The compound digestive system (abomasum, omasum, reticulum and rumen) allow feed, especially high cellulose, to be more thoroughly digested. With a horses simple stomach, as they eat it kind of pushes the partially digested feed out behind them. All ruminants have that advantage.
Because they are reumanants and have bacteria that help digest poor quality forage into high quality digestable protein, horses do not, and eat nearly 4X what a cow does, see horses laying out in the pasture for hours,? NO, they are up grazing all the time nearly, because they are so inneficient, I have 4, a waste of good grass if you ask me.