Mare Chewing On Fences

Nelly has started to chew on the cedar fence posts in her paddock and I've noticed that it's wearing down her front teeth. I'm doubtful that it's because she's hungry, we've been round bales all winter so all three horses have access to hay. Is it possible that because she's off the track that there are supplements/minerals that she's missing?

I doubt that her being off the track is the cause. Any horse can be low on vitamins or minerals. Yes, it is entirely likely this is the problem and can be solved by providing loose minerals. They'll eat what they need, most of the time.

It can also be boredom. My mare chews fencing when she's bored.

Is she sucking air when she does it? Sounds like a cribber to me... I've known many ottbs that crib, did her previous owners mention any old habits/vices?

She's biting it off and chewing on it. I rescued her from slaughter, so I don't know much about her past habits.