Ever change the diet on an infant or an elderly person, or even a dog? They can get sick because their bodies can't handle the change in diet. Also, for the deer to properly digest the corn, they need a certain kind of bacteria in their gut. The bacteria is different than what they need to digest grass, clover, brassica, etc. Ever hear about people getting deathly sick when they go to Mexico, but yet the locals are doing fine? The locals are used to the bacteria in their locale. Give them a diet from the states and they'll probably get sick.