Your horse, like every living thing, relies on vitamins for a variety of body functions and would die without them. So, yes, your horse needs vitamins. However, that doesn't necessarily mean you need to supplement them above and beyond what's in his current diet.
Whether or not your horse needs a supplement depends on two things-the level already present in his diet and what his needs are. If needs are being met by diet, he doesn't need more. It's that simple. More is not better. At best, more is a waste but harmless; at worst, more is toxic.
Common sense alone tells us that the horse evolved as a species by being able to survive on the nutrients available to him. The horse's ancestors moved from browsers to grazers (grass eaters) around 18 million years ago. Grass is a horse's natural food, and a healthy horse with an adequate supply of fresh/live grass pretty much doesn't need additional vitamins. Even in winter, a horse's diet isn't as devoid of "live" material as you might think. Dormant (not actually dead) grasses still contain active vitamins, and the diet is supplemented by browsing on the tightly closed buds and more tender distal ends of shrubs and trees. Even so, vitamins are not as available as during periods of active growth of grasses.
Vitamins with the highest potential for toxicity are the fat-soluble vitamins: A, D and K. The horse is protected from A toxicity under natural circumstances by his ability to limit how much of the abundant beta-carotene-the vitamin A precursor in plants-is actually converted to vitamin A. There are no "active" forms of vitamin D in the diet, so natural safety from that poisoning is also through regulation of how much inactive vitamin D from plants (also produced in the skin) gets converted to active D. An identical situation exists for vitamin K, where K3 is the active form but the horse absorbs either K1 (plants) or K2 (from intestinal bacteria), which the body must then "activate" to K3.
Vitamin E has little, if any, toxicity and the horse will absorb E in direct proportion to how much is present in the diet. Vitamin C and the B vitamins are also relatively nontoxic and will be absorbed in proportion to what is present in the diet. Horses on good pasture have much higher levels of these vitamins in their blood than stabled, unsupplemented horses. However, full-blown deficiencies of C or the Bs do not occur in horses. With vitamin C, the horse is protected because he's able to synthesize at least enough to prevent full-blown scurvy. All foods contain some (albeit low levels) B vitamins and synthesis of Bs by bacteria in the lower portions of the small intestine is another source. In addition, research in other species has found that absorption of Bs can be increased by activation of special carrier proteins on the intestinal lining cells.
A healthy horse on good pasture doesn't need supplemental vitamins, except for E in some instances. Vitamins B12, D and K probably never need to be supplemented. Vitamin A should be given as a supplement with caution, if at all. That leaves just vitamin C and the B vitamins.
While it's likely working horses would benefit from some supplementation of C and B vitamins when they are on hay-and-grain diets, precisely how much is largely a matter of guess work. Our chart lists some guidelines for amounts to supplement for horses in moderate or heavy (racing, endurance, upper-level eventing) work, and how the multi vitamin and mineral supplements from our previous mineral articles compare.
Requirements for vitamin C and the B vitamins may be increased in a variety of situations, including:
- Injury or surgery
- Antibiotic use
- Heavy exercise
- Bowel problems, especially involving the small intestine
- Liver disease
- Old age.