I read in a nature book about deer...that they cannot digest grain
during certain parts of the year.

http://www.gov.ns.ca/natr/wildlife/lgmams/feeddeer.htm

We feed our deer salt and some corn, as the moms wean their fawns.
We put out only a small amount...each day..and the deer leave and eat
the natural sources for the rest of the day. Since doing this we have had
several generations of heathy deer. But too much corn, and they will get sick.

Deer are ruminants too, and the locals feed hay in the "deer yards" during the winter. They have a small tract of township land that they farm just for the deer. (they do profit from the hunting season...so this is their investment).

But you know, too much omega-6 corn for cows...leads to poor fat types for us to eat. Free range beef is not only better for them, but for us, as then,
the meat has some omega-3 in it as well.