In Arkansas, there is a kind of David vs. Goliath battle underway over a weedkiller.

On one side, there is the giant Monsanto Company. On the other, a committee of 18 people, mostly farmers and small-business owners, that regulates the use of pesticides in the state. It has banned Monsanto’s latest way of killing weeds during the growing season.

Terry Fuller is on that committee. He never intended to pick a fight with a billion-dollar company. “I didn’t feel like I was leading the charge,” he says. “I felt like I was just trying to do my duty.”

Terry Fuller and his identical twin, Jerry Fuller, grow soybeans and raise cattle near the tiny town of Poplar Grove, in eastern Arkansas.

A big part of their business, though, is selling seeds to farmers. And in their storage shed, Terry Fuller shows me the product that has turned neighbors against one another and provoked that fight with Monsanto. The product is soybean seeds.

Fuller leans over and reads the label on one large bag. “7478XTS, so that is an Xtend soybean variety right there,” he says. “That’s dicamba-tolerant.”

Dicamba is a herbicide. It kills what farmers call broadleaf plants, a category that includes many weeds as well as crops like soybeans. Not these soybeans, though. Monsanto tweaked the genes of these varieties, and now dicamba doesn’t bother them at all.

It means that farmers can plant these seeds and spray dicamba on their fields as the soybeans grow, and the weeds will die, but the crops are fine.

When Fuller heard about this invention a few years ago, he thought it was great. “I absolutely wanted to spray dicamba in Arkansas and the rest of the nation,” he says.

Farmers began spraying the chemical on their fields last summer, and they say it worked splendidly. The problem was that when the weather turned hot, the weedkiller didn’t stay where it was supposed to. It seemed to evaporate and drift, sometimes for a mile or more, into fields of other crops that can’t tolerate dicamba. It left those crops stunted or with curled up leaves.

“I could not walk out of my house without seeing damage,” says Fuller.