Vitamin K in Horse Diets

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The following information from Kentucky Equine Research's Kathleen Crandell, PhD, discusses the importance of Vitamin K in your horse's diet.

The major function of vitamin K is associated with blood coagulation. In fact, the Danish discoverer of the vitamin named it after its function “koagulation” (Danish spelling). The vitamin is required for the activation of the four plasma clotting factors. Recently, vitamin K has also been found to have a role in the activation of a number of other proteins throughout the body, some specifically identified in the skin and bone.

Vitamin K1 (phylloquinone) is the compound found in green plants. It is absorbed in the proximal small intestine by an energy-requiring process. Phylloquinone also appears to be the form that is stored for a limited amount of time (less than 24 hours) in the liver. Vitamin K2 (menaquinone) is the compound synthesized by intestinal microbes, being absorbed from the small intestine by a passive, noncarrier-mediated process. Vitamin K3 (menadione) is a synthetic product and appears to be absorbed from both the colon and small intestine by passive processes.

It is generally assumed that vitamin K is synthesized by microorganisms of the cecum and colon in sufficient quantities to meet requirements. Coprophagy (eating feces) will reintroduce highly available synthesized vitamin K to the intestines. This practice has made defining absolute requirements difficult to quantify.

Phylloquinone is the safest form for supplementation of vitamin K in the animal, but it is also the most expensive. The water-soluble forms of menadione are less expensive and are used commonly in animal feeds.

Phylloquinone in pasture or in good-quality hay and menaquinones synthesized by intestinal bacteria presumably meet those requirements in all but the most unusual of circumstances. Green leaves are the richest natural source of vitamin K, and the vitamin remains present even after the green has diminished.

High-calcium diets fed to pigs have been found to greatly increase the vitamin K requirement. In horses, high-calcium diets are usually a result of high alfalfa intakes, and alfalfa is an excellent source of vitamin K. Conditions that interfere with vitamin K function are impaired fat absorption, gastric ulcers, mycotoxins in the feed, long-term antibiotic treatment, dicumarol in the feed (found in spoiled sweet clover hay) and warfarin (rat poison).

No problems have been found with excessive intakes of phylloquinone. However, menaquinones administered orally have been found to be toxic at 1,000 times the dietary requirement. Phylloquinone injectables appear safer than menadione injectables, as parenteral administration of menadione bisulfite has been found to cause acute renal failure in horses.

Reprinted from Kentucky Equine Research.