Corn is only one of many ag food sources. Soybeans are great for deer and their growth times perfectly with a buck's antler growth. Ag land in general just greatly decreases the amount of work it takes for a deer to survive most winters in the midwest.
This is probably going to degenerate into the old genetics versus nutrition debate, but I would direct everyone to a perfect example of what typical ag food sources can do. Dr. Grant Woods has turned his property down in the Ozarks into a deer paradise - only through nutrition and habitat improvement. If I remember correctly before he said before he bought the land, which is all bigwoods, [glow=red]not a single pope and young deer was registered to the entire county[/glow]. That's amazing considering what they shoot every year there now. Every deer has a certain Genetic potential but nutrition is the hatchet that trims inches off antlers most of the time. Its not just a genetic coincidence that the fertile land of the midwest grows most of the big bucks in the country.
Obviously Dr. Woods isn't harvesting anything but the deer themselves ( his website is called GROWING DEER for a reason
) but the food he provides is similar to what is available anywhere in the commercially farmed midwest. There is less waste grain and hay cured on the stem in the winter to eat on purely commercial ag land, IE no food plots, but its easier to get than what a big woods buck has to scrounge for.
So yea I have noticed a pretty big difference in antler size and body size between bucks I have seen in north central WI and down in eastern Iowa. I would say the average 2 and 3 year olds here have 10-15" bigger antlers and 10-20 pounds heavier. You go a lot further north, the bucks get bigger but I think that is getting into a completely different subspecies of whitetail.