Around here they chew wood on the outside of the barn after rain or snow storms when the wood is wet. I can usually predict when they will chew and spray liberally with Chew Stop (the stuff in the dark blue spray cans/sprayers). Nothing else works for our guys. Since ours only do it under certain conditions - and I think boredom plays a part - and most all do it, I don't think it indicates any huge problem with their diets, but in some cases it could. Chewing wood is not good for the barn OR the horses, so you need to find a way to stop it. I have also used wire netting or thin rubber matting in the stalls to cover especially easy to chew spots. Metal (i.e. angle iron) is very effective too. Slow hay feeders or more frequent feeding help too.