My youngester (cold blooded breed) was a monster on anything other than hay/haylage. Her problems manifested as bolshiness mostly, but also appeared "wired" somehow, like a kid on a sugar high. My guess was that she didn't tolerate alfalfa and/or molasses, even in very small amounts. Oh, and magnesium had the absolute opposite effect on her.
I currently have an issue with my old wb guy. He's still in ridden work and gets ad lib hay but needs more feed so he doesn't loose muscle. He does very well on alfalfa pellets - I can feed it to him in kilos and he doesn't loose the plot at all. The trouble is, he doesn't like alfalfa 🙄 and I cannot get him to eat enough of it, and the more and longer I feed, the more he leaves.
So... I've tried different hard feeds with...erm...interesting results 😅 The last one I tried was Aveve Senior mix. Well, let's put it this way - my gentle, sweet dude of a horse completely panicked everytime my friends horse snorted when we were out hacking this tuesday and almost tore my arms off when we cantered, and breaks were... uhm, seriously lacking at times. Not to mention that he got crabby to brush and tack up.
I've also tried a low starch/low sugar mix from a reputable hard feed manufacturer in Sweden (Krafft). Well, on that he piaffed (yes, seriously piaffed, no joke!) two whole miles on a hack while SCREAMING for his buddies until we got back (this is a horse you normally can leave alone in the stable and hack out on his own).
The common ingredient in these feeds is soy. So my best guess right now is that soy sends him bonkers and messes with his stomach.
Will be very interesting to test this theory since as of yesterday he is on a senior nut (also from Krafft) with a far shorter ingredient list and without soy products. Pray for me on our next hack 😅