You might also consider the TYPE of hay that you’re feeding. My barn feeds alfalfa hay and oat hay pellets (for proper balancing of Ca/P levels) and all our horses are slick and shiny and fat! Some of the horses do get supplemented with grain (in my case TC senior, which I love and have since pheets introduced me to it. I drive 1 hr+ just to get it out in CA) and rice bran. We’ve found that rice bran is a great way to add more calories in a cost effective way without making the horses high.
As a side note, the barn across the street had two older horses that the owner was convinced they had just hit the end of their life, as they were low energy and losing weight. The vet convinced her to try alfalfa hay and they perked right up and are still now going strong, years later.
Just a thought