If he is to do more work, then it is increased energy he will require rather than protein. This can be provided in the form of starch or fats.
This sentence does not make any sense to me.
Protein translates to energy ?
More excersise burns more fat, so one needs more protein ?
Starch is in plants stored as protein ?
So if you give the horse Alfafa pellents it's gets energy.
Also Alfafa pellets will suck up the extra juice coming out at the end.
I learned that the Mustang gets psylium
Psyllium is a soluble fiber used primarily as a gentle bulk-forming laxative in products,
it softens the bowles and I think that this would not be needed in treating diarrhea.
So, I think killing the bacteria causing diarrhea (watery bowel movements) with
DE would be my first thing and then remove long stapel grass second and then add Alfafa pellets to introduce to him slowly like 1/2 scoop per day for a week and thereafter 1 scoop per day.
would above be what I can recommend to the horse owner. I am the one who is exercising the poor critter.