I personally have never had much success with calming supplements. They have actually made the horse's I've used them on even more uptight. But that being said, there's something called RelaxForm EQ that was highly recommended by a friend who is very into nutrition, and has done a lot of research into horse feeds, and supplements. It's $22 for a 13 ounce container at a local tack store in town. I've also heard lots of good things about Wendells herbs as well. I would also recommend maybe doing a hair sample. There are a couple companies that will do a hair analysis for you, and tell you what vitamins your horse needs to have adjusted, and what they do for his attitude, health ect. One company, don't remember the name unfortunately actually sells supplements as well, and they'll do the analysis, its $130 so pretty expensive, and then you can order the supplements, either premixed, or mix them yourself, that you horse needs. I know someone who's horse was actually pretty uptight and easily distracted so he did the hair analysis, and adjusted his horse's feed, and he's (the horse) is doing much better.