The addition of a vitamin and mineral supplement to a horses ration is dependent on the feed being fed. Balanced rations such as those provided by most major feed companies do not need supplements and, in fact, supplements may cause an imbalance. If the ration you are feeding to your pregnant mare is designed for pregnant mares, the additional supplement is not needed. Check your feed tag to see if the ration contains 14-16 protein, has a fiber level of 10% or less, and list vitamins and minerals included. Feeds designed for the pregnant mare will not require supplementation. Have your hay analyzed to ensure that it meets the needs of your mare. Adding a supplement to a balanced ration is not needed but will not usually cause a problem if it is fed according to the manufacturer's recommendation.
