Your Horse and B Vitamins

excessive amounts of the weeds marestail horseweed bracken fern or Queen Annes Lace and an excellent source of it is brewers yeast.
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
Folate and the pregnant mare 
Also known as folate when in its naturally occurring form vitamin B9 folic acid is an important B vitamin. Folates 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.