Great for horses up here in New England. Calcium and phosphorus are better utilized by your horse when they are fed in certain ratios. Alfalfa hay is high in calcium and low in phosphorus, so a high phosphorus supplement like Select I is required when you feed alfalfa hay. Grass hay is low in calcium and phosphorus, so Select II is required to properly supplement grass hay or pasture rations. Now you can correctly supplement for the type of hay you are feeding rather than using a shotgun approach for all hays.