Dicamba: Concerns about health risks and crop damage

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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 where it is primarily used to control broadleaf weeds such as dandelions, chickweed, clover and ground ivy.

More than 1,000 products sold in the United States 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.

These are some dicamba products:

What are the health concerns of dicamba exposure?

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. Studies have also linked dicamba exposure to increased odds of hypothyroidism.

Cancer

Dicamba use and cancer incidence in the agricultural health study: an updated analysis, International Journal of Epidemiology (5.1.20).”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.”

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”

Hypothyroidism

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”

Why are farmers suing over dicamba drift damage to crops?

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.

In 2016, the U.S. Environmental Protection Agency (EPA) approved the registration of new dicamba formulations, allowing for a new use of “over-the-top” applications on cotton and soybean plants that were genetically engineered to tolerate dicamba. Scientists warned the new uses would result in damage from dicamba drifting off farm and killing crops that were not genetically engineered to tolerate the chemical.

The new uses for dicamba came about because of widespread weed resistance to glyphosate-based herbicides, including the popular Roundup brand developed in the 1970s. In the 1990s, Monsanto introduced glyphosate-tolerant crops, and encouraged farmers 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 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 to develop a cropping system of genetically engineered (or GMO) 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.

Thousands of crop harm complaints

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.The dicamba drift damage concerns have prompted lawsuits from farmers in many U.S. states. Details on the litigation can be found here.

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.

News coverage

650,000 acres of soybeans damaged by dicamba this summer, state estimates, by Stephen Steed, Arkansas Democrat Gazette (7.19.21)

Off-target dicamba damage in 2021 may be the worst year yet in the Upper Midwest, by Michelle Rook, Mikkel Pates, AgWeek (8.2.21)

‘Buy it or else’: Inside Monsanto and BASF’s moves to force dicamba on farmers, by Jonathan Hettinger, St. Louis Post Dispatch (12.6.20)

EPA documents show dicamba damage worse than previously thought, byJohnathan Hettinger, Midwest Center for Investigative Reporting(10.29.20)

Scientists warned this weed killer would destroy crops. EPA approved it anyway, by Liza Gross, Reveal (11.13.18)

Crops in 25 States Damaged by Unintended Drift of Weed Killer, by Eric Lipton, New York Times (11.1.17)

Why did an appeals court say EPA violated the law?

On June 3, 2020. the U.S. Court of Appeals for the Ninth Circuit said the EPA had violated the law in approving dicamba herbicides made by Bayer, BASF and Corteva Agrisciences. The court overturned the agency’s approval of the popular dicamba-based herbicides made by the three chemical giants. The ruling made it illegal for farmers to continue to use the product.

But the EPA flouted the court ruling, issuing a notice on June 8 that said growers could continue to use the companies’ dicamba herbicides until July 31, despite the fact that the court specifically said in its order that it wanted no delay in vacating those approvals. The court cited damage done by dicamba use in past summers to millions of acres of crops, orchards and vegetable plots across U.S. farm country.

On June 11, 2020, the petitioners in the case filed an emergency motion seeking to enforce the court order and to hold the EPA in contempt.More details can be found here.

Is the government doing anything to stop crop loss?

In October 2020, the U.S. EPA announced that it will allow U.S. farmers to continue spraying crops with Bayer AG’s weedkiller used on dicamba-resistant GMO soybeans and cotton, despite a court order blocking sales. In June an appeals court ruled that EPA “substantially understated the risks” of dicamba weed killers.

In October 2018, U.S. 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:

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.

Is dicamba in our food?

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 in 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 at 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.”

For more reporting and perspectives on the topic of EPA tolerance levels, see our reporting in Environmental Health News:

Losing effectiveness

“Crop-killing weeds such as kochia are advancing across the U.S. northern plains and Midwest, in the latest sign that weeds are developing resistance to chemicals faster than companies including Bayer and Corteva can develop new ones to fight them,” reports Rod Nickel and Tom Polansek in a January 16, 2024 Reuters investigation based on two dozen interviews and a review of academic papers published since 2021.

The International Survey of Herbicide Resistant Weeds, a global database maintained by a group of scientists in over 80 countries, records “reduced effectiveness for glyphosate, one of the most common herbicides, against 361 weed species, including 180 in the U.S., affecting corn, soy, sugar beets and other crops,” Nickel and Polanski wrote. “Some 21 weed species globally showed resistance to dicamba, the most recent major U.S. chemical, which launched in 2017.” See our glyphosate fact sheet here.