I've driven around the country side lately and noticed the same thing, deer feeding in corn fields. I think that the does are feeling the need to nourish the developing fawns and waste corn provides more punch per pound than wheat/grass/insert green crop here. If I could leave 20+ acres (I tried 3 acres and it didn't see December) of corn stand, that would be what I would do. It provides what deer need, food and cover. If one is worried about a balanced diet, make a mineral lick next to the standing corn.
Iowa's ag base (corn and beans) coupled with the minerals in our soils is what makes us a trophy state in the first place, right?