One thing we notice down here in Costa Rica is that horses will gnaw on things if they lack minerals - probably doesn't mean anything up there, but sometimes a horse will chew because they are lacking something in their diet.
The Doc says the same thing - that cribbing is often caused by a mineral deficiency.
Some horses knaw on things from boredom too. Giving them a "playtoy" is a good idea.
I've used oak, poplar, pine and cedar in our horse stalls. They don't seem to prefer one over another in terms of chewing.
