Last spring, for the first time in 20 years, Indiana farmer Jim Benham planted his fields entirely with soybean seeds that hadn’t been genetically modified to withstand herbicides. It wasn’t because the 63-year-old suddenly had embraced the anti-GMO movement. Instead, he was drawn to a nearly 14% per-bushel premium for non-GMO soybeans offered by a local grain terminal, which sells them to Asian feed processors. Mr. Benham is among a small but growing number of Midwestern farmers moving away from biotech seeds developed by Monsanto Co. , DuPont Co. and other companies in response to lower crop prices over the...