The winter coat is a natural insulation that protects the horse's body from the cold and helps regulate the horse's temperature. But when the spring comes, horses will start to shed their thick winter coats. The actual process of shedding is triggered by the amount of light. The horse’s pituitary gland will recognize that days are longer and it's time to shed. The pituitary gland is a small gland located at the base of the brain. When the horse's pituitary gland is stimulated by light, it will produce hormones that cause the hair follicles to open up. The hair follicles are glands that secrete the hair shaft. Once the hair follicles open up, the hair shafts will start to come out of the skin. The hair follicles will continue to open up and shed hair for about six months until the summer.