The U.S. government has been protecting farmers against unpredictable hardships such as bad weather since the 1930s, when drought and the Great Depression devastated the nation’s agriculture industry.

Today, agricultural subsidies and insurance cost the U.S. taxpayers about $20 billion annually, according to the U.S. Government Accountability Office. That support has come under great scrutiny in recent years, with opponents complaining that most of the money goes to millionaire farmers and giant agribusinesses, not small...