Does your horse need a Vitamin E supplement added to his or her feeding program? Hay is the most important component of a horse's diet, and contributes almost all the necessary macronutrients (protein, fat and carbohydrates) and micronutrients (vitamins and minerals) that our horses need. Notice how I said almost? Hay does not cover all our horsesâ€™ nutrient requirements and there is one micronutrient in particular that is often deficient in horses on a hay-based diet. That essential nutrient is vitamin E.