LITTLE ROCK, Ark. (AP) — A longtime Arkansas soybean farmer, Mike Wallace thought of his neighbors as a community and always was willing to lend a hand if they faced any hardships with their crops.

"Mike would do anything for any farmer," his wife, Karen, said. "If there was a farmer who got sick in harvest time or planting time or whatever, he would say, 'What can I do to help? Here's my equipment. Here's my guys. Let's go do it.'"

But across much of farm country, a dispute over a common weed killer is turning neighbor against neighbor. The furor surrounding the herbicide known as dicamba has...