I grind my feed in my JD 400 with a Farmall M. A 50 HP tractor give or take. I put corn into as fast as it can go up the auger. It never has a problem. I also put a bale of alfalfa in it. That will make it bark if I add it more than a flake at a time. I enjoy grinding so it doesn't bother me to slow down a little then.
I grind corn for one other neighbor and use the same tractor - probably 150 bushels at a time.
The M's only other job is as my loader tractor. It's probably good for it to get worked hard every few weeks.