Although plenty of equine vitamin supplements are available, vitamin supplementation is an individual decision based on the horse's health, where the horse lives, its activity level and the kind of diet the horse is fed.
Usually suupplemental vitamins are not necessary unless a low quality forage is being fed, the horse is involved in strenuous exercise, the horse is ill or recuperating from surgery, or is being fed a high grain diet.
During fall and winter months, when days are shorter and horses don't get as much sunlight, some horses may need supplemental vitamin D. Vitamin D is obtained from sunlight, so only horses that are kept in their stalls 24 hours a day may need supplemental vitamin D.
Other vitamins such as vitamin E which is found in fresh green forage and vitamin C which is found mainly in fresh vegetables and fruits and is produced in the liver of the horse may need supplementation depending on the advice of a veterinarian familiar with the horse.
Vitamin K and B-complex vitamins are produced by microbes in the horse's intestines. Severely stressed horses may benefit from B-complex and vitamin C supplements, and performance horses may need additional vitamin E.
According to Merck's Veterinary Manual, supplemental vitamins should be used with caution, especially the fat-soluble vitamins that include vitamin A, D, E, and K. These may reach toxic levels if over ingested because they are stored in the body if not needed immediately.
Water-soluble vitamins include C and the B-complex vitamins and don't usually pose any problems because any excess vitamins are excreted in urine and through sweat.
Descriptions of uncomplicated nutrient deficiencies in horses are rare. The nutrients most likely to be deficient are caloric sources, protein, calcium, phosphorus, copper, sodium chloride, and selenium, depending on age and type of horse and geographic area.
Signs of deficiency are frequently nonspecific, and diagnosis may be complicated by deficiencies of several nutrients simultaneously. The consequences of increased susceptibility to parasitism and bacterial infections may be superimposed over still other clinical signs.
Simple excesses are more common. Nutrients most commonly given in excess of needs, leading to toxicity or induced deficits of other nutrients, are: energy, phosphorus, iron, copper, selenium, and vitamin A.
If your horse has an adequate diet of good forage and spends time in open sunlight, additional vitamin supplementation should be based on veterinary advice, not the claims made in a vitamin catalog.