It will take care of itself
I wouldn't worry about American farmers producing too much food. With global agricultural output not increasing fast enough to feed the world's growing population, it won't be long before food will become a lot more expensive and even Americans will have to cut back. We see it now in the increased imports of American grain to China.
One day Americans will look back to when we wasted food with a great deal of fondness, and terms like "quarter pounder" will fade into distant memory.