Horse owners often receive conflicting advice on the level of vitamin E needed. That's largely because there are two levels to consider--the amount needed to prevent deficiency signs and the amount needed to permit optimum overall horse health. Furthermore, it can be difficult to determine exactly how much vitamin E a horse is getting from feedstuffs.
In the past 10 years, the National Research Council (NRC) increased its recommended minimum daily dietary level from 15 IU/kg to the current 50 to 100 IU/kg. The higher levels--for foals, pregnant and lactating mares, and working horses--are 800 to 1,000 IU per head daily.
These levels may still be only half the optimum, say Oregon State University (OSU) veterinary researchers. They note that the NRC didn't evaluate how much vitamin E above the basal level would enhance immune status through the vitamin's antioxidant nature. This is especially important during stressful periods such as weaning, breaking and transporting, they point out.
Researchers, who have compared the typical diet of today's horses with what horses historically consumed, also caution that feedstuffs vary widely in vitamin E content (Figure 1). For example, lush pasture is relatively high in vitamin E, but mature plants less so. Fescue or orchardgrass hay has a fairly high level, but not if it has been rained on or overcured. Also, vitamin E activity has been shown to decrease in barn-stored hay. Thus, dry-lotting with no access to green pasture is a major risk factor for vitamin E deficiency in today's horses, the OSU researchers say. 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.
