Dicamba

Use as an herbicide

Dicamba controls annual and perennial rose weeds in grain crops and highlands, and it is used to control brush and bracken in pastures, as well as legumes and cacti. It kills broadleaf weeds before and after they sprout. In combination with a phenoxy herbicide or with other herbicides, dicamba is used in pastures, range land, and noncrop areas (fence rows, roadways, and wastage) to control weeds. Dicamba is toxic to conifer species but is in general less toxic to grasses.

Dicamba functions by increasing plant growth rate. At sufficient concentrations, the plant outgrows its nutrient supplies and dies.

The growth regulating properties of dicamba were first discovered by Zimmerman and Hitchcock in 1942. Soon after Jealott's Hill Experimental Station in England was evaluating dicamba in the field. Dicamba has since been used for household and commercial weed control.

Increasing use of dicamba has been reported with the release of dicamba resistant genetically modified plants by Monsanto. In October 2016, the EPA launched a criminal investigation into the illegal application of older, drift prone formulations of dicamba onto these new plant. Older formulations have been reported to drift after application and affect other crops not meant to be treated. New formulations of dicamba designed to be less airborne and inhibit unintended drift between fields are currently under review for EPA approval in the United States.

Resistance

Some weed species have developed resistance to dicamba. Dicamba resistance in Bassia scoparia was discovered in 1994 and has not been explained by common modes of resistance such as absorption, translocation, or metabolism.

Genetically modified crops

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, but lacks herbicidal activity. Little information is available on the toxicity of this breakdown intermediate. The enzymes responsible for this first breakdown step is a three-component system called dicamba O-demethylase.

Monsanto recently incorporated one component of the three enzymes into the genome of soybean, cotton, and other broadleaf crop plants, making them resistant to dicamba. Monsanto has marketed their dicamba resistant crops under the brand name Xtend.

Volatilization

Dicamba came under some scrutiny due to its tendency to vaporize from treated fields and spread to neighboring crops. Monsanto began offering crops resistant to dicamba before a reformulated and drift resistant herbicide, which they claimed was less likely affect neighboring fields, had gained approval from the Environmental Protection Agency. Incidents in which dicamba affected neighboring fields led to complaints from farmers and fines in some US states. "Super weeds" that are resistant to Roundup have been observed, leading to a concern about the unintended consequences of its use; however, this phenomenon is not unique to glyphosate, as herbicide resistance has been observed in hundreds of weed species.

Toxicological effects

Dicamba does not present unusual handling hazards. It is moderately toxic by ingestion and slightly toxic by inhalation or dermal exposure (oral LD₅₀ in rats: 757 mg/kg body weight, dermal LD₅₀ in rats: >2,000 mg/kg, inhalation LC₅₀ in rats: >200 mg/L).

In a three-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. U.S. Environmental Protection Agency (EPA) has set the NOAEL for this study at 3 (mg/kg)/day.

In dog tests, some enlargement of liver cells has occurred, but a similar effect has not been shown in humans.

Soil

Dicamba is released directly to the environment by its application as an herbicide for the control of annual broadleaf weeds. It may cause damage to plants as a result of its absorption from the soil by plant roots. Dicamba is 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. The prevalence of dicamba in groundwater from agricultural areas (0.11%) did not correlate with nonagricultural urban areas (0.35%).

Dicamba was tested for acute toxicity in a variety of aquatic animals. The studies accepted by the U.S. EPA found dicamba acid and DMA salt to be practically nontoxic 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.