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ALTON, Iowa – The puny, yellow corn stalks stand like weary sentries on one boundary of Dennis Von Arb’s field here.

On a windy day this spring, his neighbor sprayed glyphosate on his fields, and some of the herbicide blew onto Mr. Von Arb’s conventionally grown corn, killing the first few rows.

He’s more concerned, though, about the soil. During heavy rains in the summer, the runoff from his neighbor’s farm soaked his fields with glyphosate-laden water.

“Anything you put on the land affects the chemistry and biology of the land, and that’s a powerful pesticide,” Mr. Von Arb said.

But 20 miles down the road, Brad Vermeer brushes aside such concerns.

He grows “traited,” or biotech, corn and soy on some 1,500 acres and estimates that his yield would fall by 20 percent if he switched to conventional crops and stopped using glyphosate, known by brand names like Roundup and Buccaneer.

In short, it is just too profitable to give up.