Personally I would never use wood either, but its what the OP has to work with and doesn't want to change it. I was just making suggestions to her.
On the tires, I was referring to an article I had read some time ago about horses eating that belted fencing made from recycled tires. This is from the article from The Horse: Your Guide to Equine Health Care
This is a very forgiving fence, as horses bounce off of it without being hurt. Nevertheless, Harper doesn't like it. "It's very difficult to keep taut, and if you don't keep it taut, horses can crawl through the strands. Also, horses pull and chew on the rubber and the little cords that stick out of it. The result is colic with a number of deaths reported."