Do you have her in a large paddock? Is she by herself? I would guess that she is doing it out of boredom. There are products that you can buy that are deterrents for wood chewers. I think they are a form of pepper spray that you put on the wood and when a horse starts to chew, they get their mouth burned. You might look into something like that. Or, if she plays with things, maybe get her a couple of toys to keep her mind occupied.