Twenty-five million acres have been planted with genetically modified seeds to encourage the spraying of the chemical. Farmers worry about damage to crops.

Farmers planted a new kind of seed on 25 million acres of soybean and cotton fields this year. Developed by Monsanto, the seeds, genetically modified to be resistant to a weed killer called dicamba, are one of the biggest product releases in the company’s history.

But the seeds and the weed killer have turned some farmers — often customers of Monsanto, which sells both — against the company and alarmed regulators.

Farmers who have not bought the expensive new seeds, which started to appear last year, are joining lawsuits, claiming that their crops have been damaged by dicamba that drifted onto their farms. Arkansas announced a 120-day ban of the weed killer this summer, and it is considering barring its use next year after mid-April. Missouri briefly barred its sale in July. And the Environmental Protection Agency, not known for its aggressiveness under President Trump, is weighing its own action.

“I’m a fan of Monsanto. I’ve bought a lot of their products,” said Brad Williams, a Missouri farmer. “I can’t wrap my mind around the fact that there would be some kind of evil nefarious plot to put a defective product out there intentionally.”

Yet he has been dismayed both by damage to his soybean crops, which were within a wide area of farmland harmed by dicamba, and by the impact even to trees on his property. Leaves, he said, were “so deformed you couldn’t even really identify the differences between them.”

The dispute comes as American agriculture sits at a crossroads.

Genetically modified crops were introduced in the mid-1990s. They made it possible to spray weed killers — chiefly Monsanto’s Roundup — on plants after they emerged from the ground, ridding fields of weeds while leaving crops undamaged.