The problem is that some climates are massively more efficient at producing one crop over another. Switching corn subsidies to vegetable subsidies would reduce

The problem is that some climates are massively more efficient at producing one crop over another. Switching corn subsidies to vegetable subsidies would reduce the value of farms in Iowa and increase the value of farms in California. Explicitly regional subsidies might work (CA farmers get $0.01 per carrot while IA gets $0.05). Some people will decry this as another market distortion, but that's the point.