50lb Equine Senior - Purina’s Ph.D. Equine Nutritionists and Veterinarians continually research new ways to address the specific needs of aging horses, helping your senior horse live a long, healthy life. Purina® Equine Senior® horse feed has been scientifically studied at the Purina Animal Nutrition Center for over 20 years. With this formulation, our nutritionists have combined all the benefits of the trusted original Equine Senior® horse feed together with discoveries from the most cutting-edge equine science.