Dicamba, a weedkiller used on genetically modified, dicamba-resistant crops (such as cotton and soybeans) to kill weeds that have developed a resistance to glyphosate, became a hot-button issue in 2017, when thousands of farmers and environmentalists filed complaints about dicamba’s propensity to drift, sometimes moving up to a mile from where it was originally sprayed. In 2017, there were 132 investigations in Tennessee regarding dicamba drift, affecting roughly 400,000 acres of crops, according to the University of Missouri’s Department of Agriculture.
Dicamba drift occurs when it fails to properly stick to the crops where it’s applied, sometimes drifting up to a mile from where it was originally sprayed and having catastrophic effects on crops that aren’t resistant to the weedkiller. New research from UTIA though, suggests there’s two main contributors to dicamba volatility: temperature and the addition of another weedkiller, glyphosate.
“What we found was that when one mixes Roundup (glyphosate) with dicamba, the pH is lowered, and this causes more potential volatility and may end up with more off target movement,” said Tom Mueller, a professor with UT’s Department of Plant Sciences and one of the lead researchers on the study.
Glyphosate, which has been the subject of lawsuits alleging the product causes cancer and is made by the same company, was one of the most popular weed killers on the market, and farmers would often mix glyphosate and dicamba to control a wider range of weed species. With the new data, however, Mueller and other scientists who worked on the study are suggesting farmers no longer mix the two herbicides.
“I think when a farmer is spraying near sensitive areas, like near other crops or type of sensitive plants, then I would advise them to not add glyphosate into the tank, based on our studies,” Mueller said.
Tennessee — one of the first states to enact restrictions on dicamba use — has restrictions still in place, but largely follows the EPA guidelines put forth in October, which label dicamba as a “restricted-use” product.
“We at the Department (of Agriculture) have advised and fallen in line with federal guidelines for application of over-the-top use of dicamba,” said Will Freeman, public information officer at the Tennessee Department of Agriculture. “The federal guidelines do not require limitations for this topic, but we’re working with UT to recommend it.”
Those recommendations include not using dicamba in conjunction with glyphosate, and avoiding its use when temperatures rise above 85 degrees. Still, these are just recommendations, though Freeman noted that “there’s potential for change after this year.”
The Tennessee Department of Agriculture says that there have been four complaints regarding dicamba drift in 2019, mostly affecting trees and personal gardens — not large-scale farms. That number is a “good amount” compared to previous years, which officials say proves the training and protocols in place have been effective. Freeman did note that those statistics could change “next week,” but those numbers are in line with a decline in dicamba drift-related complaints. At this time last year, there were 14 investigations of alleged dicamba drift, affecting 2,000 acres.
As for whether this data will further impact glyphosate’s popularity, Freeman said the Department of Agriculture isn’t “able to speculate on upcoming changes in glyphosate usage among Tennessee farmers.” Mueller, meanwhile, said that glyphosate is still an “extremely important herbicide” and that he believes it has a “long history ahead it of it.”
Lawsuits have been filed against both the EPA and Bayer — the parent company of Monsanto, which makes both glyphosate and dicamba — with the one against the EPA being dismissed earlier this year. The lawsuit against Bayer, however, alleges that the company violated antitrust laws when it introduced DT soybeans, which make up well over half of the U.S. soybean crop.
Editor’s note: This story has been corrected from an earlier version to note there were four complaints of dicamba drift so far in 2019, not five, as was originally reported.