Herbicides

HERBICIDES

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

In the United States in 1997, there were an estimated $6.8 billion in sales of herbicides and plant growth regulators. Herbicides constitute a large percentage of total pesticide use. Worldwide in 1997, there were 5.7 billion pounds of pesticides used, of which 2.2 billion were herbicides. Of the1.2 billion pounds of conventional pesticides used in the United States in 1997, a total of 568 million pounds of herbicides were used—470 million pounds in agriculture, 48 million pounds in industry and government, and 49 million pounds in households. The largest quantities are associated with on crops planted to large acreages, such as soy, cotton, corn, and canola.

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

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

Table 1

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

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

LYNN R. GOLDMAN

(SEE ALSO: Farm Injuries; Pesticides; Toxic Substances Control Act; Toxicology)

BIBLIOGRAPHY

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

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

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