my horses chewed a hole IN MY BARN wall last winter, in one day. It was chip board and I've heard that something in the glue appeals to them. I chalk wood chewing up to boredom. My horses do it in winter, when they are not at pasture (they get pasture spring/summer/fall). I have found that mixing sunlight dish detergent with a lot of cayenne pepper works really well as a deterrent. And small mesh haynets have helped a lot, too, as well as installing angle iron on some surfaces (gate tops, edges, etc) as was mentioned above. Some folks I know put a log or two (usually cottonwood) on their paddocks to give their horses something to chew on.