First off: Cribbing and wood chewing are 2 TOTALLY different things. I know this first hand because I know a wood chewer, and my horse is a cribber (well used to be anyways. Got him a collar, and it's totally under control now (: SCORE!) It sounds more like your horse is wood chewing. It can be caused by 2? Things, boredom, and lack of nutrition. I don't know what you feed him, but I personally think that when a horse is in a field and he starts chewing bark off of trees and such, it's because there is a problem in his diet. Not boredom. There is a supplement called Quitt or something like that (yes with 2 T's) it's by Farnam, but it's supposed to get your horse to stop. You could spray the trees with Farnam No Chew or whatever. But before doing any of those 2 things, I recommend changing his diet to fit his needs a little better.