In the decades-long chemical arms race between farmer and weed, among the grower’s heavy weapons is dicamba, a powerful herbicide. Dicamba use has gained prominence recently in part due to the evolved resistance of major weed pests like waterhemp, Palmer amaranth (both Amaranthus spp.) and marestail, Conyza canadensis, to other popular herbicides, particularly Roundup.
In 2015, the U.S. Department of Agriculture (USDA) deregulated the sale of dicamba-tolerant (DT) cotton and soybean seeds. Monsanto Co., Determination of Nonregulated Status of Herbicide Resistant Soybean and Cotton, Notice, 80 F.R. 2675, Jan. 20, 2015. And in a parallel action, on November 9, 2016, the U.S. Environmental Protection Agency (EPA) authorized using dicamba as an “over-the-top” spray on growing plants, on DT cotton and soybean, Final Registration of Dicamba on Dicamba-Tolerant Cotton and Soybean, www.regulations.gov/document?D=EPA-HQ-OPP-2016-0187-0959.
Spray drift from dicamba use onto sensitive crops in adjacent fields has stirred complaints and even lawsuits, at times pitting farm neighbor against neighbor. Dan Charles, Despite a Ban, Arkansas Farmers are Still Spraying Controversial Weedkiller, National Public Radio, Oct. 9, 2018, www.npr.org/sections/thesalt/2018/10/09/654847573/despite-a-ban-arkansas-farmers-are-still-spraying-controversial-weedkiller.
Drift complaints spiked in 2017, after the USDA and EPA approvals. A University of Missouri weed scientist who has tracked such complaints, Professor Kevin Bradley, reported 2.5 million acres of non-DT soybeans injured with dicamba in 2017 and, as of mid-2018, about 1.1 million damaged acres being investigated by state departments of agriculture. Kevin Bradley, July 15 Dicamba Injury Update: Different Year, Same Questions, Integrated Pest Management Newsletter, Univ. of Mo., July 19, 2018, https://ipm.missouri.edu/IPCM/2018/7/July-15-Dicamba-injury-update-different-year-same-questions/.
Nevertheless, on October 31, 2018, the EPA extended its 2016 registration of dicamba from late 2018 to December 20, 2020. www.regulations.gov/document?D=EPA-HQ-OPP-2016-0187-0968. Professor Bradley says he and other weed scientists were “pretty shocked” by an extension that, in his opinion, added “nothing substantive” to the product label (telephone conversation with author, Dec. 21, 2018). Beyond clarifying the terms of its prior restrictions, Professor Bradley says, the EPA has been “largely MIA” on the dicamba damage issue.
Dicamba is extraordinarily potent. At a concentration as low as 0.005 percent of its use rate (0.5 lb. per acre), it can produce injury, notably to soybeans, which are particularly sensitive to it. Prof. Robert Hartzler, Dicamba: Past, Present and Future, ICM-Blog, Iowa State University, Dec. 27, 2017, https://crops.extension.iastate.edu/blog/bob-hartzler/dicamba-past-present-and-future. And it is both volatile and prone to drift, particularly as summer temperatures rise and rainfall decreases. Robert Hartzler, id., Factors Influencing Dicamba Volatility, Aug. 16, 2017, https://crops.extension.iastate.edu/blog/bob-hartzler/factors-influencing-dicamba-volatility.
Dicamba is not new. Approved in 1962, it has since been marketed under several tradenames and formulations. Robert Hartzler, id., A Historical Perspective on Dicamba, Dec. 19, 2017, https://crops.extension.iastate.edu/blog/bob-hartzler/historical-perspective-dicamba (Historical Perspective). What is new is its use in tandem with DT seed. In areas where dicamba drift is likely to be a problem, says Professor Hartzler, farmers are more likely to stay with, or feel pressure to switch to, DT seed (telephone conversation with author, Jan. 9, 2019).
Unsurprisingly, Monsanto/Bayer’s dominance of the DT soybean seed market has raised antitrust issues. See, e.g., Master Antitrust Class Action Complaint, In Re Dicamba Herbicides Litigation, No. 1:18-md-02820-SNLJ, Doc. 138 (E.D. Mo. Aug. 1, 2018), www.moed.uscourts.gov/sites/moed/files/documents/118md2820-0138.pdf.
Equally unsurprisingly, weeds are now evolving dicamba resistance. In 2017, the International Survey of Herbicide Resistant Weeds reported 36 species with evolved resistance to Group 4 herbicides, the group that includes dicamba. Seven species are resistant to dicamba itself. Hartzler, Historical Perspective, supra.
Different groups of herbicides attack different biological functions, or sites of action, in weeds. In a conversation with me, Professor Hartzler observed that weeds are rapidly evolving resistance to all widely used products. The last new site of action was introduced more than 30 years ago. It is only a question of time—about 10 years, in his estimation—before U.S. agriculture confronts what Australia faces today: the need for new strategies beyond chemical controls (telephone conversation with author, Jan. 9, 2019).
One of Australia’s emerging solutions is “harvest weed seed control” (HWSC), the physical (mechanical) destruction of weed seeds before they enter the seedbank. Among the methods being developed is a combine attachment called a chaff mill, which separates out weed-seed containing chaff at harvest and destroys the seeds. Robert Hartzler, id., Harvest Weed Seed Control, Dec. 13, 2018, https://crops.extension.iastate.edu/blog/bob-hartzler/harvest-weed-seed-control. HWSC is as-yet untested in the United States and will add to production costs. But, as Professor Hartzler notes in his blog post, if farmers are to prevent further loss of effective herbicides, they must view reduction of the weed seedbank as just as important as protecting crop yields.