Dicamba

Dicamba is a highly volatile chemical that can damage non-target plant species through spray drift (particle drift) and/or volatilization (vapor drift). New formulations were designed to reduce volatility. However, since OTT dicamba was registered for use on DT soybeans, the MDA had received complaints each year for off-site movement onto neighboring property. 

Annual Total Reports for Alleged Dicamba Drift

Forms of Dicamba Registered for Use (list is not all-inclusive)

*No endorsement is implied in the referencing of trade names.

Dicamba is a systemic herbicide that functions as a plant growth regulator. Following application, dicamba is absorbed through leaves and roots of target weeds and is translocated throughout the plant. In the plant, dicamba mimics auxin, a type of plant hormone, and causes abnormal cell division and growth.

Dicamba belongs to the benzoic acid chemical class and is a Site-of-Action 4 herbicide. Other herbicides belonging to Group 4 (synthetic auxins) with a similar mode-of-action include 2,4-D, MCPA, clopyralid, halauxifen-methyl, and aminopyralid.

In Minnesota agriculture, dicamba is used to control weeds in various crops including corn, small grain, soybeans, and hay, as well as on fallow cropland, pastures, and land enrolled in conservation reserve programs. Prior to 2016, dicamba was registered for pre-plant and pre-emergence applications in conventional soybeans, Roundup Ready soybeans, and Liberty Link soybeans. From 2016 to 2024, Minnesota registered dicamba products for over-the-top (OTT) application to dicamba-tolerant (DT) soybeans to control broadleaf weeds such as pigweeds (Amaranthus spp.), ragweeds (Ambrosia spp.), horseweed (Conyza spp.), and Kochia spp. The MDA has worked with the EPA and the registrants of the three dicamba products to include the following restrictions on the product labels during 2022. Cutoff date:  Do not apply south of interstate 94 after June 12. Do not apply north of interstate 94 after June 30. Cutoff temperature for the entire state: Do not apply if the air temperature of the field at the time of application is over 85 degrees Fahrenheit or National Weather Service’s forecasted high temperature for the nearest available location for the day exceeds 85 degrees Fahrenheit. Forecasted temperature must be recorded at the start of the application.

Non-agricultural applications of dicamba include uses on residential lawns and on golf course turf throughout the state.

The MDA tracks the sale of pesticide active ingredients in the state. The graph below shows annual dicamba sales in Minnesota between 1996 and 2022 (Figure 1). Dicamba sales data are pooled for all forms of dicamba and reported as the pounds of dicamba acid equivalents (a.e.) sold. Notably, dicamba sales increased significantly in 2017, rising from 428,000 pounds in 2016 to 910,000 pounds in 2017, coinciding with the release of new OTT dicamba products. Sales data are available through the  Pesticide Sales database.

The various salts of dicamba are considered to have a similar toxicity to dicamba acid. Furthermore, Dicamba salts are rapidly transformed into the acid form in the environment. Therefore, toxicity information is listed for dicamba acid.

Human Health

Dicamba has a low acute toxicity via oral, dermal, and inhalation routes. It is an eye and dermal irritant, but it is not a skin sensitizer.

Human Health Values for Dicamba Acid. Data from US EPA^₁ and the Minnesota Department of Health (MDH).

Non-target Organisms

Dicamba toxicity is low for aquatic organisms, mammals, and honeybees, and it is moderately toxic to birds. Non-target plants exposed to dicamba may be damaged by this herbicide.

Dicamba acid toxicity values for aquatic and terrestrial organisms. Data from US EPA^₂

*ALB value based on toxicity data from US EPA³.

Dicamba is quickly biodegraded in soil under aerobic conditions (with oxygen) but is more persistent under anaerobic conditions (without oxygen). It is not likely to leach to groundwater due to its fast degradation; however, dicamba may reach surface water via run-off, spray drift during application, or vapor drift. Vapor drift from the volatilization of dicamba can result in injury of nearby non-target plants.

Dicamba salts are rapidly transformed into the acid form in the environment; therefore, properties are listed for dicamba acid.

Chemical Properties

Soil

 Water

Air

Degradates

3,6-dichlorosalicylic acid (DCSA) is the major degradate, or breakdown product, of dicamba. Because DCSA is primarily formed in plants, the EPA does not expect DCSA to reach groundwater at levels that would be of concern. However, DCSA is more toxic than the parent compound to certain species of birds and mammals; thus, there could be potential for adverse effects to certain species of birds and mammals. Mitigations are imposed on registration to alleviate these risks. Other minor degradates of dicamba include 3, 6-dichlorogentisic acid (DCGA) and 5-OH-dicamba.

Dicamba product labels include the following statements; however, the language may vary among products. See individual product labels for full list of environmental hazards and spray drift management restrictions or recommendations. Always read the label before applying a pesticide.

Do not apply directly to water, or to areas where surface water is present or to intertidal areas below the mean high water mark. Do not contaminate water when disposing of equipment washwaters or rinsate. Apply this product only as directed

This chemical is known to leach through soil into ground water under certain conditions as a result of agricultural use. Use of this chemical in areas where soils are permeable, particularly where the water table is shallow, may result in ground water contamination.

Ground and Surface Water Protection

• Point source contamination- Do not mix or load this pesticide within 50 feet of wells (including abandoned wells and drainage wells), sink holes, perennial or intermittent streams and rivers, and natural or impounded lakes and reservoirs.

• Movement by surface runoff or through soil- Do not apply under conditions which favor runoff. Ground water contamination may occur in areas where soils are permeable or coarse and ground water is near the surface. Do not apply to soils classified as sand with less than 3% organic matter and where ground water depth is shallow.

• Movement by water erosion of treated soil- Do not apply or incorporate this product through any type of irrigation equipment nor by flood or furrow irrigation. Ensure treated areas have received at least one-half inch rainfall (or irrigation) before using tailwater for subsequent irrigation in other fields.

Endangered Species Concerns

Use of this product in a manner inconsistent with its labeling may pose a hazard to endangered of threatened species.