My horse is older, but quite active. I just moved him from our quiet home barn to a stables with other horses. He was in great shape, or even a little overweight when we took him there a month ago, but now he's dropping weight quickly with being so active with the other horses.
They have plenty of hay to eat, free choice all day, so that isn't an issue. What is the best feed for putting on and mantaining weight for him?