So I'm thinking my horse could use a supplement. His food had to be cut down because of excess energy, and as a result he has lost some weight. Ideally I'd like something that would help him keep his weight and also maybe act as a "calming supplement" or something similar. He gets somewhat anxious/unpredictable at times. I know this won't solve all our problems, but I'm thinking he should have some sort of supplement regardless to add to his diet.
He is at a boarding barn and that's how much they give them. Which is not to say I couldnt pay more and have him get an extra few flakes, which I would have no problem with, but I was hoping to get him something that had some sort of nutrition besides his usual.
I would spend that supplement money on the hay. He likely doesn't need "nutrition", he needs more food. Horse's diets should begin with hay. And most boarding barns are going to be reluctant to deal with the hassle of beet pulp if that isn't their routine, even if that is another good way of getting more food into him without adding a lot of energy.
I know some people find that it makes their horses hot, but a flake or two of alfalfa a day should help to put weight on and can also be soothing to a nervous horse's stomach.
I'm also in the camp of more/better quality hay. At my boarding barn, I have had to buy some of my own hay because I did not like what they were feeding and I was having trouble putting weight/muscle on my 17.2hh TB mare. Plus the barn was used to dealing with ponies and quarter horse crosses and couldn't get their heads around how much hay a big TB can eat. That way I didn't have to deal with any stinginess as to how much hay she got and had more control over its quality.
I've also had good experience with flax seed. Not only does it help to put weight on but gives them shiny coats and is supposed to be good for joint health.
Do you have any say in what type of grain your horse gets? It would probably be helpful if you could switch to a higher fat/calorie, lower starch grain.
As for calming supplements one of the horses in our barn gets a valerian/chamomile herbal supplement, which seems to help, but it does test if that is a concern.
I would also start with more hay at night . . . but boarding barns can be tricky. You could also try putting the hay in a slow hay feeder (net with very small holes) so hay lasts longer. When my horse was stalled at night I always fed a bit of alfalfa hay. It's soothing for their stomachs and is higher in calories.
I'd also look at what else they are feeding. For my own horse, high starch feeds are like rocket fuel. I've had to work on his diet to ensure that he gets the calories & nutrition he needs (he's a bit of a hard keeper) without turning him into a nut case.
I am in a self-care facility so have complete control over what I feed, but what works for me is beet pulp + alfalfa pellets + Triple Crown low starch.
I also feed flax seed but don't think it makes that much of a difference for weight gain (I feed 2 cups/day).
Last summer I fed Purina Amplify when my horse was really fretting off weight because of the bugs but it's expensive and I've found that he's holding his weight now on the above diet.
We feed hay on an almost free choice basis (about 22 lbs/horse/day) but put it in slow feeders (Nibblenets) to reduce waste and make it last longer.