There are many causes for a horse to chew wood. These include everything for bordom from being stalled for too long to vitamine/mineral needs,
The first thing I would suggest, is evaluate your "routine". How often do you get the horse out of the stall per day and do something with it. Simply letting it graze is not enough. Horses, need action. They are natural wild animals and their body chemistry needs lots of movement and exercise. If I horse does not get this, it becomes bored and creates bad habits as a way of coping.
The second is to have a complete vet exam of the food percentages. This will indicate if the horse is getting enough of the proper vitamins and minerals. I hope this helps.

I don't have an answer as to why a horse does that. I think they do just 'cuz they can. My equines are in open pasture with shelters available. In the pen area I have sheathing affixed to the metal pen frames to act as a windbreak. The horses and the donks will chew on the 2 x 4s I use in the shelters and on the particle board sheathing in the pens. They have mineral blocks and sulfur blocks available, so I don't see where it's a mineral deficiency. They have free choice hay available in the winter, as well as pasture. They get grain every other day. In summary, I get back to the answer, It's just 'cuz they can... Years ago I had a parrot. The parrot used to chew thru his perch about every third day. That got to be quite annoying. I mentioned it to another parrot owner and he told me they do it for exercize. He advised me to affix a hunk of 2x4 at each end of the perch (which was a 1.25" dowell rod). Sure enough, after I attached the 2x4s he stopped chewing on the perch. I had to change out the pieces of 2x4 occasionally, but that was easier than replacing the perch. Sometimes I think it's the same with equines.

Hey, LA Cowgirl.

Hope your Christmas was merry for you and yours. Try using Bitter Apple which can be bought at most tack shops and feed stores. If not, try using cayenne pepper with water and paint in on any boards within reach. Boredom can be a cause. If you horses need to be stall kept for most of their days, try hanging a jolly ball horse toy or a plastic gallon water/milk jug.

Keep us posted.

This is a very common problem that as been addressed by many. It goes way back.

I believe with some horses it is simply a matter of being bored. Some times a toy of some sort hanging from the ceiling will help. I had one mare that played with a teather ball. It stopped her chewing the boards.

The idea of a spicy mix spread on the boards will work with many.

Other times it may be something lacking in their diet. Maybe place a small protein lick in the feeders.

And, finally, I have seen some horses that no matter what they do, the horse still eats down the stall.

Good luck.

If you change the boards don't use woanized or treated lumber. They treat them with a petrolium based solution that could make an animal real sick.

I try to feed my animals as low as I can. Since they are grazeing animals they were ment to eat with their heads down. When they eat by lifting their heads all the hay dust and seeds has a tendancy to get in their eyes and cause problems.

I also get the coursest hay I can find. If it ain't mouldy I feed feeder hay when I can get it. We have a mule that is 30YO and never had to have his teeth floated. I lay it to feeding good course ruffage.

I keep some rough hay hanging in front of them at all times.

gives them something to do and chew on besides my facilites.

Now I buy bottom bale that most horse folk would not touch with a ten foot pole. Have been buying them for years for the cattle.

when i got the equines I started them on the same hay and they have done well.

I use hay nets and hang it high and the little bit of gunk from being on the ground falls into the stall as they pull on the bales.

since I switched to hay nets and keep hay up my chewing problems have gone away.

Go down to your local supermarket and visit the spice rack.Get you a bag of bulk chillie powder. Mix it with water to a thick past and paint it on their favorite chew spots with a paint brush. It will flat change their minds about chewing.