The increased exposure to wood preservatives and insecticides associated with dry-lotting may also increase formation of destructive free radicals and oxidative stress within the horse's cells, they note. As a result, the Oregon State researchers recommend 2,000 IU of supplemental vitamin E per head daily from six weeks to two years of age. To decide whether a horse may benefit from vitamin E supplementation, the blood level of vitamin E as alpha-tocopherol should be determined. A composite of three samples spread across a day should be taken. That's because vitamin E levels naturally vary by 12 percent at different times daily, according to OSU researchers.
Vitamin E is absorbed in the small intestine, so oral supplementation is an excellent method.
But the OSU researchers warn against using an unbalanced supplement that includes selenium to get the higher vitamin E levels. That's because higher levels of E could accompany harmful overdoses of selenium. Horses and Vitamin E. College of Veterinary Medicine, Oregon State University.