Every few years, the National Research Council (NRC) publishes updated nutrient requirements for horses. The current vitamin E requirement is 1 IU/kg of body weight for an adult horse at maintenance. A requirement is the minimum amount needed by all horses within a certain category (such as weight or exercise level) to maintain a standard level of health. It does not take into consideration the individual’s needs.
A recommendation, on the other hand, goes beyond the basic requirement. It addresses any additional needs based on such things as the horse’s genetic makeup, environment, disorders (such as arthritis or injury), or disease state. Anything that compromises the immune system, such as stress or illness, will increase the need for vitamin E well beyond the required amount. Treatment of severe disorders such as equine motor neuron disease (EMND) or equine protozoal myeloencephalitis (EPM) necessitates high dosages of vitamin E.
Horses that graze on pasture usually get all the vitamin E they need. Fresh grass contains approximately 110 IUs of vitamin E per kg of dry matter. If your 1100 lb (500 kg) horse spends all day on a healthy field of grass, a generalized level of consumption would be 22 lbs (10 kg) of dry matter each day. That equates to 1100 IUs of vitamin E, providing more than the NRC requirement. However, keep in mind that even if your horse is fortunate enough to have access to fresh grass at all times, the vitamin E content can vary with the grass’ maturity, the season, and amount of rainfall. Overgrazing can also reduce vitamin E levels.
Once fresh grass is cut, dried, and stored to make hay, it loses virtually all of its vitamin E content, making supplementation a necessity. Adding a commercially fortified feed will offer some vitamin E to your horse’s diet -- the average feed contains 150 IU or less per pound. Therefore to obtain the same 1100 IUs as from healthy pasture, you would need to feed more than 7 pounds of feed per day – an amount too high for most horses.
Healthy, adult horses, without any demands on them, who are free to roam, socialize, and graze will do just fine with the minimum requirement of 1 IU per kg of body weight (0.455 IU per pound of body weight). However, most horses are confined to varying degrees and experience common stressors. Work, performing, travel, or the physical and mental discomfort of a periodically empty stomach, lead to the formation of free radicals – volatile, destructive molecules that can potentially damage every tissue in your horse’s body. While extra vitamin E won’t remove the cause of the problem, it will offer a level of protection against disease and disorders through its antioxidant activity – the ability to neutralize free radicals.
Because most horses do better with more than the minimum requirement, here are some realistic daily supplementation recommendations for the 1100 lb (500 kg horse):
More daily vitamin E is needed for specific health issues:
Many vitamin E supplements are packaged with selenium, since these two nutrients form an “antioxidant team.” However, selenium has a narrow range of safety (between 1 to 3 mg for most horses), and you may already be providing enough from other sources. If your horse needs extra vitamin E, but no extra selenium, choose a supplement that only provides vitamin E.
Vitamin E is available in natural or synthetic forms. In its natural state, it is listed as “d-alpha tocopherol” whereas in its synthetic form, you’ll find it listed as “dl-alpha tocopheryl acetate.” While natural vitamin E is more effective than the human-made vitamin, the synthetic form is more stable and therefore has a longer shelf-life. If you choose to supplement dl-alpha tocopheryl acetate, increase the amount by one-third to make it comparable to d-alpha tocopherol’s activity.
A note on IU (international units)… An IU is a unit of measurement based on vitamin E’s biological activity and can be converted to mg. One IU is equivalent to 0.667 mg of d-alpha-tocopherol or 1.0 mg of dl-alpha-tocopheryl acetate.
Vitamin E is not toxic at high doses. However, more research is needed on its potential to interfere with beta-carotene absorption. Your horse converts beta-carotene (found in plants) to vitamin A. Therefore, when supplementing high doses of vitamin E, it is best to offer adequate Vitamin A (or beta-carotene) until we know more about this potential interaction.
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