22/12/2025
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Thermoregulation in the Horse
Brian S. Burks, DVM
Diplomate, ABVP
Board Certified in Equine Practice
To blanket or not to blanket, that is the question. Mammals require internal body temperature to stay within a fairly narrow range. In horses, this is 99.0 to 100.5 F. Temperatures above or below this range alter the biochemical reactions that occur on a cellular level. Death can be the result. Most horse owners are aware of the damage and crisis inherent with fever states. Few horse owners realize how well adapted horses are to deal with cold when certain aspects of their lifestyle are in place for them.
Over thousands of years, wild horses roamed the earth, exposed to all sorts of temperatures and climates. Horses have adapted to these climates, and can survive in very hot climates to those well below zero, although they are isothermic at 40-500 F. Think about Arabians in the Egyptian desert (1200 F) or the Yakutian horses in northern Siberia where temperatures may be -940 F Whether it is the north of Europe, or Australian deserts, the horse is exposed to all of Nature’s changing elements — wind, sun, rain, snow, fluctuating temperature, etc. Actually, horses thrive in moderately cool climates. They have survived without blankets.
Genetically, the domestic horse is the same as its wild counter-part, and they require little more forage and basic shelter. They need freedom of movement, 24 hours per day, with access to food, mainly in the form of hay or grass. Humans tend to anthropomorphize, changing stabling conditions, eating habits, blanketing, etc. This is not to say that horses do not require shelter- they do as outlined below.
The hypothalamus controls the set-point, or normal temperature. There are both central and peripheral receptors, the former in the hypothalamus, and the latter in the skin. Horses detect changes in ambient and core body temperature (which is not the same as re**al temperature) and activate feedback mechanisms to bring body temperature back to the set-point. Central receptors are most responsive to heat, while skin receptors respond mainly to cold. These peripheral receptors may also be found in the spinal cord, abdominal organs, and around larger veins. Body temperature is regulated by efficient anatomical, physiological and behavioral thermoregulatory mechanisms. It is easier for a horse to warm up in cold weather than to cool down in hot weather following intense exercise.
The skin and hair coat of the horse are excellent insulators, preventing heat loss. Check out the back of an un-blanketed, but well-fed horse and you will see that the snow does not melt, or at least only slowly as the heat is being kept internally by natural insulation. Most of the heat created in the horse comes from digestion in the colon, or hindgut. As microbes digest forage, a byproduct is heat. Muscular movement generates heat, so moving through the pasture to the next pile of hay helps keep them warm.
The skin protects the interior of the body from environmental temperatures- both cold and hot. It keeps heat in, but helps to dissipate heat when the internal temperature increases above its set-point. Heat is lost from body surfaces by radiation, conduction, and convection.
The skin’s thermoregulatory mechanisms consist of four major factors: skin, coat, arteries and sweat glands. The hypothalamus responds to low body temperatures by causing cutaneous vasoconstriction and piloerection.
1. The skin itself works as an insulating layer through its relative thickness.
2. The coat insulation depends on the length and thickness of the hair layer, the wind speed, and the temperature and humidity gradients within the coat. The hair coat of the horse changes twice annually, adapting mainly to the photoperiod, and, to a lesser degree, temperature. Piloerection refers to the use of skin muscles to raise or lower the hair coat. This varies the amount of air trapping over the skin, thus forming an insulating layer, preventing heat loss. Piloerection increases coat depth by 10-30%. The hairs are covered with sebum- a greasy substance from skin glands- keeping the horse dry during rain or snow by its water-repelling effect, allowing water to run off the outer hair, keeping deeper hair dry. Longer coats keep out more water. Horses with flat coats are warm; if the coat is puffed up, they are protecting themselves from cold.
3. Arteries in the skin have muscles that allow them to constrict or dilate, regulating blood flow to the skin. Constriction reduces the amount of warm blood at the surface, preventing heat loss. Dilation allows for a larger amount of hot blood from over-heated interiors to reach the body surface and to be cooled. The cooled blood lowers internal body temperature when it returns to the body interior. This is why hypothermic patients should not imbibe alcoholic beverages- it causes vasodilation and loss of internal heat. Under normal conditions, the skin receives 10 times more blood than is required for its nutrition. This can change 20 fold in response to heat or cold.
4. The horse uses sweat glands to cool down when external or internal temperatures are too hot. When the outside temperature is too high for the air to cool the blood through the skin, the sweat glands secrete fluid. Evaporation cools the skin surface and the blood in the surface arteries, allowing cooled blood to lower body temperature even when it is hot outside. Evaporation also occurs due increased respiration; there is always some evaporative loss from the respiratory tract. The horse stops secreting sweat when body temperature returns to normal. A sweaty horse turns its coat hairs in various directions to allow cooling, and, given freedom, seeks a windy spot to effectively cool and dry itself.
Body fat is also important in thermoregulation. Body fat serves as an energy reserve, but it is also three times the insulating capacity of other tissue types. This is because it does not conduct heat well and has relatively fewer capillaries. Horses in the wild have excessive forage in the spring and summer, which becomes stored as fat, to be used in the winter. This is a natural rhythm that has been disturbed by many modern day practices, and it leads to more cases of laminitis and other obesity related problems. Fat gets distributed more evenly over the body surface in cold conditions instead of being concentrated in some particular areas as in hot conditions.
Forage intake increases heat production in the horse’s body. Heat is a by-product of hind gut fermentation. It is important that every domestic horse has access to hay 24 hours a day during cold weather to increase heat production by continuously consuming and digesting long fiber. When weather changes cold suddenly, this is especially important as other thermoregulatory mechanisms are not prepared.
This is called climatic energy demand. Horses require about 0.2 to 2.5% more energy for maintenance per degree Celsius drop in outside. Smaller horses and neonates have heat loss that is relatively greater compared to larger horses. Thus smaller horses actually need proportionally more additional hay. Smaller horse breeds lose more heat than larger horse breeds in the same temperature conditions. Large horses have less relative surface area available for heat exchange, and thus lose less heat in the cold compared to smaller horses. In addition to large body size, a spherical body shape reduces the surface area to body mass ratio, allowing greater heat retention.
Kept in the same conditions, smaller horse breeds have a longer/thicker coat compared to larger breeds. Typically foals also have a thicker coat. Generally, large body size is an advantage with respect to thermoregulation in the cold, because the ratio of heat-dissipating surface area to heat-producing/retaining body mass decreases with increasing body size.
Feral horses reduce locomotor activity during the winter to decrease energy expenditure during times of reduced internal heat production from less forage intake. Domestic horses have the same decrease in exercise, but humans provide them with food, negating the need to look for food as would their wild counterparts. Therefore exercise of domestic horses during the winter may need modification, or coats may need to be clipped to prevent overheating during heavy work.
Horses can be observed standing or lying close to one another to reduce heat loss via radiation. This position reduces the body surface area exposed to the external environment. Close position also helps those that cannot produce enough body heat. Horses can also be observed to sunbathe in the direct sun, absorbing solar radiation, instead of eating on short, sunny winter days, only to return to eating when the sun sets.
Snow along the horse’s back provides an extra layer of insulation against internal heat loss.
On windy, rainy days horses can be observed standing with their tails to the wind and their heads held low, keeping their heads and necks out of water and wind, while using their tails to protect their hind quarters, as the hairs on the dock deflect snow and wind. Horses will use natural windbreaks such as trees or hills.
Under extreme circumstances, horses generate heat by shivering. During shivering, heat is rapidly produced by breaking down ATP in the muscles. Shivering is usually an acute response to sudden cold exposure, but it may occur during extended periods of exposure to cold in rainy weather. It increases heat production by five times normal. In healthy animals, shivering is replaced by normal internal heat production as they adapt to new weather conditions. Shivering is a sign of extreme cold in horses.
So do horses require blanketing? The simple answer is no, blanketing is not necessary. With proper amounts of forage and shelter from wind and rain, most horses do just fine. This is not true of young, geriatric, or sick horses; these types may not be able to produce enough heat and may need blankets and additional shelter. Most unclipped horses can tolerate temperatures down to 14 F.
Blanketing a horse defeats the use of piloerection. The horse may actually be colder covered with a blanket. Remember that blankets do not generate heat; they simply keep in body heat. Horses in heavy work during the winter may need to be clipped (trace, torso, body, neck, etc.) to prevent overheating during work. These horses require blanketing as their primary means of thermoregulation has been removed- the hair coat.
When it is raining, snowing, and blowing, it is harder for a horse to stay warm; a blanket is advisable. Being wet and cold is obviously not a good thing. Your horse cannot take in enough roughage to generate enough heat to stay warm. You might think to feed extra grain, but remember that extra grain will predispose your horse to digestive disorders. If you are going to increase the grain ration, adding more times per day, i.e. 4 feedings instead of two, is advisable.
In general, horses do well when they can move about and have lots of fresh air. Moving keeps muscles limber and helps to keep the digestive tract moving. It also promotes respiratory health. Taking a few simple precautions can help your horse healthy during the cold winter months.
Fox Run Equine Center
www.foxrunequine.com
(724) 727-3481
