Vitamin E has even been linked to fertility issues.

Vitamin E and selenium supplements abound—and for good reason. Severe deficiencies can cause life-threatening muscle and nervous-system diseases. But these nutrients do far more in the horse’s body than protect muscles.

As critical antioxidants, they protect the immune-system cells from suffering “friendly fire” damage as they go about their work, especially areas that have a high reliance on oxygen-fueled metabolism, like in the heart, muscle and brain. They also protect the red blood cells from damage during exercise. Even reproduction can be negatively affected by the inadequate intake of vitamin E and selenium, and these nutrients are an important part of detoxification systems.

All vitamins and minerals identified as “essential” have critical roles to play, but what makes vitamin E and selenium particularly important is how likely deficiencies are.

The selenium level in feeds is a function primarily of soil levels. With the exception of a band of states through the lower two-thirds of the middle of the country, most areas are moderately to severely deficient in selenium.

Vitamin E is abundant in fresh grass, but most of it is destroyed by curing and baling, with the small amount remaining also rapidly destroyed during storage. Intact seeds and grains have some vitamin E, but not enough to fulfill requirements and this, too, declines with storage or any break in the integrity of the grain or seed.

Adding vitamin E to packaged feeds or multi-ingredient vitamin/mineral mixes is routinely done at low levels. Higher supplementation probably wouldn’t help much anyway because E is fragile, and its activity is readily destroyed by exposure to low-level bacterial or fungal activity, air, sunlight and the presence of free minerals. The solution is to meet the horse’s requirements by a separate supplement.

How Much Vitamin E?
While most of the recommended feeding levels for other nutrients are based more on flat-out deficiency-disease standards—the level that actually can cause a deficiency disease—vitamin E is an exception. Even the 1989 National Research Council (NRC) recommendations for feeding horses  focuses more heavily on feeding adequate vitamin E to help avoid exercise-related muscle damage and pain, and to support good immune response to vaccines. Because maintaining optimal health is a more complicated process than inducing a full-blown deficiency disease, the numbers really haven’t been strictly formalized yet, and may never be, but present thinking is:
• Healthy adult horses, with no stress from disease, injury, infection or exercise, not reproductively active: bare minimum 800 to 1000 IU/day vitamin E.
• Healthy horses in regular exercise: 1800 to 3000 IU/day.
• Correction of vitamin E deficiency, or supplementation during diseases where higher requirements are likely—such as chronic infections, nervous system diseases, muscular disorders, chronic lung disease like heaves—3000 to 5000 IU/day.

If you follow discussions on human supplements, you’ve likely seen arguments over what form of vitamin E is best. Water-soluble vitamin E is E that has been suspended in tiny droplets of fat. It doesn’t really dissolve, but it does remain in suspension inside the intestine and is immediately ready for absorption. Mixed tocopherol vitamin E are supplements that contain many or all of the different forms of vitamin E found naturally in plants. “All natural” E is E in the form of all d-alpha-tocopherol, while “synethetic” is a mixture of d and l forms.

Which is best?
A Kentucky Equine Research study showed blood levels of vitamin E are twice as high following supplementation with water-soluble E than with natural or synthetic vitamin E.  However, studies have also shown that single blood draw levels may not correlate well with tissue levels, and that animals on identical diets can show variations as high as 150 to 200%, making it difficult to say with certainty the differences are significant.

Mixed tocopherols are a little easier to sort through. The only form of vitamin E found in the horse’s tissues is the alpha-tocopherol. The other forms may be converted to alpha, probably by intestinal organisms, but the one the body needs and wants is alpha. Note: An advantage to the synthetic d,l-alpha-tocopherol is that this form is more stable and slower to degrade over time than the other forms.

Natural versus synthetic vitamin E also creates a lot of argument related to the catch word natural, but the bottom line is that natural vitamin E is about 36% more bioavailable than synthetic. So as far as forms of vitamin E goes, you should remember that:

• The NRC recommendations are based on the synthetic, d,l-alpha-tocopherol form.
• If you’re using natural vitamin E, for every 1000 IU of requirement, substitute 750 IU of natural E.
• If you’re using water-soluble vitamin E, it may take only 500 IU to get the same benefit as 1000 IU of a synthetic.
• There’s no strong advantage to supplements with mixed tocopherols; the horse’s body wants alpha-tocopherol.

Toxicity isn’t a concern with vitamin E in any of the dosages we’ve mentioned. In fact, vitamin E toxicity has never been observed in horses. Although this vitamin is fat-soluble and found in the highest concentration in body fat, it isn’t stored at the same high levels as the other fat-soluble vitamins A and D, which can reach toxic levels.

Depletion studies have found that it’s easy to rapidly induce drops in vitamin E by feeding diets low in E. This may be a difference between how vitamins A and D are stored compared to vitamin E or may reflect the horse’s high requirements for E compared to what they get in the diet.

How Much Selenium?
Most people are cautious about feeding selenium, and many aren’t aware that selenium deficiency is widespread and poses potential problems for their horse.  Like any nutrient, selenium in excess can be toxic. However, the safety margin for feeding selenium is wider than for vitamin A.

The estimated minimum requirement for a 1,000-lb. horse is 1 mg/day. The proposed upper safe limit of intake for long-term use is 20 mg/day. Even if your horse is eating hay and grain from a selenium-adequate area, with a level in the feed and hay of as high as 0.5 ppm, 22 lbs./day of this diet (10 kg.) would provide him with only 5 mg of selenium, which is plenty but still leaves a very wide safety margin of 15 mg/day.

If you’re still nervous about selenium, you don’t have to guess. Your local agricultural extension agent can give you details on soil and feed/forage selenium levels in your area. If you have a stable pasture or source of hay, selenium assays can be done on these. Another approach is simply to test the horse himself. Whole blood selenium assays are generally believed to be the most accurate. You can base your decision on whether to supplement or not on those results.

If you don’t do any testing, guarantee supplemental intake of at least 1 mg/day for inactive horses, 2 to 4 mg/day for exercising horses. There’s no solid evidence in any species that exercise increases selenium requirements. However, many factors can influence selenium absorption (such as the levels of other trace minerals in the diet), and the exercising horse has a more critical need for this important antioxidant. As above, unless your horse is eating a diet with an extremely high baseline level of selenium, this level of supplemental intake (1 to 4 mg/day) is safe.

The only form of selenium currently approved for use in horses in sodium selenite. Organic forms of selenium such as selenium yeast or the chelate selenomethionine have been shown in many species to be better absorbed. This may be true in the horse as well, but horse-specific information simply isn’t available.

Bottom Line
As even a quick glance through the supplements shows, E-Se combination supplements come in a variety of dosing packages for both vitamin E and selenium (Se). The best product depends on your needs.

First, determine your horse’s current selenium intake. It’s safest to assume that your base diet provides little vitamin E if you’re not supplementing it, so begin by looking for a product that provides around 1000 IU of vitamin E if your horse is not in work, at least 2000 IU of vitamin E if working.

If you don’t need any more selenium, use a product that contains only vitamin E. In addition to the synthetic E supplements, you can now choose natural and water-soluble forms of E. Unless the horse has liver disease, or you know from blood tests your horse absorbs E poorly, we’d pass on the water-soluble E.

Even if you adjust the dose down to allow for as much as 100% better absorption, we don’t think the expense is warranted. Synthetic vitamin E isn’t quite as pricey as natural E, but again the higher price doesn’t warrant the small increase in bioavailability.

If you still want to go natural with your E source, Smart Pak E is a Best Buy. At 50¢/day for 2500 IU of natural E, it’s a bit over twice as expensive as the best buys for synthetic E, with the added advantage of individual daily dose packs for the best possible preservation of potency.

The two best prices on synthetic E we found were Uckele’s Liquid E-50 and Animed’s Vitamin E Concentrate. The Uckele product is in an oil base, which eliminates the need to add oil to help absorption, and the oil also protects the E from exposure to air and moisture which will hasten the breakdown.

If you also need to add selenium, first locate the products that meet your selenium needs at the lowest dose then compare the vitamin E levels. We can’t give you a specific product recommendation here because it  depends on your selenium requirement. That said, for highest potency of both E and Se at the best price it’s a photo finish between Uckele’s E + Se, Uckele’s E + Se 10X and Equerry’s Vitamin E and Selenium. All three provide a minimum of 2000 IU E and 2 mg Se at daily prices within pennies of each other.

Scientific Literature

Veterinary Journal, January 2005: Thoroughbreds in training show significant drops in antioxidant status, which is largely prevented by supplementation, including E. Veterinary Journal, November 1997: Study of plasma levels of vitamin E in horses with EMND (equine motor neuron disease) confirmed the neurological horses had significantly lower vitamin E levels than normal controls. American Journal of Veterinary Research, January 2006: Low vitamin E is likely a cause rather than an effect by feeding diets low in vitamin E (aged hay, no pasture access). 50% of test horses developed signs of EMND  (equine motor neuron disease) compared to no cases in control horses with access to pasture. Canadian Veterinary Journal May 1994: Horses grazing fresh pasture have vitamin E levels 63% higher than horses eating harvested/stored feeds.

Why Feed Vitamin E?

The research is convincing, and the benefits are high.
Most domesticated horses require daily vitamin E supplementation. This vitamin plays a key role in immune-system function, protecting the membranes of red blood cells and nerves, protecting cell energy generating machinery from damage and in detoxification reactions. Vitamin E’s role in peak fertility is also suspected. Despite these findings, vitamin E levels in feeds and supplements usually remain too low to get the job done.

Fresh pasture grasses are high in vitamin E, levels ranging from 150 to 200 IU/kg. Horses at pasture take in several thousand IU of vitamin E per day. (Note: IU stands for International Units, the accepted method of measuring vitamins.)  However, when the grass is baled, the vitamin E level drops to less than 10 IU/kg.

An inactive horse being maintained on 10 kg (22 lbs.) or even 20 kg (44 lbs.) of hay a day would take in 100 to 200 IU of vitamin E, a level documented to eventually cause vitamin E depletion in horses.  Grains aren’t much better, with up to 20 IU/kg, and any processing of the grain or defects in the husk or outer seed coat lead to loss of the vitamin E.

Not surprisingly, studies on the vitamin E requirements of horses have found they need an intake closer to those naturally present in pasture diets than are found in routine hay and grain diets. A January 1986 study published in the Equine Veterinary Journal found diets providing between 100 to 200 IU/day led to rapid vitamin E depletion.

The 1989 National Research Council recommendation for vitamin E intake is 80 to 100 IU/kg of diet as a minimum (10 times more than that in hay), with a ceiling for daily intake between 1,000 and 3,750 IU/kg of diet. The minimum recommended intake translates to at least 1000 IU/day of vitamin E for an average-size adult horse not on pasture. Pregnant and working animals may benefit from twice this amount. Horses with medical conditions associated with oxidative stress, such as chronic infections, insulin resistance and Cushing’s disease, may benefit from higher intakes.

Supplementing Vitamin E
With little risk involved in supplementing vitamin E, and a long list of negative effects from vitamin E deficiency, this is one nutritional base you want to be sure to cover. However, not all forms of vitamin E are created equal.

We do understand why manufacturers of supplemented feeds and multi-ingredient vitamin and mineral products often neglect vitamin E. The vitamin is basically “fragile,” and it will not interact favorably with many of the other ingredients found in these combo supplements.

Pure vitamin E and vitamin E-selenium products avoid this problem, but even with specially stabilized powder forms of vitamin E, the shelf life only applies to unopened containers protected from extremes of heat. The acetate forms of vitamin E, dl-alpha-tocopheryl acetate or d-alpha-tocopherol acetate have a shelf life of two years in unopened containers, but for maximum potency it’s best not to buy more than a three-month supply at a time.

Dl-alpha-tocopherol acetate is a synthetic form of vitamin E that is used in most equine supplements. The d-alpha-tocopherol acetate is the natural vitamin with an acetate group on it for added stability. This is available in this country from some human supplements. Both of these forms require the presence of fat and bile in the intestinal tract to be absorbed. A third equine product, available through veterinarians, is Kentucky Equine Research’s Elevate W.S. This is the water-soluble d-alpha-tocopherol acetate form suspended in tiny fat micelles (eliminating one of the steps required to prepare vitamin E for intestinal absorption).

A study performed by KER found this form resulted in blood levels that were twice as high as those after the dl-alpha-tocopherol acetate form, but the increased cost may wipe out this advantage. Since horses on pasture do just fine in absorbing the vitamin E naturally present in grass, it’s unlikely that special processing of vitamin E is needed to get good absorption from normal digestive mechanisms.

Vitamin E should be fed with a meal that contains a little fat and/or provided in liquid oil-based form, such as Liquid E-50 from Uckele. Supplementing vitamin E alone avoids pushing selenium intakes to toxic levels, which can be the disadvantage when feeding vitamin E-selenium combination supplements.