Some research indicates that tail chewing can be related to a lack of certain minerals. You might try a commercial mineral supplement and add it according to the label to his feed. However, if you are feeding a commercially prepared ration, you most likely don't have a mineral imbalance and would not need a mineral supplement. There is a commercial product called Stop Chew that you can purchase and apply to the other horses' tails to prevent him from chewing on them. You can also sprinkle cayenne pepper on the other horses' tails to prevent him from chewing on them.
