Vitamin E is an important nutrient for your horse’s health because it supports muscle health and protects cell membranes. This nutrient is an essential vitamin, meaning your horse cannot synthesize it and it must be provided in the diet, and it is fat-soluble. 

Vitamin E is perhaps best known for its antioxidant properties. When the horse metabolizes oxygen, free radicals are produced, which can overload the body and cause oxidative stress. “For horses, vitamin E is the most popular antioxidant (among horse owners), it will scavenge the free radicals to reduce any cellular damage,” says Carey Williams, MS, PhD, professor of animal sciences at Rutgers University, in New Brunswick, New Jersey. Green pastures are great sources of vitamin E but, when this is not available, how do we ensure the horse is consuming the amount he needs?

How Much Vitamin E is Recommended for Horses? 

The National Research Council’s (NRC) Nutrient Requirements of Horses, published in 2007, provides a recommended vitamin E intake level based on your horse’s body weight and workload to prevent deficiency. “An average size horse that is not in work is going to require about 500 IU (international units) of vitamin E daily; however, I recommend 1,000 IU for the average horse as this level to help to accommodate any deficiencies,” says Williams. Horses in heavy exercise or breeding have increased requirements for vitamin E. 

“There isn’t a true known optimal amount for all horses,” says Rachel Mottet, PhD, an equine nutritionist and the owner of Legacy Equine Nutrition, in Ocala, Florida. “There is an established requirement that is recommended to prevent deficiency, yet ‘optimal,’ especially as it relates to equine athletes, needs more work.” 

Further research is required to fully elucidate the optimal vitamin E dose for equine athletes. When a horse exercises, there is exercise induced oxidative stress. This is due to increased amounts of free radicals. Therefore, feeding these animals more vitamin E is thought to be beneficial due to its antioxidant qualities. However, the ideal supplementation amount to optimally support equine athletes has yet to be determined. 

Horses metabolize vitamin E at different rates, Mottet adds, so research into optimal supplementation levels is challenging. “I periodically see horses getting the recommended amount of vitamin E (yet), based on their serum samples, are still deficient,” she says. “Part of the widespread deficiencies I see may be in part attributed to nonviable forms of vitamin E in different supplements, as the vitamin itself is prone to quick degeneration.”

How to Evaluate Your Horse’s Vitamin E Status

Ensuring your horse’s daily ration is meeting the NRC requirements for vitamin E is a crucial first step in evaluating your horse’s vitamin E status.

Because it is not realistic to measure the vitamin E content in pasture and hay, look at what the horse gets in the rest of his daily ration—ration balancer, concentrate, and any top-dressed supplements—especially if he’s on a primarily hay diet or has period of the year where pastures are scarce. Talk to an equine nutritionist or your veterinarian about what you find.

“If you are concerned about your horse’s vitamin E status have the veterinarian pull blood to evaluate,” says Mottet. “As long as the sample is handled properly and submitted quickly, this is a great (and the most accurate) way to determine if your horse is getting an adequate amount.”

If there is a deficiency detected via blood serum, then your horse likely needs additional vitamin E supplementation. “10,000 IU doses are used short term (to rectify a deficiency) as it can interfere with beta carotene absorption (at sustained levels),” says Williams. “Normally, it is not toxic, but you can get to a point that you interfere with other fat-soluble vitamins.”

Mottet adds that it is unnecessary to supplement your horse’s vitamin E intake if blood serum tests do not detect a deficiency.

Best Sources of Vitamin E for Horses

Again, pasture is the best source of vitamins for horses. Sun-cured hay is inevitably vitamin-deficient. “I personally like to make sure that my ration is providing my horse’s daily vitamin E requirement and whatever I get in pasture or hay is a bonus,” says Mottet.

If it’s clear the horse needs vitamin E supplementation, work with an equine nutritionist to be certain your horse is consuming a high-quality bioavailable vitamin E source.

When evaluating vitamin E in a product, there are two main types—natural and synthetic. The synthetic form of vitamin E is known as dl-alpha-tocopherol, and the natural form is d-alpha-tocopherol. “Both natural and synthetic sources work, however, the natural vitamin E is more bioavailable (usable) to the animal,” says Williams. “About 20 years ago my lab looked at heavily exercising horses and if greater vitamin E supplementation would decrease oxidative stress. It was shown that intensely exercising horses that were not housed on pastures would benefit from 5,000 IU per day for a 1,000-lb horse; the synthetic form was used for this research, therefore, if you were using the natural form, you could likely get by with 2,500 IU instead.” 

The NRC’s recommendation is a helpful starting point, but it is important to remember that more is not always better. “If the horse has adequate serum vitamin E levels, there isn’t strong data that says more is better,” says Mottet. “However, as a potent antioxidant it’s possible that additional vitamin E could provide benefits to equine athletes.” 

More research is needed to confirm the efficacy of this supplementation method, she adds. Additionally, although toxicity risk is low, there is a potential for interference with other fat-soluble vitamins when significantly over-supplied to the horse. 

Take Home Message 

Vitamin E is an important antioxidant for horses. Equine athletes in heavy work, broodmares and breeding stallions, or those without daily pasture might need additional supplementation. Natural and synthetic forms of vitamin E are available and, while both are effective, the natural form (d-alpha-tocopherol) is more bioavailable to the horse. Therefore, if your horse has a deficiency, your veterinarian or an equine nutritionist might recommend supplementing him with a liquid form of natural vitamin E from a reputable company. Before adding a supplement to your horse’s diet, consult your veterinarian for blood serum testing, which can help you obtain accurate information about your horse’s vitamin E status.