One of my least favorite equine vices is wood chewing. The reason that it is my least favorite is because it is destructive and can end up being expensive. Wood chewing can be caused by a lack of fiber in the diet or even just boredom.
Once a horse starts wood chewing, it is very difficult to get them to stop because usually they are surrounded by it, depending on their environment. The best thing that you can do for you and your horse is to protect exposed wooden edges and protect wooden fences with something like hot wire. Make sure that you also analyze your horses diet to determine whether or not the wood chewing is a result of a lack of fiber.