Dicamba

Dicamba (3,6-dichloro-2-methoxybenzoic acid) is an herbicide used to control annual and perennial broadleaf weeds in grain crops and grasslands, and it is used to control brush and bracken in pastures. It will kill broadleaf weeds before and after they sprout. Legumes will be killed by dicamba. In combination with a phenoxyalkanoic acid or other herbicide, dicamba is used in pastures, range land, and non-crop areas (fence-rows, roadways and wastage) to control weeds. Brand names for formulations of this pesticide include Banvel and Vanquish.

It is in the benzoic acid family of herbicides.

It is a light tan slightly phenolic crystalline solid. It is stable to oxidation and hydrolysis and melts at temperatures between 90 and 100 degrees Celsius (194 to 212 °F). Dicamba is nonflammable and does not present any unusual handling hazards.

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Toxicological effects

Dicamba is moderately toxic by ingestion and slightly toxic by inhalation or dermal exposure (oral LD₅₀ in rats: 757 mg/kg body weight, dermal LD₅₀2,000 mg/kg, inhalation LC₅₀200 mg/l). Symptoms of poisoning with dicamba include loss of appetite (anorexia), vomiting, muscle weakness, slowed heart rate, shortness of breath, central nervous system effects (victim may become excited or depressed), benzoic acid in the urine, incontinence, cyanosis (bluing of the skin and gums), and exhaustion following repeated muscle spasms. In addition to these symptoms, inhalation can cause irritation of the linings of the nasal passages and the lungs, and loss of voice. Most individuals who have survived severe poisoning from dicamba have recovered within 2 to 3 days with no permanent effects.

Dicamba is very irritating and corrosive and can cause severe and permanent damage to the eyes. The eyes should be flushed with running water for at least 15 minutes if any dicamba is splashed into them. The eyelids may swell and the cornea may be cloudy for a week after dicamba is splashed in the eyes.

Dermal and inhalation exposure to humans may occur during application, particularly via splashing during dilution, mixing, and loading. Application by aircraft increases the potential for exposure of humans, livestock, and wildlife due to spray drift and ventilation.

Chronic exposure can lead to the development of the same symptoms as described above. In a 3-generation study, dicamba did not affect the reproductive capacity of rats. When rabbits were given doses of 0, 0.5, 1, 3, 10 or 20 (mg/kg)/day of technical dicamba from days 6 through 18 of pregnancy, toxic effects on the mothers, slightly reduced fetal body weights, and increased loss of fetuses occurred at the 10 mg/kg dose. EPA has set the NOAEL for this study at 3 (mg/kg)/day.

Health effects

Most of the available data on potential human health effects come from laboratory animal studies. These data are evaluated and used to make inferences about potential effects on human health. In a 3-generation study, dicamba did not affect the reproductive capacity of rats. When rabbits were given doses of 0, 0.5, 1, 3, 10 or 20 (mg/kg)/day of technical dicamba from days 6 through 18 of pregnancy, toxic effects on the mothers, slightly reduced fetal body weights, and increased loss of fetuses occurred at the 10 mg/kg dose.

Dicamba is suspected of being a human teratogen. No teratogenic effects have been shown in lab animals such as rabbits and rats.

Dicamba has not been shown to be a mutagen.

In mice, some enlargement of liver cells has occurred. A similar effect has not been shown in man.^[2]

Environmental impact

Soil

Dicamba is released directly to the environment by its application as a herbicide for the control of annual broadleaf weeds. Dicamba may cause damage to plants as a result of its absorption from the soil by plant roots. Halftimes of dicamba in soil usually are between 1 and 6 weeks. Dicamba is very mobile in most soils and significant leaching is possible. The adsorption of dicamba to organo-clay soil is influenced by soil pH with the greatest adsorption to soil occurring in acidic soils. Dicamba is moderately persistent in soil. Its reported half-life in soil ranges from 1 to 6 weeks. Dicamba is likely to be more rapidly degraded in soils with high microbial populations but dissipates more slowly in hardwood forests and wetlands than would be expected from the results of laboratory studies.

At a level of 10 mg/kg in sandy loam soil, dicamba caused a transient decrease in nitrification after two but not three weeks of incubation. The investigator determined that the decrease in nitrification is not substantial and does not suggest the potential for a prolonged impact on microbial activity. In the same study, dicamba did not affect ammonia formation or sulfur oxidation. In a more recent laboratory study, dicamba, at a concentration of 1 mg/kg soil, did not affect urea hydrolysis or nitrification in four soil types.

Water

Dicamba salts used in some herbicides are highly soluble in water. A recent study conducted by the U.S. Geologic Survey (USGS 1998) found dicamba in 0.11%-0.15% of the ground waters surveyed. The maximum level detected was 0.0025 mg/L. There was no apparent correlation between the prevalence of dicamba in groundwater from agricultural areas (0.11%) compared with non-agricultural urban areas (0.35%).

Air

Dicamba is relatively volatile, and this process may be a significant factor in the dispersion of dicamba in the environment. Brown-and-burn operations may result in the formation of considerable quantities of combustion products. The combustion products of dicamba are not identified.

Environmental fate

In soil, dicamba breaks down to very simple substances like carbon dioxide and water. The soil bacterium Pseudomonas maltophilia (strain DI-6) converts dicamba to 3,6-dichlorosalicylic acid (3,6-DCSA), which is adsorbed to soil much more strongly than is dicamba and lacks herbicidal activity. Very little information is available on the toxicity of these breakdown intermediates. The enzymes responsible for this first breakdown step is a three-component system called dicamba O-demethylase. One component of the three has recently been incorporated into the genome of a variety of crop plants making them resistant to dicamba (Behrens et al., Science, 2007, 316, 1185-1188). Dicamba is toxic to many terrestrial broadleaf and conifer species, but is generally less toxic to grasses. Dicamba is relatively toxic to some species of cacti. Dicamba was tested for acute toxicity to a variety of aquatic animals. The studies accepted by the U.S. EPA found dicamba acid and DMA salt to be practically nonionic to aquatic invertebrates. Studies accepted by the U.S. EPA found dicamba acid to be slightly toxic to cold water fish (rainbow trout), and practically nontoxic to warm water fish. Although the toxicity of dicamba to experimental mammals has been well characterized, little information is available on toxicity to wildlife species. Based on acute toxicity tests dicamba is classified as slightly toxic to experimental mammals.

References

http://www.fs.fed.us/r6/nr/fid/pubsweb/dicamba_99.pdf http://pmep.cce.cornell.edu/profiles/herb-growthreg/dalapon-ethephon/dicamba/herb-prof-dicamba.html http://www.speclab.com/compound/c1918009.htm