Maybe you could try putting a vit / mineral supplement in the field like Horslyx original. I've always been led to believe chewing wood is a lack of something in their diet.
Ditton the vit & min supplement. Wood chewing is usually a sign that they need a supplement. You could put it in the field in the form of a block which I find works well.
My mare is geting hard feed and benevit as well as ad lib hay. She still embarrases me by chewing the wood on my friend's new stables when I go round to visit her though!!
Wild horses make up quite a proportion of their diet from chewing wood. Some people put logs in the stable/field for their horses. Just make sure it's not poisonus like yew.