Dicamba is a selective systemic herbicide for post-emergence control of broadleaf weeds. It was first registered by the US Environmental Protection Agency (EPA) in 1967 and is available in several formulations (see table below). Dicamba is registered for use on a variety of food and feed crops, including corn, barley, wheat, and dicamba tolerant (DT) soybeans. It is also used to control weeds in golf courses, sports fields, and parks.

Prior to 2016, dicamba was registered for only preplant and preharvest applications to soybeans. However, in 2016 the US EPA approved post-emergence, or over-the-top (OTT) use for three dicamba products (Engenia, XtendiMax, FeXapan) on DT soybeans and cotton. In 2019, EPA registered a product containing a combination of dicamba and S-metolachlor for over-the-top use on DT cotton and DT soybeans.

Dicamba is a highly volatile chemical that can damage non-target plant species through spray drift and/or volatilization (vapor drift). New formulations were designed to reduce volatility. However, 265,000 acres of non-DT soybeans were damaged because of off-site movement of dicamba in the US in 2017.  Following reports of damage resulting from the off-site movement of dicamba, EPA amended the dicamba registration labels in 2017 and in 2018. In October 2020, the EPA amended the labels again to address drift and volatility and extended conditional registration for OTT applications to DT soybeans through December 2025. Dicamba products registered for OTT applications on soybeans are Restricted Use Pesticides (RUPs) and have specific application and record keeping requirements including: a June 30th application cutoff date, completion of dicamba specific training, use of volatility reducing agent, application during a specific time of day, wind speed and direction restrictions, and maintenance of buffer distances. See product labels for all application and record keeping requirements.

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

Dicamba Form CAS # /
PC code
Trade Names* Soybean OTT Products
Acid 1918-00-9
029801
Celius; Vision;
Saddle-Up
none
Dimethylamine (DMA) salt 2300-66-5
029802
Banvel; EndRun;
Cimarron Max
none
Sodium (Na) salt 1982-69-0
029806
Status; Rave;
Overdrive
none
Potassium (K) salt 10007-85-9
129043
Lawn 3iP
Herbicide
none
Diglycolamine (DGA) salt 104040-79-1
128931
Clarity; DiFlexx;
Clarifier
XtendiMax
Tavium
Bis aminopropyl methylamine (BAPMA) salt 100094 Engenia Engenia

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


How it Works

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.

Use in Minnesota

In Minnesota agriculture, dicamba is used to control weeds in crops including corn, small grain, soybeans, and hay, as well as on fallow cropland, pastures, and land enrolled in conservation reserve programs. In 2016, Minnesota registered three new dicamba products (XtendiMax, Engenia, and FeXapan) for OTT application to DT soybeans to control broadleaf weeds such as pigweeds (Amaranthus spp.), ragweeds (Ambrosia spp.), horseweed (Conyza spp.), and Kochia spp. Prior to 2016, dicamba was registered for pre-plant and pre-emergence applications in conventional soybeans, Roundup Ready soybeans, and Liberty Link soybeans. New DT soybeans are tolerant to dicamba and glyphosate herbicides.

In 2017, the MDA received 253 complaints of off-site dicamba damage. To curb off-site movement, the MDA set June 20 as the annual application cutoff date for registered dicamba products from 2018 to 2020. This date was based on research and pesticide misuse complaints. In October 2020, EPA set June 30 as the application cutoff date on federal labels for OTT dicamba applications to DT soybeans. Currently, three dicamba products, XtendiMax with VaporGrip Technology (EPA Reg. No. 264-1210, Bayer), Engenia (EPA Reg. No. 7969-472, BASF) and Tavium Plus VaporGrip Technology (EPA Reg. No. 100-1623, Syngenta) are registered for use in Minnesota. These products are approved for use on DT soybeans and DT cotton only.

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 2017. Dicamba sales data are pooled for all forms of dicamba and reported as the pounds of dicamba acid equivalents (a.e.) sold. Sales data are available through the Pesticide Sales database.

Bar graph of dicamba sales in Minnesota from 1996-2017. Sales peak in 1998 (nearly 1,600,000), decrease in a downward trend to 2008-2010 (near 200,000) and gradually rise again to about 1,100,000 in 2016.


Toxicity

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 EPA1 and the Minnesota Department of Health (MDH).

Population Adjusted Dose (PAD) Acute=0.29 mg/kg/day
Chronic=0.04 mg/kg/day
Cancer Effect Not likely to be
carcinogenic to humans
MDH Health Risk Limit (HRL)
(chronic, 1993)
200 µg ae/L


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 EPA2

Aquatic Organism Toxicity Values Toxicity Level
Freshwater fish Acute LC50 = 28 mg ae/L
Chronic NOAEC = N/A
Slightly toxic
Freshwater invertebrates Acute EC50 > 50 mg ae/L
Chronic NOAEC = No data
Practically
non-toxic
Aquatic plants Vascular EC50 = N/A
Nonvascular EC50 = 0.493 mg ae/L
 
Most Sensitive Aquatic Life Benchmarck (ALB)*

61 µg/L (nonvascular plants)

 

 

Terrestrial Organism Toxicity Values Toxicity Level
Mammals Acute oral LD50 = 2,740 mg ae/L
Chronic NOAEC = N/A
Practically
non-toxic
Birds Acute oral LC50 = 188 mg ae/L Moderately toxic
Honey bee Acute contact LD50 > 90.65 µg ae/bee Practically
non-toxic

*ALB value based on toxicity data from US EPA3.


Dicamba in the Environment

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

Water Solubility 6100 mg/L
Dissociation Constant pKa = 1.9


Soil

Adsorption Koc = 3.45 - 21.1 mL/goc (mean = 13.4)
Soil Metabolism Aerobic half-life = 18 days
Anaerobic half-life = no data
Photolysis no data

 Water

Aquatic Metabolism Aerobic half-life = 72.9 days
Anaerobic half-life = 423 days
Photolysis Half-life = 105 days
Hydrolysis Stable

Air

Volatilization Vapor pressure (25°C) = 3.41 x 10-5 torr
Henry's Law constant = 1.79 x 10-8 atm m3/mol

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.


Environmental Hazards

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.


References

  1. US EPA Dicamba and Dicamba BAPMA Salt: Human-Health Risk Assessment for Proposed Section 3 New Uses on Dicamba-tolerant Cotton and Soybean (2016). [Docket ID: EPA-HQ-OPP-2016-0223-0002]

  2. US EPA Problem Formulation for the Environmental Fate, Ecological Risk, and Drinking Water Assessments in Support of the Registration Review of Dicamba (2016). [Docket ID: EPA-HQ-OPP-2016-0223-0004]

  3. US EPA Ecological Risk Assessment for Dicamba DGA Salt and its Oegradate, 3,6-dichlorosalicylic acid (DSCA), for the Proposed Post-Emergence New Use on Dicamba-Tolerant Cotton (2016). [Docket ID: EPA-HQ-OPP-2016-0187-0005]