Eating the Fence?

At the barn I volunteer at, the pasture fences are made out of wood (old wood, not rotting but kind of weak). When I go to bring horses in, I notice that the fence has been chewed on before. At first I thought the horse that cribs in his stall must be doing it out there, too. However, the other day, I watched two horses literally bite little splintery pieces off the fence and eat them. If this gives any clue, I've only seen them do it when the fence is wet, but the fence is almost never dry because of rain.
Is this dangerous behavior, or is it normal and I've just never seen it before? Could it be caused by stress? Poor diet? I'm worried that they might ingest some splinters or something and hurt themselves, and all of the pastures with vinyl-type fences are flooded. I haven't asked anyone else about it and I won't be back until after the holidays. What should I do once I get back? Thanks!

Wood chewing isn't cribbing. Cribbing is when a horse bites down on a hard surface a sucks in air. My horses will wood chew from time to time. Some say it is boredom, lack of forage, mineral deficiency....personally, the more room to roam and graze the better off they are mentally and physically. But those aren't always options so, make sure their diet is balanced and they have access to enough hay/forage.

I agree with what goneriding has said. If limited space is all that is available for these horses perhaps a slow hay feeder system would help keep them occupied longer and away from the fences. Also, I've always suspected that some woods have a taste factor to them (it's either in the wood itself or from the sap that the wood produced) and depending on what's happening with the weather, etc., the "flavour" is heightened and thus becomes of interest to the horses. I don't know what to say about the splinter possibility. I would hope the horses have the sense to spit them out or else grind and mash them with their teeth to the point they're not troublesome.

My pony started eating wood, boredom caused it and frustration of being penned up. I put up a strand of hot wire, problem solved. I do not need him destroying the property when he can find other things to do other than eating all the time! [he's fat...all he does is eat, it's annoying]

If you don't want the fence to be eaten, add hot wire or see if they will move him to where there is fencing made from something else.
Also give him more to do out there.

Some horses just chew. My Arab colt is on 20+ acres of grass during the day, yet he still chews the fencing. The BO puts a metal muzzle type thing on his halter when he goes out that allows him to graze but not chew the fences. It's similar to this:

I think it boredom my horses could chew down a fence line in one night. We were ready to kill them all one winter. Started feeding hay free choice and put out mineral blocks and now the fence eating has stopped.

Thanks for the suggestions! I will try them! :)

Most horses the eat and/or chew wood are missing enough Calcium (Ca) in their diets. They are usually eating grass or grass hay and not much alfalfa -- if any.

All grains and most grasses have way too little Ca in them to balance out the Phosphorus (P) in their diet.

We feed a loose mineral that has 4-5 times more Ca in it than P. It is also 25% salt and has 2% Magnesium (Mg) in it. Horses that come in new usually eat it like candy for a week or two. Horses that have previously devoured trees and every wood fence they got near, all quit when they filled up on the high Ca mineral. Horses that need Ca will eat the bark off of older trees, eat young trees up completely and will eat any wood they can chew. They will eat paint and will lick dirt. All these things are called 'pica' and feeding a mineral free choice that they need will stop it.