A Confused Hair Coat

Why would a 9-year-old horse grow winter hair during the summer months?
Please login

No account yet? Register


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

Create a free account with TheHorse.com to view this content.

TheHorse.com is home to thousands of free articles about horse health care. In order to access some of our exclusive free content, you must be signed into TheHorse.com.

Start your free account today!

Already have an account?
and continue reading.


Written by:

Susan L. White, DVM, MS, Dipl. ACVIM, is the Josiah Meigs Distinguished Professor, Emeritus of Large Animal Medicine at University of Georgia’s College of Veterinary Medicine. White has a long-standing interest in equine dermatology, lectures on the topic extensively nationally and internationally, and maintains an equine dermatology consulting service.

Related Articles

Stay on top of the most recent Horse Health news with

FREE weekly newsletters from TheHorse.com

Sponsored Content

Weekly Poll

sponsored by:

What signs does your horse show when he has gastric ulcers? Please check all that apply.
86 votes · 215 answers

Readers’ Most Popular

Sign In

Don’t have an account? Register for a FREE account here.

Need to update your account?

You need to be logged in to fill out this form

Create a free account with TheHorse.com!