Chewing wood...i.e fence rails, timber frame in stables or the bark of trees can become a habit due to boredom, for horses held in stables,yards or a bare / overgrazed paddocks...not so much 'windsucking or cribbing on the trees, although that can be the case on rails and timber they can grasp and suck back on.
We separate our horses to just one horse to a 5 acre paddock here so there is plenty of grazing, and we have found that during the day when they rest in the shade of a paddock tree, they would occassionally start to work on the bark. ...so all trees at risk where protected....new plantings of trees are kept outside the paddock fence perimeter or in the corners of the paddocks, with timber rails and off set electic wire to keep them off. This works well for us.
Lack of fibre in the diet is also thought to be a contributing factor...ours are given ample hay, pasture and minerals but we still would not leave a tree un protected in the paddock! Not all horses will do it, but the risk is always there, particularly if several are sharing the paddock and one starts on the trees...the others will join in copying the habit. Too many trees are lost to lack of management.
Even new fence rails, painted with a black wood perservative is not a deterrent....we have had them chewed down...but in yards only....which brings back the boredom issue I guess.
Some people wrap wire netting around the trees to keep horses off, this will be effective so long as it is adjusted as the tree mature to remain effective.
Sheryl
highlandparkshowhorses.com