A defoliant is any herbicidal chemical sprayed or dusted on plants to cause their leaves to fall off. Defoliants are widely used for the selective removal of weeds in managing croplands and lawns. Such use worldwide, along with the development of other herbicides and pesticides, allowed for an increase in agricultural production around the mid-20th century during what was called the Green Revolution. Defoliants have also been sometimes used in warfare as a means to deprive an enemy of food crops and/or hiding cover.
Use and application
A primary application of defoliants is the selective killing of plants. Two of the oldest chemical herbicides used as defoliants are 2,4-Dichlorophenoxyacetic acid (2,4-D) and 2,4,5-Trichlorophenoxyacetic acid (2,4,5-T). 2,4-D and 2,4,5-T are absorbed broad-leafed plants, killing them by causing excessive hormonal growth. These phenoxy herbicides were designed to selectively kill weeds and unwanted plants in croplands. They were first introduced at the beginning of World War II and became widespread in use in agriculture following the end of the War.
Defoliants have a practical use in the harvesting of certain crops, particularly cotton, in the United States of America as well as a number of other cotton-producing countries. The use of defoliants aids in the effective harvesting of cotton and the cultivation of finer lint quality. The effectiveness of defoliant use in cotton harvesting relies on the type of defoliant(s) used, the number of applications, the amount applied, and environmental variables. Common harvest-aiding, chemical defoliants include tribufos, dimethipin, and thidiazuron. According to a 1998 report by the U.S. Department of Agriculture National Agricultural Statistics Service (NASS), tribufos and thidiazuron accounted for 60% of crop area that was treated by defoliants during that crop year.
Health and environmental effects
In 1998, the U.S. Environmental Protection Agency (USEPA) claimed that the use of agricultural defoliants led to increased risks of water contamination and dangers to freshwater and marine life. High doses of tribufos were labeled as a possible carcinogen and a toxin to freshwater and marine invertebrates. Dimethipin has also been labeled as a possible human carcinogen.
A published study in the Journal of Agricultural and Food Chemistry reported that through successive surface runoff events in defoliated cotton fields, the concentration of defoliants within the test area decreased exponentially. Thus they concluded that the use of defoliants in cotton fields could negatively affect marine life in runoff zones.
Agent Orange, a defoliant used by the U.S. Military during the Vietnam War to defoliate regions of Vietnam from 1961 to 1971, has been linked to several long-term health issues. Agent Orange contains a mixture of 2,4-D and 2,4,5-T and contains dioxin contaminants. Members of the Air Force Ranch Hand and the Army Chemical Corps who served in the Vietnam War were occupationally exposed to Agent Orange and they have shown a positive relationship between occupational exposure and diagnosis of diabetes, heart disease, hypertension, and chronic respiratory diseases.
Those who are occupationally exposed to defoliants also are at a higher risk of developing Alzheimer's disease. Among other occupations, a study has shown that farmers are at a significantly higher risk of developing Alzheimer's disease due to a greater chance of defoliant exposure.
- Zierler, David (2011). Invention of Ecocide. Athens, Georgia: University of Georgia Press. ISBN 9786613110404.
- Snipes, Charles E.; Cathey, George W. (1992). "Evaluation of defoliant mixtures in cotton". Field Crops Research. 28 (4): 327–334. doi:10.1016/0378-4290(92)90018-5. ISSN 0378-4290.
- Potter, Thomas L.; Marti, Luz; Belflower, Sally; Truman, Clint C. (2000). "Multiresidue Analysis of Cotton Defoliant, Herbicide, and Insecticide Residues in Water by Solid-Phase Extraction and GC−NPD, GC−MS, and HPLC−Diode Array Detection". Journal of Agricultural and Food Chemistry. 48 (9): 4103–4108. doi:10.1021/jf9909104. ISSN 0021-8561.
- NASS (1999). Agricultural Chemical Usage 1998 Field Crops Summary. Washington, D.C.: U. S. Department of Agriculture National Agricultural Statistics and Economic Research Service. pg 141.
- Tow Fawthrop (June 14, 2004). "Vietnam's war against Agent Orange". bbc.co.uk.
- "Agent Orange Linked To Skin Cancer Risk". Science 2.0. Retrieved 17 September 2014.
- Kang, Han K.; Dalager, Nancy A.; Needham, Larry L.; Patterson, Donald G.; Lees, Peter S.J.; Yates, Katherine; Matanoski, Genevieve M. (2006). "Health status of Army Chemical Corps Vietnam veterans who sprayed defoliant in Vietnam". American Journal of Industrial Medicine. 49 (11): 875–884. doi:10.1002/ajim.20385. ISSN 0271-3586.
- Tyas, Suzanne L; Manfreda, Jure; Strain, Laurel A; Montgomery, Patrick R (2001). "Risk factors for Alzheimer's disease: a population-based, longitudinal study in Manitoba, Canada". International Journal of Epidemiology. 30 (3): 590–597. doi:10.1093/ije/30.3.590. ISSN 1464-3685.
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- Defoliants and Desiccants by Frederick M Fishel, Pesticide Information Office, Institute of Food and Agricultural Sciences, University of Florida
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