High-quality hay can be an important source of essential nutrients in your horse’s diet. A horse’s protein and energy requirements depend on age, stage of development, metabolism and workload. A mature horse will eat 2 to 2.5 percent of its body weight a day, and for optimum health, nutritionists recommend that at least half of this should be roughage such as hay. For a 1,000-pound horse, that means at least 10 pounds of roughage each day.