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Dicamba Has Been Around For Years. Why Would It Now Be Causing Problems?

Genetically engineered cotton seeds delivered to Missouri farmers in 2015 featured a warning not to spray them with dicamba. The corresponding dicamba herbicide was not approved by regulators until 2017.
File: Kristofor Husted
Harvest Public Media
Genetically engineered cotton seeds delivered to Missouri farmers in 2015 featured a warning not to spray them with dicamba. The corresponding dicamba herbicide was not approved by regulators until 2017.

The herbicide dicamba is thought to have been the culprit in more than 3 million acres of damaged soybeans across the country, destroying plants and leaving farmers out millions of dollars in crops.

The chemical has been in use for decades, so why is it today apparently causing farms so much damage?

The answer is two-pronged, according to Kevin Bradley, a University of Missouri assistant professor and weed specialist who has studied the reported damage. Here’s what he says:

Reason 1: Farmers Are Spraying More

In recent years, many farmers used glyphosate -- the active ingredient in the weed killer Roundup produced by agribusiness giant Monsanto -- to destroy pesky weeds like palmer amaranth, or pigweed. About 90 percent of soybean plants, corn and cotton grown in the U.S. are genetically modified to withstand at least one herbicide, so that when farmers spray the fields, the weeds die and the crops survive. Roundup, in particular, grew popular.

Eventually, many pigweed plants grew resistant to formulations of glyphosate. So Monsanto developed soybean seeds genetically modified to withstand new formulations of an older pesticide: dicamba. The seeds entered the market, though, before the new version of the pesticide. Some farmers reached for old dicamba formulations and sprayed their fields. But those versions have been criticized as being too likely to drift in the wind onto neighboring farmland, and onto plants not genetically engineered to withstand the chemicals.

There have always been drift issues with dicamba, Bradley says, but the sheer amount of dicamba sprayed over the past two years has pushed the problems up a notch. He anticipates farmers will continue to spray dicamba to keep pigweed out of the fields in the future, too.

Reason 2: Farmers Are Using Dicamba Later In The Year

Farmers historically sprayed dicamba in April and May around corn and other crops. Today, many farmers are spraying the new formulations of dicamba as late as June and July on the genetically engineered soybean plants introduced in 2016 and on genetically engineered cotton, which was released in 2015.

Spraying later in the year could open up plants to exposure when they are more mature, which could amplify damage.

The U.S. Environmental Protection Agency is reviewing the restrictions it places on the use of dicamba, according to an EPA spokesperson, in light of dicamba-related investigations.

Missouri and Arkansas temporarily banned dicamba sales and use in July. Regulators in Missouri quickly put together a new label with instructions on how to properly prepare and apply the chemical.  Nevertheless, reports of damaged acreage continues to stream in. There are currently more than 2,200 official dicamba-related investigations in the U.S., according to Bradley.

For its part, Monsanto wrote in an open letter to farmers that says, “The overwhelming majority of farmers are experiencing tremendous success during this first year of commercial launch.” The company says it is studying the reports of damage, and that applicator education and training is the way to improve accurate application of the dicamba.

The challenge lies in what farmers will do in 2018. As they start making their seed orders in the next few months, it will be interesting to see if more farmers pony up for more expensive dicamba-tolerant seeds in fear of drift damage. 

Copyright 2021 Harvest Public Media. To see more, visit .

Kristofor Husted is a senior reporter at KBIA in Columbia, Mo. Previously Husted reported for NPR’s Science Desk in Washington and Harvest Public Media. Husted was a 2013 fellow with the Institute for Journalism and Natural Resources and a 2015 fellow for the Institute for Journalism and Justice. He’s won regional and national Edward R. Murrow, PRNDI and Sigma Delta Chi awards. Husted also is an instructor at the Missouri School of Journalism. He received a B.S. in cell biology from UC Davis and an M.S. in journalism from Northwestern University.
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