Vitamin E is another one of the fat-soluble vitamins. Its primary function in your horse's body is as an anti-oxidant. It is fat-soluble, so it can easily penetrate cell membranes (which are made up of fat, or lipids) and serves as one of the main antioxidants that protects these membranes.
Amounts of E in the horse's diet varies considerably. The main sources of it are forage. However, when talking about forages (and therefore naturally occuring sources of E) content declines the longer the forage is stored.
The content also varies based on how mature the forage was at harvest -- older forages have less E activity than younger plants.
Grains have a lower content of E than forages. And again, the amount of E found in any given grain varies depending on harvest conditions.
To make up for all this variation, most feed companies fortify their feeds with extra E.
White muscle disease is the disease that is commonly thought of when vitamin E deficiency is being discussed. This disease is also known as nutritional muscular dystrophy, and is a degenerative disease that affects the heart and skeletal muscles of foals under a year of age.
A deficiency of E has been implicated in white muscle disease, however, the available studies point to selenium deficiencies being the primary cause of the disease. E supplementation is used, along with selenium supplementation, to treat the disease.
Another disease in horses that may be related to E deficiency in horses is Equine Motor Neuron Disease (EMND). EMND is a degenerative neuron disease that affects lower motor neurons of horses 2 years and older. This disease is characterized by sudden and severe onset of trembling, a constant shifting of the back legs when the horse is standing, more than usual lying down, and muscle wasting. It can also affect the eyes. Several studies have shown that this disease occurs after a prolonged E defeciency.
And more good news for horse owners...
...vitamin E toxicity is not a concern in horses, even with large amounts of supplementation. The upper safe dietary limit, which has not been determined specifically in horses, is set at 1,000 IU per kg of dry matter (1000 IU/kg DM) based on observations in other species.