Ad-lib forage. Not only will it take them longer to masticate and digest, it will also encourage the digestive system to keep working effectively (particularly important when grazing is limited).

I also really like the snak-a-balls, it keeps my lot entertained for hours - okay, so you may get a messier bed but I'd rather sacrifice that in order to keep them happy.

Maintaining social interaction with other horse's will also help - even if just stabling next to another horse will help.