Your Horse and B Vitamins

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Are you providing enough? 

By Shelagh Niblock, PAS

Today’s horse owners are no doubt aware of the importance of vitamins in the diets of their horses, but many may not be aware of the role vitamins play in equine nutrition. Vitamins in their fresh natural form are organic substances found in grains and forages. They are important as cofactors, or facilitators, for different metabolic function, and deficiencies of them can cause disease conditions. Vitamins, unlike many of the nutrients we feed our horses, cannot be broken down for energy, and they provide no other nutrients to the horse. They have widely different structures and are soluble in either fat or water depending on their structure. Vitamins are classified according to their solubility. The fat-soluble vitamins include vitamins A, D, E, and K, and the water-soluble vitamins include the large group of B vitamins and vitamin C (ascorbic acid).

The natural forms of most vitamins are present in plant material or the plant material contains the precursors necessary for the horse to manufacture them, either metabolically themselves or through a mutually beneficial association with gut microbes.

The fat-soluble vitamins are stored primarily in the liver, and for that reason, it is possible to feed the horse more than required. On the other hand, the water-soluble vitamins of the B group and vitamin C are excreted in the urine or manure if fed in higher quantities than required; consequently, toxicities of these vitamins are rare.

Fresh forage is the best source of vitamins

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Fresh forage is the best way to provide your horse with B vitamins. Photo: AdobeStock/Edoma

Fresh pasture is still the best source of vitamins and vitamin precursors for both the fat-soluble and water-soluble vitamin categories. Grains can be a good source of some vitamins as well. Differences in both processing and preserving forages and grains can affect the amount and viability of the vitamins present naturally. Similarly, storing forage as hay will result in a rapid decline of natural vitamins in forages. 

All complete feeds available for horses are supplemented with the fat-soluble vitamins A, D, and E. There is, however, no requirement under Canada’s Feeds Act and Feeds Regulations for horses to include water-soluble vitamins in complete feeds, primarily because it has been difficult to prove under research conditions that there is an improvement in equine health if they are added. B vitamins, even if they are supplemented, cannot be listed on the tag of the feed you buy if the feed is not registered with the Canadian Feed Inspection Agency (CFIA), but manufacturers of high quality equine feeds often do include at least B1 thiamine, B2 riboflavin, and B3 niacin in the formulation of their equine complete feeds. There is no requirement for vitamin C supplementation in equine feed in Canada because horses are capable of manufacturing their own.

Is my horse getting enough vitamin C?

Horses manufacture their own vitamin C in the liver, but because vitamin C is a powerful antioxidant, it has been of interest to researchers as a potentially valuable addition to the diets of performance horses. What research has shown, though, is that it takes a lot of vitamin C (over three grams daily) to actually make a difference in serum levels of ascorbic acid, and as a result, it is difficult to provide it in amounts great enough to make a significant difference to the horse. In addition, there has been some suggestion in research that supplementing performance horses with vitamin C may actually down-regulate their own ability to manufacture it. For this reason, it is advisable to step-down gradually the inclusion of vitamin C in the diet of your horse if you have been supplementing it.

Is my horse getting enough B vitamins?

The group of B vitamins of interest in equine diets includes choline, B1 thiamine, B2 riboflavin, B3 niacin, B5 pantothenic acid, B6 pyroxidine, B7 biotin, B9 folic acid, and B12 cobalamin. All except B12 are provided in fresh forage or by the mutually beneficial metabolic processes of the gut microbes. Cobalamin B12 is manufactured by the microbes in the small and large intestine of the horse only and not found in forage. Horses need a source of cobalt in their diets to facilitate the manufacture of cobalamin by the gut microbial population, while niacin can be manufactured by the horse or by the gut microbes. Sufficient supplies of B vitamins for most horses basically boils down to the provision of good quality forage and healthy hindgut function.

Related: Are You Feeding Yoru Horse Enough Vitamin E?

What role do the B vitamins play?

The B vitamins are diverse in their function, but only two of them — B1 thiamin and B2 riboflavin — have had a minimum dietary intake recommended by the 2007 National Research Council (NRC) Nutrient Requirements of Horses. The rest have been shown to have specific functions and, in some cases, deficiencies and toxicities have been determined in research, but these have been very difficult to reproduce reliably.

Thiamin, riboflavin and niacin 

Vitamin B1 thiamin is significant in carbohydrate metabolism and manufacture of adenosine triphosphate (ATP), an important molecule in the cellular energy cycle in horses. Thiamin is perhaps best known in the equine world as a calming nutrient and is often recommended for horses with anxiety or spookiness. Thiamin deficiency can lead to bradycardia (slower than normal heart rate), anorexia (reduced appetite) and ataxia (instability caused by neurological dysfunction). Thiamin is the suggested treatment for horses that have consumed excessive amounts of the weeds marestail (horseweed), bracken fern, or Queen Anne’s Lace, and an excellent source of it is brewer’s yeast.

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A rapid decline in natural vitamins occurs in forage stored as hay. Photo: AdobeStock/Vprotastchik

Vitamin B2 riboflavin is an important cofactor in energy metabolism. Deficiencies have never been proven in horses, but in other species, deficiency of it has been linked to poor hair coat, dermatitis and eye problems. The best natural feed sources are green leafy forages, like alfalfa and clover, and to a lesser degree, grass forages.

Vitamin B3 niacin is important for energy metabolism and is believed to be important for calcium metabolism as well. Horses can manufacture niacin in the liver, but it is also supplied by the hindgut microbes. A deficiency has never been demonstrated in horses, suggesting that a sufficient supply is provided through a combination of diet, microbial growth, and healthy liver function.

The role of biotin 

Vitamin B7 biotin is essential for cell proliferation and reproduction. Biotin is most often supplemented for improved hoof wall and hair coat quality, and although NRC has not established a minimum biotin intake for horses, it does recommend that horses get at least 10 to 30 mg per day of supplemental biotin. Virtually all of our favourite hoof supplements contain biotin but do make sure when reviewing tags that the product you buy provides at least 10 mg (10,000 μg) daily.

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Horses with demanding training or competition schedules may need supplemental B vitamins. Photo: AdobeStock/Skumer

Folate and the pregnant mare 

Also known as folate when in its naturally occurring form, vitamin B9 folic acid is an important B vitamin. Folate’s role is prominent in the rapid cell growth of tissues, or where rapid turnover of cells is needed. Folate deficiency has not been described in horses, but research has shown that the need for this vitamin increases in horses engaged in intense exercise, or in a state of lactation, growth, or gestation. Folate is provided either by the microbes in the hindgut or from fresh forage, but as many of our horses do not get fresh forage in significant quantities, supplementing this vitamin can be advisable.

Of particular concern are the very significant effects the drug treatments sulfadiazine and pyrimethamine have on folate production in horses. These drugs are used in the treatment of equine protozoal myeloencephalitis (EPM), and they appear to affect the microbial synthesis of folate in horses. If treatments are short, then the horse can recover the ability to produce this important vitamin, but unfortunately, treatment for EPM can be a lengthy process, which can make this a cause for concern. Certainly, this is something that should be discussed with your veterinarian if you do have a horse with EPM, as it seems that simply supplementing your horse with synthetic folic acid is not helpful. In the case of pregnant mares treated for EPM, use of these medications runs the risk of delivering foals with congenital defects.

Other B vitamins  

The B vitamins pantothenic acid B5, pyroxidine B6, choline, and cobalamin B12 are often grouped together as there have never been deficiencies or toxicities reported in horses for any of them. All are important for energy metabolism. Choline is included in the group of B vitamins as it has many similar characteristics and is very important for cell membrane function and the transmission of nerve impulses. Cobalamin B12 is important for the function and maintenance of red blood cells, and is manufactured by microbes in the hindgut from dietary cobalt in horses. Experimental increases in cobalt intake have increased levels of serum B12, but excretion of B12 goes up when horses are supplemented with this vitamin by injection. Because of the potential increase in serum B12 through increased dietary cobalt intake, and the potential effect that might have on red blood cells and resulting athletic performance, there have been discussions at the Canadian Para Mutual Association — the Canadian agency that regulates doping in performance horses — about regulating it in performance horse diets.

Related: The How and Why of Soaking Hay

Does a mature, healthy horse eating quality forage need supplementation?

For most horses, there is little reason to supplement B vitamins; however, because not all complete feeds are actually supplemented with B vitamins, there may be a place to augment them in the diet of your horse. B vitamins appear to be well supplied by the combined sources of feedstuff and microbial production, but there is evidence that quantities may be compromised in the following categories:

  • Horses under stress due to illness or otherwise, with anorexia or reduced appetite;
  • Horses in intense training or undergoing a rigorous show schedule;
  • Horses who have undergone antibiotic treatment that might have reduced microbial populations in the hindgut;
  • Horses on high grain/low forage diets or poor forage diets;
  • Late gestation mares with compromised hindgut capacity.

If you have a performance horse, a senior horse, a growing horse, or a horse that fits into any of the above categories, then supplementation is probably advisable.

Feeding B vitamin supplements

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A rapid decline in natural vitamins occurs in forage stored as hay. Photo: AdobeStock/Vprotastchik

Likely, the easiest way to ensure your horse has sufficient B vitamins is to choose, as much as you can, leafy green forages. Brewer’s yeast is an excellent source of B vitamins and is palatable and readily available. If you do choose to supplement with purchased products, check with an equine nutritionist for help in assessing what you might want to include in your feeding regimen.

If you are feeding a complete feed, check the tag for the presence of B vitamins, and if they aren’t listed, contact the manufacturer or check the ingredient statement. Remember that unless a feed is registered with CFIA, it will not have the B vitamins listed on the guaranteed analysis. As well, you should be cautious if mixing B vitamins, as thiamin B1 is not compatible with riboflavin B2, and neither are compatible with cobalamin B12. Commercially made mixes of these vitamins will have each vitamin in coated form so as to avoid chemical interactions between them.


If your horse is mature, engaged in light work, and getting good quality forage including fresh pasture, then supplementing B vitamins may not be needed, but it may be advisable for performance horses, growing horses, and seniors. Supplementing B vitamins will not necessarily help your horse win the race or jump the jump, but it can help ensure overall health and an ability to withstand the rigours of a competitive life. Check the label of your feed bag, and if in doubt about B vitamin content, contact the manufacturer. Remember that CFIA tagging regulations prohibit the listing of B vitamins on the tag unless the feed is a registered product. As well, B vitamins are water-soluble, which means that if they are fed in excess of requirements, they are readily excreted from your horse’s body. For this reason, B vitamin toxicity is very rare and supplementing as part of a management plan for your horse is a safe and effective practice.

Main Photo: Shutterstock/Rolf Dannenberg