We dry lot ours in the winter. I call it that, its also called a sacrifice lot or pasture. There is never any grass growing there no matter the season. We keep limestone screenings on it to keep the mud at a minimum so there is no way grass will grow on it. Its where they go when I need to rest my regular pasture to keep it from being overgrazed, and where they spend the winter. It was a suggestion from our land conservation officer in a rotating pasture plan to have a dry or sacrifice lot. There, dry lot explained
Ours have all been fat and happy all winter on just plain prairie grass and pellet feed (purina strategy). You might want to have a fecal analysis done just to rule out any parasites. I'm betting that's what the problem is.