Metolachlor

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Overview

Metolachlor is a herbicide from the chloroacetanilide family, developed by Ciba-Geigy. Its mode of action is elongase inhibition, and inhibition of geranylgeranyl pyrophosphate (GGPP) cyclisation enzymes, part of the gibberellin pathway.

Agricultural use

It is used for grass and broadleaf weed control in corn, soybean, peanuts, sorghum and cotton. It is sold under brand names including Dual, Pimagram, Bicep, CGA-24705, and Pennant. It is also used in combination with other herbicides.

Metolachlor is the second most popular herbicide used in the United States.^[2]

Metolachlor products are being or have already been phased out in Europe.^[3]

Safety

Metolachlor is a slightly toxic compound in EPA toxicity class III.^[4]

Its wide use has led to the contamination of both ground and surface waters and concentrations ranging from 0.08 to 4.5 parts per billion (ppb) have been detected from normal agricultrual use in Iowa, Pennsylvania and Wisconsin.^[5] Out of 1,997 surface water sampled from 312 locations in 14 states, metolachlor was found in 1,644 of them at the maximum concentration of 138 ppb.

Exposure of metolachlor in humans commonly occurs through contact with the skin which can cause slight skin irritation or through contact with the eyes. Symptoms of metolachlor intoxication in humans include shortnness of breath, dark urine, diarrhea, dizziness, sweating, nausea, anemia abdominal cramps, and jaundice.

Metolachlor is classified as a Category C pesticide by the United States Environmental Protection Agency (USEPA) which indicates limited evidence of carcinogenicity.^[6] Evidence of the bioaccumulation of metolachlor in edible species of fish as well as its adverse effect on the growth and development raise concerns on its effects on human health. There is no set maximum concentration (maximum concentration level, MCL) for metolachlor that is allowed in drinking water, the USEPA does have a health advisory level (HAL) of 0.525 mg/L for this chemical.

Studies have shown that metolachlor induces cytotoxic and genotoxic effects in human lymphocytes.^[7] Genotoxic effects have also been observed in tadpoles exposed to metolachlor.^[8] Evidence also reveals that metolachlor affects cell growth. Cell division in yeast was reduced,^[9] and chicken embryos exposed to metolchlor showed a significant decrease in the average body mass compared to the control.^[10]

Ecological effects

Some animals have been shown to be moderately tolerant of metolachlor. Among them is wildfowl which can tolerate moderate exposure to metolachlor. Metolachlor is also moderately toxic to both cold and warm water fish including the bluegill sunfish, rainbow trout and carp. Exposure of metolachlor to some fish and algae in water show very little accumulation of the chemical and any if even accumulated is rapidly excreted upon placing the organisms in clean water.

Fate in the environment

Metabolites of metolachlor have been found in varying levels treated plants. Plants however, retain their metolachlor metabolites although animals that consume such plants are able to break down and eliminate the chemical rapidly. Some parts of plants, such as the leaves of cotton can retain much higher levels of metolachlor residues compared to other parts of the plants such as the seeds that can contain little.

The breakdown of metolachlor in the soil is affected by a number of factors such as moisture, temperature, microbial activity, soil type , concentration of oxygen and nitrification and is mobile and easily leached through soil. The effect of changes in moisture content and temperature related to microbial activity also affects metolachlor breakdown.

See also

• Acetochlor
• Alachlor

References