Good luck and I agree with what everyone else said. A calming supplement will help a horse relax, but not necessarily reduce studly behavior. Was he gelded late and could there be any hormonal/medical reasons for this? Also stress can cause this as I know a horse that is boarded here and before when he was at a friend's barn turned out with mares, he would mount them all the time, but he wasn't in as much work and didn't get to see his owner as much. Now he's here, still turned out with mares, and doesn't mount them b/c he has a few gelding playmates that he runs around and plays with - all without being studly - and he's in more work (though not always). In his case the biggest change was finding him playmates to get his extra energy out with and the studly behavior disappeared w/no other changes really - and he's a 21 year old Morgan!