Horses have a recognized metabolic need for all vitamins and minerals. If an adult horse receives a diet of high-quality cereal grains and forages, adequate amounts of ascorbic acid are synthesized in tissue and sufficient amounts of other water-soluble vitamins (other than biotin and possibly thiamine), as well as vitamin K, are synthesized by intestinal micro-flora and absorbed.
Cereals and forages for horses must be produced in good farming conditions, harvested without microbes and other damage, stored properly and not more than 2-3 years old. To know more about the protein supplements for horses, you can browse the web.
If a horse wants to live on low-quality root vegetables and forages, other vitamins will be needed in food for optimal performance.
Young foals need a food source from cyanocobalamin (B12), usually obtained from their dairy milk, and early weaned foals should be given an additional source of all vitamin B. Vitamin A supplement is needed for all horses if the forage contains an insufficient amount of carotene.
Horses convert carotene mix of grass and clover into vitamin A which is relatively inefficient (about 40 g of carotene produced by vitamin A). Vitamin D2 or D3 supplements will be needed if the forage has been artificially dried, or if the horse is kept for a long time.