Here's a fact: mules are stronger, pound-for-pound, than horses and donkeys. It has to do with smooth muscles, vs. skeletal muscles, whatever, and whatever, science. This all means that mules eat food more efficiently, and probably process it pretty well. Well enough that they just leave it all out there on the field. Such was the case during the Army - Rice game, when a whole bunch of mule shit delayed the game for several minutes. Fertilizer, IMO.