You probably have a few options, depending what would best keep weight on your horse.
Rolled oats added into a grain-free ration balancer is doing wonders for the hardest keeper I have ever owned BUT he is a 26 yr old WB and a retired show jumper. The critter chiropractor told me to remove part of my brain from managing metabolic horses and keep this fella like he is a race horse😳
Of the ration balancer options that are available within my driving distance, Nutrena Safe Choice Senior seems to be the best, looking at the ingredients.
I only added the ROLLED (not crimped) oats a few days ago and I can already see a difference. There are zero oats in his manure, so I know he is digesting everything.
I feed the rolled oats at a 20% ratio to his Nutrena Safe Choice Senior.
You can also add Succeed or Egusin 250 to the feed pan. Egusin 250 is pellets so may be better in your boarding situation; it is, however, $12? more for nine days less product.
I add Succeed to both my horses feed pans. The WB does have ulcers, which I went thru H-e-l-l with him when he first arrived. He went completely off his feed, lost ~75# and boy was I scrambling trying to find something he liked that was reasonably healthy. Thus the Nutrena Safe Choice Senior but it doesn’t have enough fat calories. Since the WB is no where near the word metabolic, I’m feeding him the maximum recommended amount of rolled oats and so far it’s working to add weight. To reiterate, ROLLED oats, not crimped.
He is not fizzy either.
I hope this helps:)