Oh, there are two of these threads! I replied in the other one.
But briefly, I have always been a firm believer the it does not heat horses up. However I recently aquired a 20 year old tb gelding for my father to learn to ride on. This horse is VERY quiet, I can do anything on him, jump on him bareback and take him around the paddock, ride him with just a rope around his neck and get a dressage test out of him... he's a lovely old boy. But give him just ONE biscuit of alfalfa, and he is a raving lunatic!! He will gallop the length of the paddocking bucking and carrying on like a 2 year old! Without alfalfa, he serenely cruises the paddock nibbling grass.
So yep, I'd say it can definitely hot them up!