During the late winter and early spring months, horses may be seen eating the bark of trees. This may be due to a nutrient deficiency, possibly of one of the minerals. If you have been providing a mineral block, check to see if it has been consumed and needs to be replaced. If not, you might try providing one to see if it provides the missing nutrients.
Wood chewing can also occur during this time due to the low fiber content of the cool season grasses that are available to them. Therefore, by eating the tree bark, they may be making up for some of the lack of bulk or fiber. It has been reported in controlled research that horses increase wood chewing during wet and cold weather. The wetness softens the wood and the coldness increases the maintenance energy requirements to maintain normal body temperature.
Fiber digestion is essentially a fermentation that produces heat which can be used to maintain body temperature or warmth. Therefore, the increased fiber intake from wood may actually increase fermentation and heat production in the horse to maintain normal body temperature. If the horse is in need of bulk or fiber, try providing a good quality hay.
However, in most cases, it seems that the horse likes the taste of bark. A lot of trees produce sap that the horse may find sweet. In this case, you might try putting something on the tree that is harmless to the horse but that he will find the taste of unpleasant.
Bark consumption is generally not harmful to the horse. Splinters from fences, however, may cause potential gastric complications that may cause a horse to colic.