I have a few trees in an area with my horse. The horse has eaten the outer bark of the trees from the ground up to five feet all the way around. I spoke to the vet and she said the horse will be ok. She said the horse is probably just bored.
I talked to a man that used chicken wire around his trees to keep his horses from eating the bark. Is it possible to put a coating of something on the tree to save them and then maybe the wire to prevent damage as the tree repairs itself?
Horses take on an affection for tree bark during the boring late winter where minerals, salt and forage are not sufficient. Your question is one I hear all the time and it commonly occurs through early Spring.
The horses will be ok but the trees may not. If the horse eats into the cambial layer all the way around, the tree will die. Normally, the horse just nibbles the bark layer (bark tastes a bit better than inner bark) and the tree heals and survives but does not thrive.
You need to protect each tree, ideally, out to the dripline or Critical Root Zone. If this cannot be done, wrap the tree with a protection barrier but do not nail into the tree. You can also obtain a chew-repellent spray, such as Bitter Apple. It will provide an unpleasant taste while it does not harm the animal.
Also, if you can, try to let the horses spend less time in the pasture during late winter. You may also want to provide a fortified salt and mineral block in the pasture and see that the horse has good quality hay and grain to eat.