Written By Dr. Kris Hiney

This month we will conclude our discussion of vitamins with the B vitamins. There are many vitamins that are traditionally referred to as the B vitamins, including thiamin, riboflavin, niacin, pyridoxal phosphate, panthothenic acid and cobalamin.  You may even associate them with their “numbers” so to speak: B1, B2 etc. These are all water soluble vitamins which can be synthesized by the microbial population of the hindgut of the horse.  In many circumstances this microbial synthesis of vitamins is adequate to support normal physiological functions in the horse.  However, under some conditions, supplementation of these vitamins becomes necessary. Unfortunately relatively little is actually known about the true requirements of the horse for many of these vitamins.  We will primarily focus on the vitamins which have the most information available; thiamin, niacin, riboflavin and biotin.

We will begin our discussion of the significant B vitamins with thiamin, one of the most commonly supplemented B vitamins.  Thiamin is a vitamin which is required in many reactions which support energy metabolism, or the production of ATP.  Deficiencies of thiamin in the horse can result in muscle fasciculation, ataxia and most frequently in appetence.   However, true thiamin deficiencies in horses are very infrequent.  Nevertheless,   it is often supplemented when horses go off feed to restore their appetite.  There is some evidence that the exercising horse may require more thiamin, which is presumably related to their higher rate of metabolism.  Dietary sources of thiamin are typically found in the concentrate portion of a horse’s diet.  Cereal grains, their by-products, and  brewer’s yeast are especially high in thiamin.   Overall, maintenance horses are currently recommend to consume 3 mg thiamin/kg of DM consumed while exercising horses should consume 5 mg of thiamin/kg of DM.  If we use a standard 500 kg horse as an example, and assume it is consuming 2% of its body weight in dry matter (or 10 kg of feed), this horse should consume between 30-50 mg of thiamin per day.

Riboflavin, historically referred to a B2, is another vitamin which is required in energy producing pathways, especially in the electron transport chain.  Riboflavin also functions in lipid metabolism and as an anti-oxidant.  Riboflavin, like thiamin, is synthesized in the hindgut of the horse through microbial fermentation.  Interestingly, no documented cases of riboflavin deficiencies have been reported in the equine.  Legumes are relatively high in riboflavin, so horses consuming alfalfas or clovers should have little difficulty in meeting their riboflavin requirements.   Even horses consuming grass sources of forages easily meet their riboflavin requirement.  The current recommendation of horses is to consume 2 mg of riboflavin per kg of DM, but even grasses contain 7-10 mg of riboflavin/kg of DM. Therefore there appears to be little reason to supplement horses with riboflavin.

Niacin, traditionally referred to as B3, participates heavily in oxidation/reduction reactions in the body which are vital to energy metabolism. Niacin can not only be produced in the hindgut, but it can further be synthesized by the horse through the conversion of tryptophan to niacin within the liver.  Like riboflavin, niacin deficiency has also not been described in the horse.  Currently, there is not even a recommended dietary intake for horses for niacin.

Biotin is a water soluble vitamin which is a co-factor in many carboxylation reactions (addition of carbon to a compound).  These are important reactions in gluconeogenesis (the synthesis of glucose by the body) and fatty acid synthesis.  Of traditional horse feeds, alfalfa supplies the highest concentration.   Once again, the microbial microflora are also quite capable of synthesizing biotin. While no distinct deficiencies of biotin have been reported, low quality hooves are often associated with low biotin.  Supplementation of biotin in the range of 15-20 mg day has been reported to improve hoof wall integrity, structure and strength.  However, when supplementing biotin, horse owners must realize that significant effects do take quite some time to be realized.  The shortest time period of supplementation which achieved positive effects on hoof growth and hardness was 5 months, with some studies reporting a need to supplement for over a year.

Finally, there are certainly many other vitamins that may be of interest to the horse owner, such as folate, lipoic acid, cobalamin etc.  We do know that synthesis of cobalamin, or B12 does require the mineral cobalt to be incorporated.  However, horses appear to be quite capable of doing so and do not appear to need any supplementation.  In fact, horses can graze cobalt deficient pastures with no ill effect where ruminants would die from deficiency diseases.  Currently there is a paucity of information available to guide the horse owner in best practices concerning many of these other vitamins. Perhaps someday we will know more about these important vitamins and can make better recommendations for dietary values to enhance the health status of the horse.  Until then, just be thankful your horse has its gut bugs, he couldn’t do it without them!