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To solve the first possible cause it's advisable that you provide suitable quantities of feed and roughage (hay). In addition if your horse is stabled often or cannot go out to pasture for most of the day, consider adding a salt or mineral lick in his stall. Not only will this help ensure your horse takes in the necessary salts and minerals, it will provide him some entertainment for those times where there is no hay available.
The solution to boredom is about the same as it is for potential mineral deficiencies. A happy horse is a horse that can eat throughout the day since unlike humans they weren't built to thrive on three large meals. Horses have small stomachs for their size, and as such they are grazing animals that prefer eating many smaller meals many times throughout the day. Salt licks and toys can also alleviate a horse's boredom when there is no hay to be had.
Wood chewing is detrimental to your horse because it can cause splinters to become imbedded between his teeth or in his gums, but even worse splinters can get swallowed and cause havoc within his stomach and intestines, potentially leading to an increased chance of colic.
If you notice that your horse has picked up this vice you can try the following remedies:
First make sure that your horse isn't chewing for a nutritional or food need. They should have plenty of roughage (hay or grass) in their diet, but if their current diet seems to be lacking you can also consider vitamin or mineral supplements to be mixed with their pellets or sweet feed. Salt licks are perfect because they serve as both a supplement and a source of distraction/entertainment.
Provide Toys And Distractions
Boredom is a primary cause for vices, so if your horse did not contain a few toys such as a Jolly Apple in his stall consider adding some. Often these distractions will keep your horse's idle mind busy, thereby stopping the urge to chew wood.
Since wood chewing is not the same as cribbing, metal strips along the wooden edges of doors and stall windows will stop a horse from chewing.
If your horse is chewing on a fence in the paddock or field just add some electric fencing along the top. This will prevent your horse from chewing on those surfaces.
Chew Stop And Related Products
Chew Stop (and similar products) are non-toxic sprays and paints that can be applied to wood, thereby giving the wood a foul taste and scent that will turn off horses. These sprays must be re-applied approximately once a month inside the stable and once every two to three weeks outdoors depending on the harshness of the elements. Chew Stop has the advantage of being usable on awkward portions of wood that can't be protected with an electric fence or metal strips (i.e., a tree).
Horse wood chewing is not a terribly worrisome vice, but it is important that you take steps to cure it immediately when you detect it. Also ensure your horse is indeed a wood chewer and not a cribber, because cribbing is an entirely different problem that is far more difficult to combat.