Horse Stocking Rate on the Pine Ridge Reservation
A horse will consume 2-3% of their normal body weight in forage per day. There for a 1000# horse need to be provided 20 to 30# of air dry forage (hay) per day or the equivalent in pasture/range grasses. The current recommendations are to figure a horse at 1.25-1.5 animal units months when comparing them to beef cattle stocking rates for range units.
The actual amount that an individual horse needs depends on that horses metabolism. Some horses are easy keepers. They seam to get fat by just looking at the grass. Other's are hard keepers. No matter what you feed them they seam to stay slim. This is where the art comes into science of feeding animals. You as the owner need to utilize your observation skills to see if your horse is under or over conditioned.
Currently the recommended stocking rate from BIA is 40 acres (A) of land per animal unit year (AUY) or 3.33 acres (A) per month for a 1000# cow with calf at side. For a 1000# horse this would be 4.17 acres (A) for an easy keeper or 5 acres (A) for a hard keeper per month. To raise a horse year round on rangeland you would need from 50 to 60 acres (A) per 1000# horse to provide the forage intake needed to maintain the horses weight. Of course this may also need to be adjusted to weather conditions as drought where you may need to decrease stocking rate due to lower forage production of your range plants.