Hi – Yes – I went through weight-gain issues with my quarter horse. The short story from my vet (teeth floated, de-wormed all that done) Feed maximum quantity of Purina Strategy GX, feed one pound of Grow-N-Win pellets twice a day. Grow N Win is by Pro Manna, feed the one pound in addition to Purina Strategy. That put weight on my 15 yr old. Plus the pasture, plus hay.
This year I am trying Cool Calories supplement. Missouri had a mild winter and since the gelding was maintaining weight well, I wanted to give Cool Calories a try.
There are so many variables that cause weight loss, so start by ruling out medical problems Good Luck!