Vitamin K has one major, and very important, role in the horse's body. Through a complex process it helps with blood clotting. The Gla-proteins (which are part of that complex process) that are made from K are also important in bone metabolism and heart health.
So, K is a pretty important vitamin to sustain life.
Similar to other vitamins, the best source of K in your horse's diet is his forage...
...and cereal grains in his diet contain very little K.
Dietary requirements for K have not been determined in the horse, but most horses in normal housing situations easily obtain all of the K they need from their diet.
Problems with blood clotting is the major symptom of vitamin K deficiency. It is caused by those Gla-proteins being formed the wrong way and not being able to do their job correctly.
In humans, K deficiency has also been implicated in diseases affecting bone and heart health.
However, thankfully for us horse owners, K deficiency due to the horse not consuming enough has never been reported. However, if the horse consumes K antagonists (substance that works against K, making it unable to do its job), deficiency symptoms can appear.
One of these antagonists, dicoumarol, is produced by moldy sweet clover hay. Problems with blood clotting from consuming moldy sweet clover hay (and thus dicoumarol) HAVE been reported in horses. Yet another reason to make sure you never feed moldy hay to your horse.
Additionally, the therapeutic use of warfarin in horses can interfere with the metabolism of K, causing blood clotting problems.
Vitamin K toxicity from overconsumption of the vitamin has not been reported in horses.
It is estimated that the toxic level of K by ingestion in the horse is at least 1,000 times the daily recommended intake. That's the good news.
The bad news is that in one study, a researcher managed to cause acute renal failure in every study horse when he administered a single dose of menadione (the synthetic form of K) by injection according to the manufacturer's recommendations.
This, paired with the fact that phylloquinone (one of the two naturally occurring forms of K) injections appear safer in newborn human babies, has led to the suggestion that phylloquinone injections be used if a horse needs to have K administered by injection.
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