Some really good replies here. My new mare wasn’t in good shape when I got her and she chewed wood. I thought when she got used to her new environment and got accustomed to plenty of hay it would stop. It didn’t. I found 3 things in her case that caused her to do this. With the resolution of each problem the habit decreased until it disappeared. First, she needed her last baby cap pulled. Second, she had ulcers. Third, pain when she came into heat. Spraying the wood works while the detective work is being done but hopefully you can track down the reason behind it.