Vitamin K for Horses

What is vitamin K?

The name ‘Vitamin K’ refers to a group of fat soluble vitamins that include:

Vitamin K₁ (phylloquinone): found in fresh green plants
Vitamin K₂ (menaquinone): produced by bacteria. There are 15 different menaquinones
Vitamin K₃ (menadione): synthetic vitamin K

What does vitamin K do?

Vitamin K is required for many functions in the body. The most important of these are:

Activation of blood clotting mechanisms
Maintaining vascular health; and
Healthy bone metabolism

How much vitamin K does a horse need?

The true requirement for vitamin K in horses is still unknown. Primary vitamin K deficiency (i.e. nutritional deficiency not caused by a vitamin K antagonist) has never been recorded in a horse.

Published information suggests that the absorption of vitamin K from fresh forages and absorption of vitamin K produced by bacteria in the gut is enough to meet requirement. However, recent studies have called into question the ability of a horse to absorb bacterial derived vitamin K₂ from the hindgut¹. Still, there are many bacteria in a horse’s small intestine and vitamin K is definitely able to be absorbed from there.

However, horses in stable conditions without access to fresh green forage may not receive optimal amounts of vitamin K in their diets. In these situations, some supplementation may be required.

What form of vitamin K is best?

There has been some debate in recent times regarding which form of vitamin K is best to supplement with. In horses, it appears that most forms of vitamin K are suitable for supplementation.

A study in horses published in the Journal of Animal Science (Terachi et al 2011)² found that supplementation with Vitamin K₃(the form most commonly used in horse feeds) was the most effective at increasing blood concentrations of the biologically active menaquinone, MK-4.

A more recent study³ has suggested that vitamin K3 is not converted to MK-4. However this study used a single dose of vitamin K₃, with blood samples only taken for 8 hours post-dosing. This is in contrast to the Terachi et al. study which supplemented horses for 7 days and found the highest levels of MK-4 in the vitamin K₃ supplemented horses. It is likely that in the study of Skinner et al, the single dose of Vitamin K3 and the short time-frame for blood sampling may not have allowed enough substrate or time for conversion to MK4 to occur.

Vitamin K₃ has been demonstrated to be converted to biologically active MK-4 in multiple animal species and it would appear horses are no exception.

Skinner JE, Cawdell-Smith AJ, Regtop HL, et al. 59 Extent of vitamin K absorption from the equine hindgut. Journal of Equine Veterinary Science;35:409.
Terachi T, Inoue Y, Ashihara N, et al. Plasma vitamin K concentration in horses supplemented with several vitamin K homologs. J Anim Sci 2011;89:1056-1061.
Skinner JE, Cawdell-Smith AJ, Biffen JR, et al. 11 Intestinal absorption of different vitamin K compounds in the horse. Journal of Equine Veterinary Science;35:387.

Did you know that many horses have vitamin or mineral deficiencies and don’t show any symptoms until the damage is serious? Learn about how to tell if your horse has a deficiency here: What are the Symptoms of Mineral Deficiency in Horses? (feedxl.com)

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