Herbicides

Introduction

Herbicides are a group of Pesticides used to kill unwanted plants. Some of these act by interfering with the growth of the weed and are often based on plant hormones. Herbicides used to clear waste ground are nonselective and kill all plant material with which they come into contact.

Use

Herbicides account for about 69% of all pesticide use in the states. They are used on around 90 million hectares (ha), around half of the country's farmlands with field corn amounting to over 50% of herbicide usage. Corn and soybeans account for 3/4 of total usage.

History

Prior to the 1930s, herbicides were non-specific in what they killed and also very toxic to humans. The 1930s and 1940s brought drastic changes the the pesticide field. Advances in synthetic chemistry led to the creation of many new Insecticides and herbicides. These new chemicals were more specific in what they targeted but still far from perfect.

Herbicidal use was not limited to agricultural purposes. The chlorophenoxy compound, made from a mixture of 2,4-D, 2,4,5-T, and the latter's contaminant TCDD commonly known as dioxin, Agent Orange was used extensively in The Vietnam War to reduce foliage by killing any broad-leaved plants it came in contact with.

Classifications

Herbicides can be grouped by activity, use, chemical family, mode of action, or type of vegetation controlled.

By activity:

Contact herbicides: destroy only the plant tissue in contact with the chemical. Generally, these are the fastest acting herbicides. They are less effective on perennial plants, which are able to regrow from roots or tubers.

Systemic herbicides: are translocated through the plant, either from foliar application down to the roots, or from soil application up to the leaves. They can destroy a greater amount of plant tissue than contact herbicides.

By use:

Soil-applied herbicides: applied to the soil and are taken up by the roots of the target plant.

Preemergent herbicides: applied to the soil before the crop emerges and prevent germination or early growth of weed seeds.

Post-emergent herbicides: applied after the crop has emerged.

Their classification by mechanism of action (MOA) indicates the first enzyme, protein, or biochemical step affected in the plant following application.

The main mechanisms of action are:

ACCase inhibitors: compounds that kill grasses. Acetyl coenzyme A carboxylase (ACCase) is part of the first step of lipid synthesis. Thus, ACCase inhibitors affect cell membrane production in the meristems of the grass plant. The ACCases of grasses are sensitive to these herbicides, whereas the ACCases of dicot plants are not.

ALS inhibitors: the acetolactate synthase (ALS) enzyme (also known as acetohydroxyacid synthase, or AHAS) is the first step in the synthesis of the branched-chain amino acids (valine, leucine, and isoleucine). These herbicides slowly starve affected plants of these amino acids which eventually leads to inhibition of DNA synthesis. They affect grasses and dicots alike. The ALS inhibitor family includes sulfonylureas (SUs), imidazolinones (IMIs), triazolopyrimidines (TPs), pyrimidinyl oxybenzoates (POBs), and sulfonylamino carbonyl triazolinones (SCTs).

EPSPS inhibitors: The enolpyruvylshikimate 3-phosphate synthase enzyme EPSPS is used in the synthesis of the amino acids tryptophan, phenylalanine and tyrosine. They affect grasses and dicots alike. Glyphosate (Roundup) is a systemic EPSPS inhibitor but inactivated by soil contact.

Synthetic auxin inaugurated the era of organic herbicides. They were discovered in the 1940s after a long study of the plant growth regulator auxin. Synthetic auxins mimic this plant hormone. They have several points of action on the cell membrane, and are effective in the control of dicot plants. 2,4-D is a synthetic auxin herbicide.

Photosystem II inhibitors: reduce electron flow from water to NADPH2+ at the photochemical step in photosynthesis. They bind to the Qb site on the D2 protein, and prevent quinone from binding to this site. Therefore, this group of compounds cause electrons to accumulate on chlorophyll molecules. As a consequence, oxidation reactions in excess of those normally tolerated by the cell occur, and the plant dies. The triazine herbicides (including atrazine) are PSII inhibitors.

External Links

• European Union - Chemical and Pesticide Information . (accessed: 4 April 2003).
• World Health Organization - WHO Pesticide Evaluation Scheme (WHOPES)\ . (accessed: 4 April 2003).
• International Programme on Chemical Safety (IPCS). (accessed: 4 April 2003).
• U.S. National Library of Medicine's (NLM) Enviro-Health Links - Pesticide Exposure . (Accessed 21 May 2006).
• USDA - National Agricultural Statistics Service - Agricultural Chemical Use (Accessed 16 May 2005).
• Ontario College of Family Physicians Pesticides Paper (accessed: 16 June 2004). An extensive review of the health effects of pesticides.
• Health Canada - Pesticide Information (accessed: 9 April 2003).
• U.S. Environmental Protection Agency (EPA) - Office of Pesticides Programs (OPP). (accessed: 19 June 2004).
• U.S. Geological Survey - National Water-Quality Assessment (NAWQA) Program (accessed: 4 April 2003).
• California Department of Pesticide Regulation (accessed: 4 April 2003).
• En espanol (Spanish Language) - Almacenar y Desechar - Pesticidas: Regulación de Pesticidas - (Storage and Disposal of Pesticides) - U.S. Environmental Protection Agency (EPA). (accessed: 19 June 2004).
• Washington State University
• Scorecard
• Food News
• Pesticide Action Network North America (PANNA)
• Pesticide Action Network International (PANI)
• Pesticide Database site
• National Pesticide Telecommunications Network (NPTN). (accessed: 4 April 2003).
• Beyond Pesticides
• EXTOXNET InfoBase Online. (accessed: 4 April 2003).
• Washington Toxics Coalition (WTC). (accessed: 4 April 2003).
• The Northwest Coalition for Alternatives to Pesticides (NCAP)
• Monsanto Company (accessed: 4 April 2003)
• Integrated Pest Management (IPM)
• University of Minnesota's electronic textbook of Integrated Pest Management
• The Consortium for International Crop Protection (CICP)
• The NSF Center for Integrated Pest Management (CIPM) - North Carolina Pest Management Information.

References

Kamel F and Hoppin JA. Association of pesticide exposure with neurologic dysfunction and disease. Environ Health Perspect. 2004 Jun;112(9):950-8. Available online at EHPonline. (accessed: 30 June 2004).

MMWR (1999). Farm worker illness following exposure to carbofuran and other pesticides - Fresno County, California, 1998. February 19, 1999, 48(6), 113-116. (accessed: 5 July 2003).

Dean, S. R., & Meola, R. W. (2002). Effect of diet composition on weight gain, sperm transfer, and insemination in the cat flea (Siphonaptera: Pulicidae). J Med Entomol, 39(2), 370-375.

Dryden, M. W., & Gaafar, S. M. (1991). Blood consumption by the cat flea, Ctenocephalides felis (Siphonaptera: Pulicidae)