Horses evolved eating small amounts of grass while walking and grazing over many miles every day. Since they evolved in this manner, you can see why we have some of the problems we have today because we feed them differently. Many horses are turned out on pasture and fed a small amount of concentrates twice daily. The other end of the spectrum is racehorses that are kept in stalls 23 hours a day, exercised for one hour and fed a large amount of concentrates and hay twice daily. This is a common reason racehorses get colic and develop stomach ulcers as they are not designed to be fed in this manner.
An 1,100 pound horse's stomach only holds about two to four gallons, which is not thatmuch for this size animal. The horse's stomach is somewhat flexible but not enough to accommodate large quantities of grain and hay at one time. And since horses cannot vomit, once the material is in the stomach there is only one direction it can travel in the GI tract.
Another concern about feeding concentrates is that horses are also fed hay and it is impossible to know how much hay is in the horse's stomach, so adding a large quantity of concentrate can be a problem. Food leaves the stomach and goes to the small intestine where digestion takes place, and the small intestine is easily filled with material. This allows food to move too rapidly to the large intestine and can lead to colic and laminitis. So to avoid these problems, I recommend feeding no more than five pounds of concentrates at one feeding. The more often you can feed and the less amounts you feed, the less likely your horse will have problems.