How Horses Grow Winter Coats

It seems logical that the temperature outside tells a horse, or any other animal, to shed their summer coat and grow a winter one. But! It’s actually a horse’s eyes, brain, and daylight hours that dictate how horses grow winter coats.

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It’s daylight hours that trigger a winter coat for horses

At the end of June, the Summer Solstice is the day with the most daylight hours. As summer turns to fall, the sun stays out less and less.

Specialized cells called photoreceptors notice daylight and relay that information to the brain. Photoreceptors are located in the retina, at the back of the eye. Through their eyes, your horse’s brain notices this change and signals the shedding of the summer coat and growing of the winter coat.

Your horse also has a few hormonal changes in the fall to prepare for winter. Cortisol and melatonin levels rise, both of which influence hair growth. Incidentally, these hormonal changes may also interfere with insulin regulation, a common component of laminitis, especially for horses with PPID (Cushing’s disease) or Insulin resistance (IR).

At any rate – most horses start shedding their summer coats in August. At first, you may notice a bit more hair coming off; then things begin to ramp up in September. In the spring, the shedding process is much more noticeable as that yak hair comes out.

As the summer coat leaves, the new winter coat comes in. And it’s a slow process, sometimes taking a month or longer to grow fully.

Do temperature and blanketing trigger a winter coat?

I can’t find any science about the exact role that temperature and blanketing have with hair growth – but all science agrees that if it’s the case, it’s pretty minimal. Daylight is the key!

Do temperatures and weather decide how a horse’s coat grows?

Let’s look at this from a few angles. If the temperature is the deciding factor about a winter coat, it’s too late to start growing longer hair when it gets cold. Consider weird temperature swings, often for days or a week. Summer can come back in fall and winter with fluke weather events. If the temperature were the deciding factor, they would shed and re-grow in that time. Does the hair stall out? Perhaps?

If the temperature was the deciding factor, it also means that horses in hot southern areas, like Florida, Texas, California, Arizona, etc., would never grow a winter coat – but they do! And clipping is often needed to make them more comfortable.

Also, consider the horse with metabolic disorders. The long, thick coat is a sign of Cushing’s disease and grows regardless of the climate. The brain produces excess hormones, and the hair keeps growing, even in the dog days of summer. The temperature may say “no winter coat,” but their body says otherwise.

These aisle lights are insufficient to stop the winter coat from growing.

What about lights in barns?

Let’s also bust this temperature discussion by examining lights in barns. Many show barns and breeding facilities use lighting in the barns to stop the winter coats from coming in. For breeding facilities, lights also keep mares cycling all year long for optimal breeding and foaling times.

As the days become shorter, the lights in barns run on timers to mimic the sunrise and sunset in the height of summer. Lights keep the summer coat all year long when done correctly with enough brightness.

The climate and temperatures do not impact coat length or estrus cycles with additional lights.

Are blankets an effective way to shorten a winter coat?

Since temperature doesn’t play a big role, neither does blanketing – which is just regulating temperature with fabric.

A horse’s genes are programmed to create summer and winter coats that fit that horse. Like the Fjord horse, horses bred to survive Artic winters will still grow their genetically programmed winter coat living in Miami or Phoenix.

Consider, too, the thin-skinned Arabian or Thoroughbred living in Canada or any other cold area. They grow what their genes tell them, often ending up short for harsh winter weather.

What else influences how horses grow winter coats?

Some things can throw a wrench into your horse’s hair growth cycle. Looking at a horse’s health holistically, their diet, metabolism, and vision can interfere with the growth of a winter coat.

Diet and nutrition

What your horse eats is how it gets the ingredients needed to keep all of their organs and body systems functioning, including the skin. Hair, like hooves, has keratin, vitamins, and minerals that become askew without the proper balance of ingredients. Note that excessive ingredients can also interfere with healthy skin and hair.

Ingredients in supplements, like Omega-3 fatty acids and biotin, support healthy skin and hair. They are usually easy to feed and tasty, too. You could also try chia seeds for horses.

Cushing’s and metabolic disorders

Metabolic disorders, as discussed, muck up a horse’s hormones, some of which are directly related to hair growth. It’s so easy for your vet to do easy and affordable blood tests to check for metabolic disorders. And, you will then have a better understanding of your horse’s laminitis risk.

Eye health

Eyesight problems may also interfere with the brain’s ability to process daylight hours, but not always. Those photoreceptors are critical, and not every eye injury or condition affects those cells.

Over time, you will start to notice when your horse begins to shed and how his winter coat grows. Patterns can help you decide when to clip and blanket if needed.

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