I doubt it's anything missing in their diet - horses will pretty much eat anything within their reach that is appealing in any way. Make sure they are getting enough roughage (i.e. hay) and maybe even give them some toys to alleviate any boredom. If your horses continue to eat bark, you can fence around the trees to prevent further damage to the trees and the horses teeth.
I love my horse from hoof to head
From head to hoof and tail and mane
I love my horse as I have said
From head to hoof and back again
- James Whitcomb Riley