I believe most horses are the healthiest when they are allowed to graze on grass and eat small amounts frequently. The horse's GI tract is designed for them to eat small meals frequently and so when they are fed one or two large meals daily, problems can develop. I know sometimes all you can do is feed once or twice daily, and not everyone has a turnout area. However, just realize this is abnormal for the horse's digestion. Kentucky Equine Research indicates in equinews.com that the horse's stomach holds about two to four gallons; it is not as large a stomach as you would expect for this size animal. And the stomach doesn't need to be that big because horses were not meant to eat large meals and therein lies the problem when we feed a horse a large meal once or twice a day.
Normally, the food eaten passes out of the stomach in 30 minutes. Chewing causes saliva to be produced that, among other things, buffers the acid in the stomach. Horses continually produce acid in their stomach so if they are only fed once or twice a day, then saliva is only produced at those times and the rest of the time acid is sitting in the horse's stomach with no saliva to buffer it. So you can easily see why race horses that are fed only twice a day and are kept in a stall 23 hours a day develop stomach ulcers. Also, the hindgut of the horse is made to digest fiber rich foods like pasture, hay, and hay cubes. When large grain meals are fed, some of the grain by-passes the small intestine and ends up in the hindgut, which changes the bacteria in the intestine leading to colic, diarrhea, and behavioral issues like wood chewing and stall walking. So horses are designed to eat small meals of forage all through the day and the further we get away from that, the more problems we see.