I have had great luck with Purina Amplify. It is pure fat and doesn’t make the horses nutty. I put weight back on my 23-year old mare by adding a cup and a half per day for a few months. Now I have a four-year-old paint gelding who needed weight and with him, 2 3/4 cups per day plus two extra flakes good grass hay on top of his regular grass hay and grain has been perfect for getting his weight up without super charging him. Good luck!