The nice thing about water soluble vitamins is that, even if they don't help, feeding them can really do no harm.
Fat soluble vitamins require just a little more discretion. As clearly indicated, fat soluble vitamins are soluble in fat (lipids). They are generally absorbed in the small intestine and travel by blood to the tissue, where they are stored. Vitamins A and E are especially prone to storing themselves in tissue. Vitamins A, D and K are prone to toxicity so care should be taken to account for all sources of these vitamins in existing feeds/conditions before supplementing them.
Let's start with the quickest one...Vitamin K. Vitamin K's major function is blood coagulation. Because quality forage and the hind-gut can typically produce all of the K that a horse needs, it is not usually supplemented for the average horse. Feed companies may throw a little in a fortified feed to help supply a performance horse with additional support, but the only real reason Vitamin K is ever specifically supplemented is for horses that 'bleed'. EIPH (exercise-induced pulmonary hemorrhage) refers to the presence of blood in the airways of the lung in association with exercise. A horse may or may not bleed out of his nostrils after a strenuous effort. If bleeding is suspected (and no blood in the nostrils is present) the only way to accurately determine EIPH is an endoscopy exam immediately following a competitive situation. Chemical compounds (namely Lasix) used to prevent bleeding in competitive horses has been banned in most cases, mainly because studies show that if given to horses that are not EIPH, it is actually performance enhancing. The use of Vitamin K has been proven to help certain horses and prevent them from bleeding and it is not banned.
Vitamin A (carotenes)- Wherever I find an article that already says what I want to say, I don't feel the need to rewrite history, so here is a good article on Vitamin A.
Vitamin E - Has long been used as an antioxidant, immune booster and is specifically good for horses that have muscle problems. A horse that needs additional Vitamin E can safely stay on as much as 5,000IU/d of it without problems (any more than that begins to inhibit Vitamin A absorption). Probably a more normal range, while still supplying benefits is around the 1,200-1,500IU/d. Wheat germ oil and lucerne forage are the best known suppliers of Vitamin E. These says, Selenium is often added to Vitamin E supplements because this vitamin and this mineral work in conjunction. However, care and thought should always be given to adding selenium to a horse's diet and a conscious effort made to add up all the sources of selenium before adding more. Selenium is toxic in high levels and a horse is better served to be slightly selenium deficient vs. slightly over-supplemented. Selenium is a mineral, so I'll discuss it more in depth in the Minerals posts.
And now for my favorite one to argue...Vitamin D!
I am not a big fan of adding a lot of Vitamin D (synthetic D3 specifically), to MY horses diets, at least not on a regular basis. My horses live a very natural, outdoorsy lifestyle, I don't keep them in barns, swaddle them in blankets or flysheets and during the growing months, they all get to eat at least a reasonable amount of green grass. Basically, their Vitamin D requirements should be fulfilled in the most natural way possible. That being said, it is almost impossible to get away from the addition of Vitamin D3 in their diets, as it is a popular additive in nearly every fortified/complete feed made, as well as a big component in many complete Mineral Supplements. Adequate Vitamin D is most certainly necessary for bone development, calcium and phosphorous depend on it to do their job in building bone mass and maintaining strong bones and there is strong evidence that Vit. D is also important to immune health and normal inflammatory response.The UE is allowed to promote that fact, the U.S....not allowed to say that, go figure!. U.S. feed companies are allowed to tell us that adequate supplies of Vitamin D will help prevent osteoporosis, which is sort of a no-brainer because after all...It works with calcium and phosphorous to build/protect the bones.
It's not that I won't feed a complete mineral supplement with a high level of Vitamin D3 in it, it's that I don't generally feed it on a regular basis, for an extended period of time. I feed more of my mineral supplement in the winter than any other time of the year or if a horse is being dry-lotted for an extended period. That is the times that I know my horses still need nutrition, but not especially 'energy'. Now that I am riding regularly and competing on them throughout the winter, with shorter daylight hours...and outside of a few weeds they nibble on in the yard, there is still no green grass, supplementation is necessary for awhile.
There is no actual evidence out there that both supplementing Vitamin D AND grazing AND sunlight will lead to Vitamin D toxicity, at least not the 'death stage' of it, but a horse that is being over-supplemented with Vitamin D will actually have the same symptoms as a horse that is lacking in Vitamin D; reduced feed intake, poor growth and an unthrifty appearance. Stiffness in body and limb may also occur. Before providing additional Vitamin D to a horse's diet, make sure to take into account the amount that is already being provided in any fortified or complete feeds and supplements that you are feeding.
The average daily requirement of Vitamin D for an 1,100lb horse is 3,300IU/day. Five to eight hours of sunlight in optimal conditions will provide this. This is also about the average you will find added to fortified feeds...perhaps a little more, perhaps a little less, depending on the maker of the feed. Additional supplements (like the fat supplement I use...It also provides an additional 1,200IU/day) can raise that level as well.
That being said, the upper level of 'safe' for a horse is around 22,000IU/day. So, as you can see there is quite a bit of a safety net. But it never hurts to know exactly what a horse should have and then be aware of exactly what you are feeding them.