Alfalfa hay is very rich in protein so you would want to be careful feeding too much to your older horse. If he is doing well on the grass mix that is probably fine. Most times staright alfalfa hay is fed in small portions even to younger horses. It is very rich and can give horses diarrhea. On the racetrack we would feed straight timothy hay and a “flake” of alfalfa in the morning and matybe at night, but not much more than that.