Feed supplements can be very helpful, but it's not wise to add them to your horse's ration without first consulting your veterinarian or an equine nutritionist. The reason is that by indiscriminately adding nutrients to your horse's diet your may inadvertently cause imbalances.
Most horses in normal activities don't need supplemental vitamins, but there are two exceptions:
Selenium, an important trace mineral that acts as an antioxidant, can occur in levels that are too low in hays grown in certain areas of the country, particularly the Pacific Northwest, New England, the upper Midwest, and the East Coast down to Florida. Too much selenium in the diet can be toxic, but cases of selenium poisoning are very rare since horses find selenium-rich foods to be unpalatable. Selenium deficiency in the diet is also fairly rare, but if you suspect a problem, you may want to consider having a sample of your hay analyzed.
Vitamin E is another antioxidant that most horses get in ample supply in their normal feed. But if you've increased your performance horse's fat intake to keep him powered up, for example, you may need to add a little more vitamin E as well. The reason is that antioxidants bind with free radicals, the damaging, oxygen-related substances that are generated as a result of fat digestion. But megadosing is not better. Some people tend to go overboard and give as much as 5,000 IUs per day, but these amounts can interfere with the absorption of other nutrients. Experts recommend 500 to 2,000 IUs per day for a horse on a high-fat diet who's involved in hard work.