Shorter days and cooler temperatures can bring on thicker hair coats in many equines—but what truly triggers the change in hair length? It actually isn't cold temperatures—if that were the trigger, horses wouldn't begin to grow their coats until it was already cold, when it would be too late to help keep them warm.
In many areas of the country, horses begin growing their winter coats as early as September, when light begins to diminish. As the days get shorter, it's actually the reduction in sunlight hours that triggers the horse's body to begin increasing the production of melatonin, which prompts hair growth. The reverse is also true: When days get longer in the spring, melatonin production slows and the coat sheds out.
So what about horses in the south that grow less of a hair coat than their northern brethren? Here too, it's all about the daylight hours. Horses that live in regions that are closer to the equator actually have less of a difference in daylight/nighttime hours than more-northern regions, so they produce less melatonin. Because of this, horses tend to grow coats suitable for their climates.
When cold weather does strike, you may notice that your horse looks fuzzier than normal, and you aren't wrong. The individual hairs spring up in cold weather, trapping air pockets to provide extra insulation and keep him warm. This is the same theory as putting on a jacket, which helps to contain the body heat you are already producing.
While this is all true, some breeds tend to grow heavier coats than others, no matter where they live. One way to control the length and thickness of a horse's winter coat is to put the horse “under lights,” meaning lights in the barn are left on to trick the horse's brain into thinking the daylight hours are longer.
Read more at Horse Channel.
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