Useful Reference Works
Horse Sense: 4. How Much Does a Horse need to Eat?
The average I have been told all my life, in feeding stock with hay, is that a horse needs 25 pounds of rough feed a day. That means, in order to maintain his weight, fitness, and good tone, he needs to consume 25 lbs of roughage, meaning grass or hay, each day. Grain is NOT a substitute for good feed. It is merely a supplement, and does not fill the ol' belly any more than a granola bar. Of course, like people, individual horses differ in their actual calorie requirement, and the richness of the feed must also be taken into account. (i.e. alfalfa has more calories per pound than plain grass hay.) Adjustments must therefore be made according to the type of feed, the animal's level of activity, and how easily he maintains his body weight.