Here a couple of articles based on scientific studies explaining how complex and efficient the horse's body is at adapting to a wide variety of climatic conditions and very well adapted to cold climates... providing it is allowed to live in conditions approximating those in nature! Yes, that means outside 24/7, in a herd, free choice access to forage and without a blanket!
I am always amazed to see horses in FLORIDA wearing a blanket! Our horses here in Canada live outdoors in temperatures often dipping well under -30C for long periods of time (weeks), and they are absolutely fine as long as they are allowed to grow a winter coat, have room to move, are getting plenty of free choice hay and have access to a wind breaking shelter. They are very healthy and thriving, and much less sensitive to infection and disease as their immune system is fully functional. They do grow lots of hair, and yes, we do ride them and drive them despite the winter coat! As long as their breathing and heart rate are back to normal, a good roll in the snow is the horse's way to dry off and fluff the coat. We have yet to ever seen a horse catch a cold as a result of being turned out damp!
This has been widely attributed to a Colorado State University study, but CSU has actually denied doing such a study. However, there are other studies to corroborate the information below, and years of field observation of horses kept in natural environments will also validate.
Horses have the ability to loft and lower their coats to many different levels, so it's like exchanging different thermal weights of blankets off and on them all day and night, depending on what they need, without human interference. Their 'self-blanketing' process works a little like 'chill bumps' do on human skin. That is why long-haired horses may seem fluffier on some days than on others. Only three things make the 'self-blanketing' process not work: blanketing, clipping, and wind. Not even snow or rain stops their internal thermostats from doing the job.
The thermoneutral zone (TNZ) for horses is defined as the range of temperatures in which the horse maintains its body temperature with little or no energy expenditure. Essentially, the TNZ is the temperature range wherein the horse does not have to work to raise or lower its body temperature. TNZ for horses is when the ambient temperature lies between 5 and 20-25 degrees Celsius (depending on circumstances *). Outside of the TNZ, they are using energy to control their body temperatures. Since they are cooling their bodies when the temp is over 20-25C, they are having to use extra energy to cool themselves when blanketed in warm temperatures.
Shivering is one of the ways the horse's body uses to warm itself; movement is the other one. Moving generates a considerable amount of heat for a horse, but they can also stand and shiver while napping or resting. It does not necessarily mean that they need to be blanketed. However, a horse MUST have a way to get out of the wind in order for their 'self-blanketing' abilities to function fully. While blanketing makes the human feel good about horse care, it does not fill a need of the horse. Many horse owners think that a blanket is a necessary part of good horsekeeping. It is actually an item that is very seldom required - provided the horse is allowed to grow a seasonal coat, is healthy and in good weight, has had time to acclimatize to the weather and is kept in conditions allowing lots of movement day and night, ample forage and the ability to interact with other horses.
* The wide range of the LCT (lower critical temperature) in adult horses extends from 41° F (5° C) for horses in mild climates to 5° F (-15° C) in natural-coated/unclipped horses adapted to very cold temperatures. Young horses and foals have higher LCT than adult horses. Defining the UCT (Upper Critical Temperature) is more difficult and researchers have suggested that it can range from 68-86° F (20-30° C). Source - Equinews, Kentucky Equine Research Staff.
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