Horses have a metabolic requirement for all vitamins and minerals recognized. If the adult horse receives a diet of cereal grain and forage quality is high, an adequate amount of ascorbic acid is synthesized in the network and sufficient quantities of vitamins are water-soluble other (apart from biotin and maybe thiamine) and vitamin K, which is synthesized by intestinal microflora and absorbed. You can get to know about racehorse supplements through an online search. 

Thus, dietary requirements are for vitamins A, D, and E, biotin and thiamine possible.

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Cereals and fodder for the horses must be produced under conditions of good breeding, and other microbes harvested without damage, stored deep and aged no more than 2-3 years.

The young foal in need of food sources of cyanocobalamin (B12), usually obtained from their dam's milk, and early-weaned foals should be given additional resources from all vitamin B. Vitamin A supplementation is required for all horses if forages contain adequate amounts of carotene.

The horse had to change the mix of grass and clover carotene to vitamin A relatively inefficient (about 40 g of carotene vitamin A produced). Vitamin D2 or D3 supplements will be needed if the forage has been artificially dried, or if the horse is stored for a long time.

Outside the temperate latitudes, or in high temperate latitudes, vitamin D supplements are necessary.