I've exercised some tough love with them to get them to eat a new hay. Fill up their hay racks with only that for a day or two and see what they think once they are a bit hungry. If they have pasture available, they won't starve if you even give it a few days with only that hay. As far as if they need it, I have a few questions to help work that our. How's their condition? Are they wethers, bucks or does? If we are talking does, are they pregnant, in milk, or dry?