The Minnesota Department of Agriculture is investigating about two dozen complaints from farmers about a weed killer used on genetically modified soybean fields that can tolerate the herbicide.
Minnesota agriculture department supervisor Greg Cremers said the complaints about dicamba started coming in earlier this month.
"They're coming from all across the southern part of the state," Cremers said. "Starting to see a trend where a little more moving up into the central part of the state."
Minnesota farmers told Minnesota Public Radio the herbicide drifts into non-resistant fields and hurts crops. Damaged vegetation will be tested in the coming months to see if diacamba is to blame.
Tim Carlblom, who owns farmland in the southern Minnesota town of Jeffers, said his soybeans have rounded, cupped leaves that he believes are caused by the weed killer. He said he's worried it may impact the fall harvest.
Agriculture company Monsanto introduced dicamba-resistant soybeans to the market. Robb Fraley, the company's chief technology officer, said dicamba has small-scale drift issues just like any herbicide, but that most of the damage is caused by farmer error.
Some farmers may be using generic versions of the herbicide that have high volatility while others may not be properly cleaning their herbicide tanks, he said.
"The vast majority of the farmers who've used this tool have used it safely and effectively," Fraley said.
Private crop consultant Stephan Melson said thousands of acres of soybeans may be damaged, but that many farmers won't report the problem because they don't want to hurt relations with their neighbors.
"Personally I've counted, I guess, 1,300 or so acres, but I know it's much higher than that," Melson said. "There are a lot of fields that we personally don't work with that have this injury."
Arkansas, Missouri and Tennessee are seeing similar problems and have taken steps to restrict diacamba usage.
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