Ruminants need a certain percentage of long forage in their diet. For cattle, I've always figured 30% will keep you out of trouble all the time. If you're careful, you can get down to about 20% but you gotta watch em close. I saw some university research at 10% forage but that was really pushing it.
Deer can free choice on browse, acorns, grass or your corn. And like muleshoe said, it takes time for their digestive system to get out of whack.
Dale
