Horses' hair coats grow and shed primarily in response to ambient temperature and photoperiod (hours of daylight).
Photo: Alexandra Beckstett, The Horse Managing Editor
By Susan L. White, DVM, Dipl. ACVIM, and Josiah Meigs, Distinguised Professor, Emeritus, Department of Large Animal Medicine, College of Veterinary Medicine, University of Georgia
Q. Why would my 9-year-old horse grow winter hair in the summer? He was nice and slick all winter under blankets and around the first of June his hair grew like it was winter and then shed out in July. Nothing has been changed as far as food or turnout, and it has been hotter than Hades here. He acts normal, too.
Kathryn Krouse, via email
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.
Horses kept blanketed in the winter fail to develop a dense winter coat. One could hypothesize that the blanket keeps the skin temperature high in comparison to environmental temperature. Thus, a winter coat does not develop despite changes in photoperiod. Once the blanket is removed, lower night temperatures (as found in spring) may activate the periodic hair cycle and stimulate hair growth (winter coat) despite the change in photoperiod. Subsequent shedding (in July) might be the hair cycle normalizing to the influence of photoperiod and ambient temperature.
I have experienced the opposite effect in some horses turned out in winter with a heavy winter coat and subsequently brought in the barn and put under lights midwinter. These horses shed in January, despite cold ambient temperatures and no significant change in photoperiod, demonstrating that managerial factors can disrupt the hair’s normal periodic cycling.
If your horse is on a balanced diet with no current or previous illnesses, he will most likely grow his winter coat properly in the fall if you don’t blanket him. If your horse continues to show abnormal hair cycles throughout the year, you should consult with your veterinarian, because a systemic abnormality may be influencing hair growth cycles.