Serious wood chewing is always a lack of a mineral. If your horses are eating grass hay, they would be missing Calcium (Ca). If you give them a free choice loose mineral that is at least 20% Ca and is less than 6% Phosphorus (P), you usually fix the imbalance and stop the wood chewing almost overnight. It also stops horses from stripping bark off of trees and killing them, eating dirt and chewing tails. All are symptoms of a Ca deficiency, almost always in horses eating grass hay.
Our loose mineral that we keep out also has Magnesium and Zinc in it as well as 200,000 units of Vitamin A per pound. When we have it out (have fed it for 35 or 40 years) we have had NO cases of rain rot and NO dirt, wood, tree or tail eaters. We have 60 head right now and none of them chew wood -- at all.
If yu think you are feeding minerals to a horse by keeping out a mineral salt block, you are badly mistaken. These block only have 'trace minerals' or 'micro minerals' which are almost never missing.
Macro Minerals -- the ones needed in greater amounts are Ca, P, Magnesium and Zinc. You have to buy them to fit your deficiency and to balance out your forage. Grass and grass hays are very Calcium and Magnesium deficient unless they are grown on limestone soils that are not depleted.