When your horses are eating fence posts , it is probably because they like the taste. They do not lack nutrients.It is a natural part of equine behavior to eat wood, but of course it's not nice or very secure with "half" fence posts. You can lubricate them with Cribox, antler oil or similar anti-bite products.Horses in a paddock need environmental enrichment for example. sand piles where they can roll, a shift between the fold, fender hung in trees, balls or treesRemember incidentally that a fold might be too small but not too big!
Horses in the wild will strip bark off trees, they know what to eat naturally for pain or anti inflamitories.... horses would eat natural garlic and berries from the hedgerows... It is because we have taken horses out of their natural enviroments, but they will instictively find what they need if they were set loose in a forest....
they normally eat bark cos they are lacking vits,and sometimes it because they get bored.silver birch is pain killer bark.
The best way to avoid eating fence by horses is to have plastic fence, when you choose one of them make sure it is the safest electric fence www.equisafe.co.uk/offer.html