Horses eating bark of trees
I have two horses; the first on a Morgan/Arabian 7yrs. and the other a Quarter Horse 4yrs. The problem with them is that they are eating the bark off of all the young trees in their pasture (most under 15" in diameter). This is the first time they have ever done this, it has never occured in previous years, they also do not chew on the wood in their stalls at all.
There could be a number of reasons for this behaviour. One reason horses often start chewing wood is a lack of fibre in the diet so you may want to add a few biscuits of hay to their daily feed intake. This may also keep them entertained for a bit of time and take their minds of tree eating. The young trees could also taste nice, sweet and juicy and if this is the case then you will have a hard time stopping them and will have to wrap protection around the trunks or fence them off. Because they are not eating any other forms of wood then this may be the case. Some references do say that a lack of copper can cause wood chewing and you can always add some Rosehip tea to the feed as a safe form of providing this. The Rosehip tea will also provide Vitamin C, iron, biotin and cobalt and is an excellent circulatory and kidney tonic. There is also an article attached about DIY supplements and natural sources of vitamins and minerals which may be of interest and information. Include some kelp (seaweed meal) for them to cover all other necessary trace elements and minerals or perhaps a good mineral block for them to access. If you think it might be boredom and they are the playful types then provide a few safe toys in the paddock for them, such as the playballs they now make.