ashes and common salts for centuries. In 1896, a fungicidal compound known as Bordeaux mixture (a combination of copper sulfate, lime, and water) was found also to be effective against some weeds. Subsequently, copper sulfate was employed as a selective weed killer in cereal crops. By the early 1900’s, sodium arsenate solutions and other selective inorganic herbicidal mixtures had been developed. In 1932, dinitrophenol compounds were introduced.
In the early 1940’s, a new generation of herbicidal compound emerged. In an attempt to mimic natural plant hormones, the defoliant 2,4-D was created. At low concentrations 2,4-D promotes retention of fruit and leaves; at higher concentrations, it overstimulates plant metabolism, causing the leaves to drop off. A related chemical, 2,4,5-T, came into general use in 1948. The years after World War II saw the first large-scale application of herbicides in agriculture and other areas. The new defoliants rapidly gained acceptance because of their effectiveness against broad-leaved weeds in corn, sorghum, small grains, and grass pastures.
A few years after their development, these defoliants were employed as chemical weapons. During its conflict with Communist guerrillas in Malaya during the late 1940’s and early 1950’s, Britain sprayed 2,4,5-T on crops and jungle foliage to deprive the guerrillas of food and cover....
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U.S. Regulation of Herbicides (Encyclopedia of Global Resources)
In 1947, the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) authorized the United States Department of Agriculture (USDA) to oversee registration of herbicides and other pesticides and to determine their safety and effectiveness. In December, 1970, the newly formed United States Environmental Protection Agency (EPA) assumed statutory authority from the USDA over pesticide regulations. Under the Federal Environmental Pesticide Control Act of 1972, an amendment to FIFRA, manufacturers must register all marketed pesticides with the EPA before the product is released. Before registration, the chemicals must undergo exhaustive trials to assess their potential impact on the environment and human health. The EPA’s decision to grant registration is based on the determination that unreasonable adverse effects on human health or the environment are not anticipated within the constraints of approved usage. Beginning in October, 1977, the EPA classified all pesticides to which it has granted registration as either a restricted-usage (to be applied only by certified pest control operators) or unclassified (general-usage) pesticide.
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Further Reading (Encyclopedia of Global Resources)
Clark, J. Marshall, and Hideo Ohkawa, eds. Environmental Fate and Safety Management of Agrochemicals. Washington, D.C.: American Chemical Society, 2005.
_______. New Discoveries in Agrochemicals. Washington, D.C.: American Chemical Society, 2005.
Crone, Hugh D. Chemicals and Society: A Guide to the New Chemical Age. New York: Cambridge University Press, 1986.
Monaco, Thomas J., Stephen C. Weller, and Floyd M. Ashton. Weed Science: Principles and Practices. 4th ed. New York: Wiley, 2002.
Vencill, William K., et al., eds. Herbicide Handbook. 8th ed. Lawrence, Kans.: Weed Science Society of America, 2002.
Ware, George W. Complete Guide to Pest Control: With and Without Chemicals. 4th ed. Willoughby, Ohio: MeisterPro Information Resources, 2005.
_______. Fundamentals of Pesticides: A Self-Instruction Guide. 2d ed. Fresno, Calif.: Thomson, 1986.
Zimdahl, Robert L. Fundamentals of Weed Science. Boston: Elsevier/Academic Press, 2007.
Agriculture and Agri-Food Canada. Manure, Fertilizer, and Pesticide Management in Canada. http://www4.agr.gc.ca/AAFC-AAC/display-afficher.do?id=1178825328101&lang=eng
Health Canada. Pesticides and Pest Management. http://www.hc-sc.gc.ca/cps-spc/pest/index-eng.php
U.S. Environmental Protection Agency. Pesticides....
(The entire section is 177 words.)
Herbicides (Encyclopedia of Public Health)
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 used70 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
|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|
|Heavy metals||Lead arsenate, arsenicals|
|Nitrophenolic and dinitrocresolic herbicides||Dinitrophenol, dinitrocresol, dinoseb, dinosulfon|
|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)
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):5485s5488s.
Herbicides (Encyclopedia of Food & Culture)
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 (37287 B.C.E.) mentions killing trees by pouring olive oil over their roots. Cato (23449 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; ; Ecology and Food; Government Agencies; Pesticides; Safety, Food; Toxins, Unnatural, and Food Safety.
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