Your horse’s vitamin needs don’t really change in the winter – but the supply available from the diet does. This is because there is a big difference between fresh grass versus hay and also because hay vitamin levels decrease with time. The main vitamins of concern are A, E and C.
Full blown vitamin A deficiency is rare but suboptimal levels of this important antioxidant can negatively impact fertility, vision, skin health, cellular division and the immune system.
When hay has reached 6 months from cutting it will begin to show significant drops in vitamin A. At this point the horse’s diet should contain around 10,000 IU of supplemental vitamin A (less for ponies, more for large breeds). By the time the hay is a year old, supplemental levels should increase to 20,000 IU/day.
While vitamin A is vital for health, it can also be toxic in excessive amounts. Limit the dose of vitamin A from all sources to 40,000 IU/day for an average size horse. Many feeds and supplements have added vitamin A so check labels.
Vitamin E is another important antioxidant which becomes incorporated into the membranes surrounding cells as well as the cellular machinery inside them, like the mitochondria which generate energy. Vitamin E plays a strong protective role in muscle and the nervous system, as well as the immune system.
Vitamin E content in hay is very low and drops quickly. An average size horse should get at least 1000 IU/day with evidence to suggest best immune system function is from 2000 IU/day. Horses with muscular or nervous system issues may be prescribed more. Vitamin E is best absorbed if mixed directly into some oil before feeding. Vitamin E is nontoxic even at high doses.
We’re all familiar with the benefits of vitamin C for the immune system but it has other important functions. Vitamin C is concentrated in the adrenal glands, where stress hormones are produced. It is necessary for the formation of collagen, which forms the framework for bone, tendons, ligaments, cartilage and the connective tissue which gives structure to all tissues throughout the body. Vitamin C deficiency interferes with healing.
While horses can manufacture enough vitamin C to avoid having full blown C deficiency (scurvy), when eating fresh grass they get large amounts of vitamin C and levels in their body drop drastically in winter or when they are stalled and fed hay. A reasonable level of supplementation is 3000 to 10,000 mg per day.
Higher doses of vitamin C can cause diarrhea. The vitamin itself is not directly toxic but it can increase the absorption of iron so keep dosages on the low end in older horses, horses with equine metabolic syndrome or any horse known or suspected to have iron overload. Combining low dose C with other sources of water soluble antioxidants such as bioflavonoids and grape seed meal is a safe way to provide protection without high doses of vitamin C.
These key vitamins are relatively inexpensive to supplement and a good way to help your horse stay healthy and vibrant all winter.
Eleanor Kellon, VMD