Vitamin K (menadione)
Fast fact: Occasionally, foals are born with a vitamin K deficiency simply because their guts have not had time to produce it.

How your horse uses it: For blood clotting and activating a number of proteins so they can be utilized by the body.

Where it's found: Of the three forms of vitamin K, those most important to your horse are present in fresh and dried green leafy plants (K1) or produced in the gut by bacteria (K2).

Dietary requirements: There are no established minimums for vitamin K. It has been estimated that horses need less than 0.5 milligrams per kilogram (mg/kg) of dry weight diet per day, an amount they can readily synthesize or derive from hay.

If he doesn't get enough: Horses deficient in vitamin K are prone to hemorrhaging, both internal and external. A deficiency is most likely to occur when the gut bacteria can no longer synthesize the vitamin or the body cannot absorb or utilize it due to gastrointestinal upset, a disruption of gut flora or compromised liver function. Certain anticoagulant drugs, such as warfarin, and large amounts of moldy sweet clover also interfere with vitamin K production. In these cases, injections of the synthetic version of the vitamin, K3, can resolve the problem.

If he gets too much: Dietary sources of vitamin K have very low toxicity levels, and natural poisoning has never been reported in horses. Injections of synthetic K3 when the horse is not deficient can lead to renal failure, laminitis and possibly death within 12 hours.

Excerpted from an article by Christine Barakat in EQUUS Magazine, Issue 263 (September 1999), copyright 1999 by PRIMEDIA Enthusiast Publications, Inc. All rights reserved.

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