That would drive me crazy too. It sounds funny, but put a pile of sticks, like willow branches and such in their pens. We had trouble with our horses being bad,( chewing on trees, each other,etc,) a few winters ago too. We had cleaned up our tree line and put the sticks in their pen to burn later, but they ATE them all that winter. The whole pile was gone by spring. They would stand there like it was a bale of hay, toss them around, chew on them, what ever suited them that day. They didnt wreck as many blankets that winter or chew on the living trees either.