Now some farmers say they are being forced to use the new GMO seeds to guard against dicamba.

Nathan Reed, a farmer in Marianna, Arkansas, whose crops were damaged by dicamba from fields more than two miles away, worries about his business.

"We use overwhelmingly non GMOs, not because we are anti-GMO but because we found some niche markets," Reed said at a public meeting last month. "We are in the business of making money, just like Monsanto is."

"It is going to put that ability at risk for us," he said.

Farming states Missouri, Minnesota and North Dakota have imposed restrictions on dicamba, though they permit farmers to use the herbicide one or two times at the start of the season.