cattle proof fencing around trees
 

I've just planted some walnut trees in an open field which will be used to graze cattle. Ther's one 200year old walnut tree there already so I decided to give it some company. I'm wondering if anyone has ideas on how to fence each individual tree so that the cattle won't break the fencing. They're planted a good distance apart so electric fencing is out. They tend to eventually break wooden fencing leaning on it and scratching off it. Someone suggested rail fencing with support stakes but it looks very expensive from what I've priced. Any ideas would be welcome.