Horses' hair coats grow and shed primarily in response to ambient temperature and photoperiod (hours of daylight). | Photo: Alexandra Beckstett/The Horse

A: Hair growth in mammals is not continuous, but cyclic, and has three periods. During its anagen phase, the hair is actively growing (elongating) from the base (dermal papillae) of the hair follicle. Growth is followed by a transitional stage called catagen, where the hair bulb narrows gradually until it separates from the hair, transitioning to the telogen or resting stage. During telogen the hair remains in the hair follicle but no longer has a “root” or hair bulb.

In humans this process occurs continuously, with some of the hairs in all three stages and a small number of hairs shed daily. In horses, hair coat growth and shedding is periodic. A variety of physiologic factors modify hair growth, including photoperiod (hours of daylight), ambient temperature, nutrition, hormones, general state of health, and other more poorly understood intrinsic factors. Of these, the two most important are photoperiod and ambient temperature. However, the details of hair follicle cycling and growth are very complex and poorly understood.

Generally, horses begin to shed in the spring when the photoperiod increases (which subsequently has an effect on many hormones) as does ambient temperature. In fall, a longer, thicker hair coat replaces the short, fine-haired summer coat as the photoperiod decreases and ambient temperatures become cooler