If a horse is fed a balanced ration at the desired level for the activity of the horse, vitamin supplementation is not required. Vitamin supplementation may be required only in very stressful situations or when feed quality is poor.

Below is additional information on vitamins in general.

A vitamin is an organic compound that is needed in small quantities to help run the many chemical reactions that occur in the body. Vitamins have been one of the most oversold, misfed feedstuffs. A little is necessary, but a lot of vitamins do more harm than good. In truth, under normal feeding programs with good-quality feeds, horses do not need extra vitamins added to their ration.

However, if the feedstuffs are of low quality or the horse is under stress, vitamins may be added as follows: for vitamins A, D, E, and K, add five pounds of a vitamin premix per ton of mixed feed. (If you are using a commercial feed, it probably already has the vitamins added). If B vitamins are desired, add five pounds of brewers dried yeast per ton of feed.

Vitamin A – Beta-carotene
Vitamin A is common in green forages in the form of beta-carotene, and the body easily changes beta-carotene to vitamin A. Vitamin A is necessary for the health of the cells that line all tissues of the body: skin, respiratory system, eyes, reproductive organs, the alimentary canal, etc. Deficiency may result in poor hair coats, reproductive and respiratory tract infections, night blindness and/or excessive tearing, and diarrhea.

The horse can store excess vitamin A in the liver and is capable of storing enough to last four to six months. Forages made into hay contain vitamin A, but it dissipates quickly. After six months of storage, hay has very little vitamin A. Horses fed poor-quality hay or hay stored for more than six months, especially if the horse also received no green forage, would need supplementation. Exceeding the needs will be of no benefit, and levels 10 times the normal level will cause toxicity.

Vitamin D
Vitamin D is called the sunshine vitamin because the ultraviolet rays of the sun will convert a compound (7-dehydrocholesterol) in the skin into vitamin D. Horses exposed to four to six hours of outdoor light (even on cloudy days) will make sufficient vitamin D. Also, sun-cured forages contain the vitamin. If a horse is not exposed to the sun or is fed poor-quality hay, a deficiency could occur. However, levels 10 times the normal level will be toxic, causing calcium deposition in soft tissues, which can damage muscle, blood vessels, kidneys, and the heart.

Vitamin K
This vitamin is highly supplied in all forages, green or dried. It is also produced by the microbes in the cecum and colon and absorbed there. Vitamin K is necessary for normal blood coagulation. A deficiency is recognized by failure of the blood to clot when a cut or injury occurs.

Vitamin E
Vitamin E deficiency has not been reported in horses, and an ample supply is found in the normal feedstuffs of the horse. Vitamin E has been linked with selenium in normal muscle function and has been given in treating "tying up" syndrome. Because of the benefit of vitamin E to muscle function, feeds for performance horses often have increased levels of vitamin E. This would be most beneficial to horses working several hours a day. The vitamin has also been given to improve reproductive performance, but there is no evidence that this helps in horses. Use of the vitamin to improve the horse’s immunity to infectious disease has not been proven effective, either.

B Vitamins
These vitamins are available in the normal feedstuffs and are made by the microbes of the cecum and colon. As a result, only horses on low roughage diets and/or under severe stress are likely to become deficient. Deficiency results in poor appetite, sour attitude, and anemia. Vitamin B12 has commonly been sold to horsemen to improve the horse’s performance, but research has shown that B12 supplementation is not needed by the horse.

If a horse is under a lot of stress, such as a foal out in terrible weather or an endurance trail horse that is being worked very hard, it may be beneficial to add B vitamins to the ration. Brewers dried yeast is an excellent source and can be added to the ration at the rate of five pounds per ton of feed to meet the horse’s needs.

Vitamin C
This vitamin is not needed in the horse’s diet because sufficient amounts are synthesized in the liver. Many claims for vitamin C have been made but none substantiated.