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To Jody Herr, it was a telltale sign that one of his tomato fields had been poisoned by 2,4-D, the powerful herbicide that was an ingredient in Agent Orange, the Vietnam War defoliant.

“The leaves had curled and the plants were kind of twisting rather than growing straight,” Mr. Herr said of the 2009 incident on his vegetable farm in Lowell, Ind. He is convinced the chemical, as well as another herbicide called dicamba, had wafted through the air from farms nearly two miles away.

Mr. Herr recalled the incident because he is concerned that the Dow Chemical company is on the verge of winning regulatory approval for corn that is genetically engineered to be immune to 2,4-D, allowing farmers to spray the chemical to kill weeds without harming the corn stalks.

That would be a welcome development for corn farmers like Brooks Hurst of Tarkio, Mo., who are coping with runaway weeds that can no longer be controlled by Roundup, the herbicide of choice for the last decade