No idea whether you have it in the US, but we use SpeediBeet. It's sugar beet that you soak, and it helps horses put on weight. Its really quite economical too, you pay like A$30 for a 20kg bag, and we use like two handfuls a day for three horses, one of which is in ..oh, I wouldnt say heavy work,but not light either..he's a competition showjumper, and it helps him keep on weight without making him go mental all the time :)
I throw my heart over the fence, and he follows it. ~Sir Louie VI- my showjumper, my best friend, my everything-iloveyou.