Dicamba (3,6-dichloro-2-methoxybenzoic acid) is a broad-spectrum herbicide first registered in 1967. The herbicide is used on agricultural crops, fallow land, pastures, turfgrass and rangeland. Dicamba is also registered for non-agricultural uses in residential areas and other sites, such as golf courses. At these types of sites, it is primarily used to control broadleaf weeds such as dandelions, chickweed, clover and ground ivy.
Dicamba can be found in liquids, dusts and granule products. There are more than 1,000 products sold in the United States that include dicamba, according to the National Pesticide Information Center.
Dicamba’s mode of action is as an auxin agonist: it produces uncontrollable growth that leads to plant death.
Older versions of dicamba were known to drift far from where they were applied, and typically were not used widely during warm growing months when they could kill off-target crops or trees.
The Environmental Protection Agency approved the registration of new dicamba formulations in 2016, however, allowing for a new use of – “over-the-top” applications on growing dicamba-tolerant cotton and soybean plants. Scientists warned the new uses would result in dicamba drift damage.
The new uses for dicamba came about because of the development of widespread weed resistance to glyphosate-based herbicides, including the popular Roundup brand, introduced by Monsanto in the 1970s. In the 1990s, Monsanto introduced glyphosate-tolerant crops, and encouraged famers to use its “Roundup Ready” cropping systems. Farmers could plant Monsanto’s genetically engineered glyphosate-tolerant soybeans, corn, cotton and other crops, and then spray glyphosate herbicides such as Roundup directly over the top of the growing crops without killing them. The system made weed management easier for farmers as they could spray the chemicals directly over their entire fields during the growing season, wiping out weeds that competed with the crops for moisture and soil nutrients.
The popularity of the Roundup Ready system led to a surge in weed resistance, however, leaving farmers with fields of hardy weeds that would no longer die when sprayed with glyphosate.
In 2011 Monsanto announced that glyphosate, had been “relied on too long by itself” and said it planned to collaborate with BASF and develop a cropping system of genetically engineered crops that would tolerate being sprayed with dicamba. It said it would introduce a new type of dicamba herbicide that would not drift far from fields where it was sprayed.
Since the introduction of the new system, complaints about dicamba drift damage have surged in several farm states, including hundreds of complaints from Illinois, Indiana, Iowa, Missouri and Arkansas.
In a report dated Nov. 1, 2017, the EPA said it had tallied 2,708 official dicamba-related crop injury investigations (as reported by state departments of agriculture). The agency said there were more than 3.6 million acres of soybeans impacted at that time. Other impacted crops were tomatoes, watermelon, cantaloupe, vineyards, pumpkins, vegetables, tobacco, residential gardens, trees and shrubs
In July 2017, the Missouri Department of Agriculture temporarily issued a “Stop Sale, Use or Removal Order,” on all dicamba products in Missouri. The state lifted the order in September 2017.
These are some dicamba products:
- FeXapan herbicide plus VaporGrip Technology, EPA Registration Number 352-913;
- Tavium – EPA Registration Number 100-1623
- Engenia Herbicide, EPA Registration Number 7969-345; and
- XTENDIMAX with VaporGrip Technology, EPA Registration Number 524-617
On Oct. 31, 2018, the U.S. Environmental Protection Agency (EPA) announced an extension of Engenia, XtendiMax and FeXapan registrations through 2020 for “over-the-top” use in dicamba-tolerant cotton and soybean fields. EPA said it had enhanced the previous labels and put in place additional safeguards in an effort to increase the success and safe use of the product in the field.
The two-year registration is valid through Dec. 20, 2020. The EPA has stated the following provisions:
- Only certified applicators may apply dicamba over-the-top (those working under the supervision of a certified applicator may no longer make applications)
- Prohibit over-the-top application of dicamba on soybeans 45 days after planting or up until the R1 growth stage (first bloom), whichever comes first
- Prohibit over-the-top application of dicamba on cotton 60 days after planting
- For cotton, limit the number of over-the-top applications from four to two
- For soybeans, the number of over-the-top applications remains at two
- Applications will be allowed only from one hour after sunrise to two hours before sunset
- In counties where endangered species may exist, the downwind buffer will remain at 110 feet and there will be a new 57-foot buffer around the other sides of the field (the 110-foot downwind buffer applies to all applications, not just in counties where endangered species may exist)
- Enhanced tank clean-out instructions for the entire system
- Enhanced label to improve applicator awareness on the impact of low pH on the potential volatility of dicamba
- Label clean up and consistency to improve compliance and enforceability
Just as glyphosate applications in farm fields have been found to leave residues of glyphosate on and in finished foods, such as oatmeal, breads, cereals, etc., dicamba residues are expected to leave residues in food. Farmers whose produce has been contaminated with dicamba residues via drift have expressed concerns that their products might be rejected or otherwise harmed commercially because of the residue issue.
The EPA has set tolerance levels for dicamba is several grains and for the meat of livestock that consume grains, but not for a variety of fruits and vegetables. A tolerance for dicamba in soybeans is set at 10 parts per million, for instance, in the United States, and a 2 parts per million for wheat grain. Tolerances can be seen here.
The EPA has issued this statement regarding dicamba residues in food: “EPA performed the analysis required by the Federal Food, Drug and Cosmetic Act (FFDCA) and determined that residues on food are “safe” – meaning that there is a reasonable certainty of no harm to people, including all reasonably identifiable subpopulations, including infants and children, from dietary and all other non-occupational exposure to dicamba.”
Cancer and Hypothyroidism
The EPA states that dicamba is not likely to be carcinogenic, but some studies have found an increased risk of cancer for users of dicamba.
See these studies regarding the human health effects of dicamba:
Dicamba use and cancer incidence in the agricultural health study: an updated analysis International Journal of Epidemiology (05.01.2020) “Among 49 922 applicators, 26 412 (52.9%) used dicamba. Compared with applicators reporting no dicamba use, those in the highest quartile of exposure had elevated risk of liver and intrahepatic bile duct cancer and chronic lymphocytic leukaemia and decreased risk of myeloid leukaemia.”
Pesticide Use and Incident Hypothyroidism in Pesticide Applicators in the Agricultural Health Study. Environmental Health Perspectives (9.26.18)
“In this large prospective cohort of farmers that were occupationally exposed to pesticides, we found that ever-use of four organochlorine insecticides (aldrin, chlordane, heptachlor, and lindane), four organophosphate insecticides (coumaphos, diazinon, dichlorvos, and malathion), and three herbicides (dicamba, glyphosate, and 2,4-D) was associated with increased risk of hypothyroidism.”
Hypothyroidism and pesticide use among male private pesticide applicators in the agricultural health study. Journal of Occupational Environmental Medicine (10.1.14)
“The herbicides 2,4-D, 2,4,5-T, 2,4,5-TP, alachlor, dicamba, and petroleum oil were all associated with an increased odds of hypothyroidism”
A review of pesticide exposure and cancer incidence in the Agricultural Health Study cohort. Environmental Heath Perspectives (8.1.10)
“We reviewed 28 studies; most of the 32 pesticides examined were not strongly associated with cancer incidence in pesticide applicators. Increased rate ratios (or odds ratios) and positive exposure–response patterns were reported for 12 pesticides currently registered in Canada and/or the United States (alachlor, aldicarb, carbaryl, chlorpyrifos, diazinon, dicamba, S-ethyl-N,N-dipropylthiocarbamate, imazethapyr, metolachlor, pendimethalin, permethrin, trifluralin).”
Cancer Incidence among Pesticide Applicators Exposed to Dicamba in the Agricultural Health Study. Environmental Health Perspectives (7.13.06)
“Exposure was not associated with overall cancer incidence nor were there strong associations with any specific type of cancer. When the reference group comprised low-exposed applicators, we observed a positive trend in risk between lifetime exposure days and lung cancer (p = 0.02), but none of the individual point estimates was significantly elevated. We also observed significant trends of increasing risk for colon cancer for both lifetime exposure days and intensity-weighted lifetime days, although these results are largely due to elevated risk at the highest exposure level.”
Non-Hodgkin’s Lymphoma and Specific Pesticide Exposures in Men: Cross-Canada Study of Pesticides and Health. Cancer Epidemiology, Biomarkers and Prevention (11.01)
“Among individual compounds, in multivariate analyses, the risk of NHL was statistically significantly increased by exposure to the herbicides…dicamba (OR, 1.68; 95% CI, 1.00–2.81); ….In additional multivariate models, which included exposure to other major chemical classes or individual pesticides, personal antecedent cancer, a history of cancer among first-degree relatives, and exposure to mixtures containing dicamba (OR, 1.96; 95% CI, 1.40–2.75)…were significant independent predictors of an increased risk for NHL”
The dicamba drift damage concerns have prompted lawsuits from farmers in many U.S. states. Details on the litigation can be found here.