A summary of Michael Pollan, Omnivore's Dilemma, and Greg Critser, Fat Land, as they deal with agriculture programs: farming was undeveloped in the 1920's, went into crisis in the 1930's when the New Deal came up with the "Ever Normal Granary" program, after the war farming became mechanized, industrialized, using nitrogen fertilizer developed by the scientist who developed poison gas, but still was mostly okay until Nixon and Earl Butz. Butz, the racist Secretary of Agriculture destroyed New Deal farm programs, encouraging full production "fence row to fence row". This led to cheap corn, which was used by big business using the Japanese invention of high fructose corn syrup to make big soft drinks. Cheap food meant the fast food outlets could "supersize" their meals to get more business. As a result, Americans overeat and get fat.
[This is based on memory, oversimplifies, but is not totally unfair to the writers. As you can tell from my tone, I quarrel with the narrative.]