How many bales of hay does a horse eat?

That depends on the weight of the bales, the weight of the horse, the current body condition of the horse, the use of the horse and whether or not the horse has another source of forage such as pasture along with hay. Horses require 1.5%-3% of their body weight in forage per day. For a 1000lb horse that is 15 lbs of hay or grass at least per day. If the horse is working hard or needs to gain weight, then the amount of forage required per day increases.