Apple cider vinegar. Horse often will chew wood if they are low in potassium because the wood contained potassium.
 Try adding ACV to the horses water. Start off with a ratio of about 1 cup to every 50 gallons, but you can bump it all the way up to 1 cup to every 6 gallons. Apple cider vinegar is an excellent source of easily absorbable potassium and other trace minerals.