B vitamins are necessary for the metabolism of carbohydrates, proteins, fats, and nucleic acids. The bran and germ from cereal grains (such as wheat and rice) and brewer’s yeast are excellent sources of B vitamins. B vitamins (except B12) are found in good-quality forages and are synthesized by bacteria in the equine hindgut. Vitamin B12 is synthesized in adequate amounts when there is sufficient cobalt in the diet.
Dietary B vitamin supplementation might be beneficial in very young, very old, and hard-working horses as well as horses fed high-grain rations. All of these horses might have an altered population of bacteria in the hindgut.
Minimum requirements for thiamine (vitamin B1) of 3 mg/kg diet dry matter for most horses and 5 mg/kg diet dry matter for working horses have been established by the NRC. That publication also lists a minimum value of 2 mg/kg diet dry matter for riboflavin (vitamin B2). Optimum levels for supplementation have not been established, but toxicity of B vitamins is very unlikely.