I posted something about this last winter
I spent a good part of the summer cutting down dead trees that were killed by my horses eating the bark off. Mine have good hay, mineral and salt blocks they are all happy and healthy plus plenty of room to roam around. I finally wrapped the remaining trees in wire fence material. Chicken wire will work fine. Next time plant evergreens, they don't seem to be interested in the pines or cedars, I notice they don't eat a mulberry tree bark either. I think its just certain trees.
Do it sooner than later. The more bark they strip off the more likely you will lose the tree.