Vita Flex Pro E & Selenium Vitamin E Powder Horse Supplement, 4-lb bucket; Many soils in the USA and Canada are chronically deficient in selenium, which is why this Vita Flex E & Selenium Vitamin E Powder Horse Supplement is a great option for horses! This vitamin E and selenium supplement for horses provides balanced levels of vitamins E and selenium, which can help contribute to the correction of nutrient deficiencies. Selenium is an important trace mineral that acts as an antioxidant in your horse’s body. Vitamin E is another important antioxidant and selenium works synergistically with vitamin E for many body functions.