A lot of good suggestions. I might try the black seed stuff suggested above - interesting.
It comes down to availability for me. They use to sell soy meal everywhere, but now...they act like you are trying to "stump the bartender" when you ask for it. I use to feed a 1/2 cup per horse twice/day - and they shined like glass! Now, I just get the flax oil - not near as good, but it helps.