Under optimal conditions the horse's body can produce enough vitamin C and D for maintenance, but there are caveats. During times of increased stress and possibly old age vitamin C supplementation may help your horse cope with oxidative stress especially if the quality of the forage is low. Grass, legumes, and properly cured forages all are sources of supplemental vitamin C.

The production of vitamin D is dependent on regular sunlight exposure and many athletic horses are confined indoors for a majority of the day and at increased need for the bone remodeling properties of this vitamin. Though vitamin D deficiency is rarely reported in the horse there may be a benefit of supplementing the horse that receives little sunlight during the day. Vitamin D toxicity from over supplementation is more common than deficiency. It should be noted that horses can store substantial amounts of vitamin D in the liver so short periods of no sunlight are not a problem.

Green forages are good sources of vitamin B1(thiamine), B2 (riboflavin), folacin and vitamin K. The gut flora not affected by antimicrobial drugs produce much of the B vitamin and vitamin K requirements of the horse. Between the vitamins produced in the bowel and those in good quality feed stuffs, symptoms of vitamin B or K deficiencies are unreported in the horse not on antimicrobials and on good quality forages. In fact documented deficiency of these vitamins is rare at any time. Long term feeding of poor quality forages has resulted in thiamine (B1) deficiencies.

Vitamins A and E are supplied in adequate amounts by a diet of green grass or good quality hay. Just as with vitamin D, A can be stored in substantial amounts in the liver so short term deficiency in the diet is not a problem. Deficiencies of either of these vitamins will cause poor doing or disease. Supplementation should be done anytime a horse is required to consume poor quality forages for a extended period.