A horse needs to eat at least 10 to 20 percent of its body weight in hay, which averages out to around 15 to 20 lbs. of hay. A horse's digestive system is designed for roughage, which includes pasture grasses and hay. Horses with plenty of room to graze don't need as much hay as a horse that is kept in a stable with little time for grazing.
Grains are not ideal for horses, but many horse owners supplement a horse's diet with them. It's ideal to allow a horse to freely graze and only supply hay or grains if there isn't enough pasture grass.