Doug, you're off base here. Horses are grazers. Grass and clover have very little nutritional value to a horse and a horse will graze continously until all these grasses are gone. You can put a horse out in a big enough field that it cannot decimate, feed it nothing else, and it will starve to death while eating continuously. A couple cups of grain and a 10" leaf of Timothy hay a day on the other hand, will make a horse fat even if it never grazed outside again. They eat it out of boredom, but get little out of it. Deer are browsers. They don't eat continuously like a horse, because they don't eat the same things as a horse. Deer also chew cud, to get every drop of nutrients out of every mouthful. Horses don't. Horses have an extremely fast digestive flow, they don't chew their cud, and they actually waste huge amounts of nutrients in the food they eat. Ever think about that when you clean a stall? Could you imagine if deer had that volume of waste? They crap hard little pellets. They extract every bit of energy out of what they eat, and require FAR FAR less feed per pound of body weight as compared to a horse.
You are comparing tiny little apples to big jumbo oranges here.
My family has horses...I know this.
I know that there are far more deer than horses, yet nobody hauls deer manure to fertilize their gardens. WHY IS THAT?