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 pray the gentle hands may guide my feet;I ask for kind commands from voices sweet; At night a stable warm with scented hay, where, safe from every harm, I'll sleep till day.
- Pony's prayer