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The information in this profile may be out-of-date. It was last revised in 1996. EXTOXNET no longer updates this information, but it may be useful as a reference or resource.

Please visit the National Pesticide Information Center (NPIC) to find updated pesticide fact sheets. If you don't find a fact sheet related to your question, feel free to call 1-800-858-7378. NPIC is open five days a week from 8:00am to 12:00pm Pacific Time.

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E X T O X N E T

Extension Toxicology Network

Pesticide Information Profiles

A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Oregon State University, the University of Idaho, and the University of California at Davis and the Institute for Environmental Toxicology, Michigan State University. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program.

EXTOXNET primary files maintained and archived at Oregon State University

Revised June 1996

Metolachlor

Trade and Other Names: Trade names for products containing metolachlor include Bicep, CGA-24705, Dual, Pennant, and Pimagram. The compound may be used in formulations with other pesticides (often herbicides that control broad-leaved weeds) including atrazine, cyanazine, and fluometuron.

Regulatory Status: Metolachlor is a slightly toxic compound in EPA toxicity class III. Labels for products containing it must bear the CAUTION Signal Word. Metolachlor is in most cases a General Use Pesticide (GUP), although some products may be restricted use (RUP).

Chemical Class: Not Available

Introduction: Metolachlor is usually applied to crops before plants emerge from the soil, and is used to control certain broadleaf and annual grassy weeds in field corn, soybeans, peanuts, grain sorghum, potatoes, pod crops, cotton, safflower, stone fruits, nut trees, highway rights-of-way and woody ornamentals. It inhibits protein synthesis; thus, high-protein crops (e.g., soy) can be adversely affected by excessive metolachlor application. Additives may be included in product formulations to help protect sensitive crops (i.e., sorghum) from injury.

Formulation: Not Available

Toxicological Effects:

• Acute toxicity: Metolachlor is slightly toxic via ingestion. The reported oral LD50 in rats for technical grade metolachlor is from 1200 mg/kg to 2780 mg/kg [1,30,100]. It is slightly to practically nontoxic by skin exposure, with a reported dermal LD50 of greater than 2000 mg/kg [58,30]. Technical metolachlor is a skin sensitizer in guinea pigs, and causes slight irritation and mild eye irritation in rabbits [58]. The formulated products are generally not skin sensitizers, but cause a range (slight to moderate) of skin and eye irritation in rabbits [58]. The 4-hour rat inhalation LC50 of greater than 4.3 mg/L indicates slight toxicity via this route [1,58]. Compared to the technical grade, metolachlor formulations are generally of similar or lesser toxicity by all routes, except by inhalation; some formulated products may show higher toxicity by this route [58]. However, none of the formulated products for which inhalation toxicity data are available are highly toxic by this route [58]. Human exposure most commonly occurs through skin or eye contact [101]. Signs of human intoxication from metolachlor exposure include abdominal cramps, anemia, shortness of breath, dark urine, convulsions, diarrhea, jaundice, weakness, nausea, sweating, and dizziness [101].
• Chronic toxicity: While metolachlor is not readily absorbed by the skin, repeated dermal exposures may create skin sensitization, especially among those who work with metolachlor [101]. In rats fed metolachlor for 90 days, no effects were noted at about 90 mg/kg/day [87]. In a 2-year study of mice, a similar no-effect level was found, but doses of about 300 mg/kg/day caused decreased body weight gain [58]. No negative effects on mortality or organ weights were observed in male or female rats at doses of 15 mg/kg/day, but exposed females showed significantly lower weight gain and microscopic changes in their liver structure at 150 mg/kg/day [58].
• Reproductive effects: In two long-term rat reproduction studies, mating, gestation, lactation, and fertility were not affected at doses of 50 mg/kg/day [58,101]. However, pup weights and parental food consumption decreased at this low dose. In another 2-year rat study, metolachlor caused the wasting of testicles at doses of 150 mg/kg/day [101]. In studies on mice, no effects were noted on fertility, or zygote or embryo survival rates after very high single oral doses [101]. This evidence suggests that metolachlor is not likely to have an effect on reproduction in humans under normal circumstances.
• Teratogenic effects: Metolachlor caused no birth defects in rats at maternal doses of 300 mg/kg/day administered during critical periods of gestation (organogenesis), although some delayed or abnormal development in offspring was seen at this dose [58]. A decrease in food consumption was observed in the mother [100]. In rabbits, a similar pattern of effects (no defects but some delayed development) was also seen at doses of up to 360 mg/kg/day [58]. These data indicate that teratogenic and developmental effects in humans are unlikely at expected levels of exposure.
• Mutagenic effects: Metolachlor tested negative in two bacterial assays [101]. Also, no mutagenicity effects were noted in a standard mouse test [101]. From this evidence it is unlikely that the compound is mutagenic.
• Carcinogenic effects: Male and female mice exposed to doses up to 100 mg/kg/day for 18 to 20 months did not develop cancer [58,101], nor did male rats at doses of up to 150 mg/kg/day over a 2-year period [58]. Female rats given high doses for 2 years showed a significant increase in new growths, nodules, and lesions in livers at that dose [58,100]. From these data, it seems unlikely that metolachlor is carcinogenic in humans.
• Organ toxicity: Exposure to metolachlor can damage the liver and cause irritation of the skin, eyes, and mucous membranes. It has also caused skin sensitization in guinea pigs.

• Fate in humans and animals: Studies show that orally administered metolachlor is quickly broken down into metabolites and is almost totally eliminated in the urine and feces of goats, rats, and poultry. Metolachlor itself was not detected in the urine, feces, or body tissues [101]. Rats, given a single oral dose of metolachlor, excreted 70 to 90% of the metolachlor as metabolites within 48 hours [6]. In animals, trace amounts of metolachlor metabolites were found in kidneys, liver, blood, and milk; however, no residues were found in eggs, meat, or fat samples of laying chickens [101].

Ecological Effects:

• Effects on birds: Metolachlor is slightly to practically nontoxic to birds. The reported oral LD50 is greater than 2000 mg/kg in mallard ducks and is greater than 4500 mg/kg in bobwhite quail [58]. Both the mallard and the bobwhite quail show 5-day dietary LC50 values of greater than 10,000 ppm, also indicating very low toxicity to upland game birds and waterfowl [58,100]. However, although mallard ducks showed no impairment of reproductive capabilities at high level long-term exposures, bobwhite quail fed a diet containing high levels of metolachlor for 17 weeks during mating, egg laying, and egg hatching produced fewer chicks [100].

• Effects on aquatic organisms: Metolachlor is moderately toxic to both cold- and warmwater fish, including rainbow trout, carp, and bluegill sunfish. The reported 96-hour LC50 values for this compound are about 3 mg/L in rainbow trout, 5 mg/L in carp and channel catfish, and 15.0 mg/L in bluegill sunfish [58]. The 48-hour LC50 of metolachlor in Daphnia (waterflea) is 25 mg/L [58]. Studies on algae and fish exposed to metolachlor in water indicate that very little is accumulated and that any accumulated material is excreted rapidly when the organisms are placed in clean water [100].
• Effects on other organisms: Metolachlor is nontoxic to bees; its contact LC50 in earthworms is 140 ppm [58].

Environmental Fate:

• Breakdown in soil and groundwater: Metolachlor is moderately persistent in the soil environment. Half-lives of 15 to 70 days in different soils have been observed [58,100]. Soils with significant soil water content may show more rapid breakdown. Very little metolachlor volatilizes from the soil, and photodegradation will be a significant pathway for loss only in the top few inches [102]. Breakdown is mainly dependent upon microbial activity, and thus will be temperature-dependent [102]. Microorganism metabolism occurs by both aerobic and anaerobic processes, and is affected by temperature, moisture, amount of leaching, soil type, nitrification, oxygen concentrations, and sunlight [58,102]. Metolachlor is moderately well sorbed by most soils [58,102]. Soils with higher organic matter and clay content may sorb it better. It is slightly soluble in water [58]. Extensive leaching is reported to occur, especially in soils with low organic content [100]. Metolachlor was one of four pesticides that were extensively studied throughout the nation in the National Alachlor Well Water Survey. This several-year project analyzed the contents of over 6 million private and domestic wells. Metolachlor was detected in about 1% of the wells (about 60,000 wells) at concentrations ranging from 0.1 to 1.0 ug/L [64]. It has also been found in a number of surface water samples in 14 states, at a maximum concentration of 0.138 mg/L [9]. These levels may result from runoff during spring and summer applications to fields [101].
• Breakdown in water: Metolachlor is highly persistent in water over a wide range of water acidity. Its half-life at 20 C is more than 200 days in highly acid waters, and is 97 days in highly basic waters [101]. Metolachlor is also relatively stable in water under natural sunlight. About 6.6 % was degraded by sunlight in 30 days, a slow and minimal rate [100].
• Breakdown in vegetation: Metolachlor, applied before plants emerge, is absorbed through shoots just above the seed, and may be absorbed from the soil into and through the roots [58]. This chemical acts by inhibiting the production of essential plant components like chlorophylls, enzymes, and other proteins [58]. Metolachlor is a growth inhibitor affecting root and shoot growth after seeds have germinated. The breakdown of metolachlor in corn, soybean, peanuts, and sorghum is similar. Residues and metabolites are found in minimal concentrations in roots, grain, and oil, but other parts of the plants may have higher levels. Some care should be exercised when crop remnants are used as forage; cotton crops may retain very high levels of residue [100].

Physical Properties:

• Appearance: Pure metolachlor is an odorless, off-white to colorless liquid at room temperature [1]. In formulations, its color ranges from opaque white to tan.
• Chemical Name: 2-chloro-6'-ethyl-N-(2-methoxy-1-methylethyl)acet-o-toluidide [1]
• CAS Number: 51218-45-2
• Molecular Weight: 283.80
• Water Solubility: 530 mg/L @ 20 C [1]
• Solubility in Other Solvents: miscible with benzene, dichloromethane, hexane, methanol, octanol, xylene, toluene, dimethylformamide, ethylene dichloride, and cyclohexanone; insoluble in ethylene glycol and propylene glycol [1]
• Melting Point: Not Available
• Vapor Pressure: 1.7 mPa @ 20 C [1]
• Partition Coefficient: 3.4502 [1]
• Adsorption Coefficient: 200 [11]

Exposure Guidelines:

• ADI: Not Available
• MCL: Not Available
• RfD: 0.1 mg/kg/day [13]
• PEL: Not Available
• HA: 0.1 mg/L (lifetime) [101]
• TLV: Not Available

Basic Manufacturer:

Ciba-Geigy Corporation
P.O. Box 18300
Greensboro, NC 27419-8300

• Phone: 800-334-9481
• Emergency: 800-888-8372
  

References:

References for the information in this PIP can be found in Reference List Number 10

DISCLAIMER: The information in this profile does not in any way replace or supersede the information on the pesticide product labeling or other regulatory requirements. Please refer to the pesticide product labeling.