Vitamin E is a natural anti-oxidant. Horses must obtain this Vitamin from their diet, as they can not produce it in their bodies. With modern horse keeping methods, most performance and riding horses do not have access to fresh lush green pastures, where they would find enough of this fat soluble vitamin to satisfy their requirements, and must be supplemented. 
 
Vitamin E is important for the immune system, for neurological and nerve function, as well as to rid the body of damaging free radicals that can lead to tissue damage.