(Apologies in advance for the essay) I bought my OTTB last April and the first thing I noticed was how sweet, calm and gentle he is. Everyone at the barn said he seemed like an old man in a young horse body (he is 7) But now, he has become unruly, unresponsive, rude, and almost dangerous. A little background... He came to me malnourished and only partially restarted but once I began riding him he was super sweet and eager to learn. His weight kept fluctuating and realistically the barn he was kept at could not afford to feed him the amount of forage he needed, he was also mainly in a stall because the pastures had poor drainage and too many horses. At this place, he showed no signs of ill behavior. I recently moved him down the street to pasture board and he loved it! His weight is also finally going up and his topline is really looking nice. He had been bumped to 8qts of grain (Nutrena Proforce Fuel) total per day when the old place did 4qts (Nutrena Safechoice Senior) As well as 24/7 access to good quality hay in the winter, and grass in the summer. He was doing good! However, right after he had been bumped to 8qts he has become a nightmare. The past handful of rides we had I had to dismount multiple times and let him run around the ring to get rid of the energy. I also have to lunge now for 10-15 minutes before I ride which he behaves mostly. But when riding him, he is skittish, sensitive and very nervous. He started foaming and chewing at the bit, and I can physically feel his heart beat very fast. I consider myself a fairly calm rider, and I'm always talking to him and trying to be very gentle. Our rides used to consist of being able to ride on the buckle, stand in place for many minutes and I'd be able to let my toddler sister walk with him. Now, he can't even walk a lap without bolting off with me. I've tried multiple bits and nothing seemed to change. He isn't due for teeth until May-June, and he has no soundness issues or gut problems that I know of. The only thing I have not done is try a different saddle. But I feel like if it were a saddle issue he would have let me know sooner, since this attitude began after we bumped/switched his feed. His general attitude has also changed and he constantly runs away from me in his paddock, even when I'm not going to bring him to the arena, I just went to say hello to him. This issue is persisting and really making it hard for me to grow our relationship. Does anyone have any experience with this? Sorry for the essay. Maybe the Nutrena Proforce is not a good choice for him? I'm nervous to switch him since his weight fluctuates a lot but I'm open. All the other horses (OTTBS included) are fed this grain at the barn and are all very mellow. I also asked to lower his grain to 6qts total instead of 8. It's also worth mentioning we've had a frigid winter so far and he has heaps of snow around him making it difficult to burn off any energy. TL;DR- Gentle OTTB becoming very hot and unruly after bumping feed intake and switching grain.