When I had a horse that needed to put on some weight a nutritionist gave me two simple pieces of advice. 1) Feed more and 2) Feed more frequently. In fact, he said, is you feed several small meals you might not even need to feed more grain.
Most barns feed a concentrate twice a day and usually horses get hay several times. Adding a lunch time feed can really benefit a hard keeper, even if you can’t feed lunch every day. Although many people worry about changing a horse’s feeding routine, adding a snack of up to 1 pound of their normal grain a few times per week won’t do your horse any harm. Horses evolved to eat small frequent meals and they digest their food more efficiently when it’s fed in small quantities.
I don’t feed a concentrate but I almost always give Freedom an extra ration of alfalfa pellets if I ride during the day. And one of the mares at our barn is a very picky eater. Feeding her lunch when possible goes a long way toward keeping weight on her.