Farmers joined the U.S. food industry last week to launch a coalition seeking a unified federal standard for labeling food products made with genetically modified (GMO) ingredients only if there is scientific evidence of a health of safety issue.

The Coalition for Safe Afford­able Food wants to avoid a patchwork of state-by-state labeling requirements and affirm the safety of GMOs for consumers, said southwest Iowa farmer Ray Gaesser, president of the American Soybean Association (ASA). It costs farmers 15 to 30 percent more to grow...