While most producers would dispute the link between farm subsidies and poor dietary choices (and in linkage, obesity), we don't really have much research showing what does influence food choices. A new study may help.
The results, just published in Psychological Science, a journal of the Association for Psychological Science, show that taxes were more effective in reducing calories purchased over subsides. Specifically, taxing unhealthy foods reduced overall calories purchased, while cutting the proportion of fat and carbohydrates and upping the proportion of protein in a typical week's groceries.
By contrast, subsidizing the prices of healthy food actually increased overall calories purchased without changing the nutritional value at all. It appears that mothers took the money they saved on subsidized fruits and vegetables and treated the family to less healthy alternatives, such as chips and soda pop. Taxes had basically the opposite effect, shifting spending from less healthy to healthier choices. [More]
I think what this may suggest is modifying the farm bill to subsidize fruits and vegetables rather than corn and wheat wouldn't be that effective. Taxing food choices that are deemed less healthy (soda pop, snack foods, etc.) at the point of purchase would be more likely to alter diets.
Of course, it also suggest from a public health viewpoint, grain subsidies are wrong no matter how you look at them.