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Should I blanket?

Last revised April 24, 2017.

G, my Merry Go-Round horse, stands in the snow, blanketed. A nearby run-in shed is empty. All of the horses in his pasture have chosen the outdoor life on this day.

Before we get going: do horses have hair or fur? The answer is that they have both. Fur is made up of hair. A horse sheds its winter coat, but retains its mane and tail hairs when it does so, so the hair of mane and tail are different than the hair on the rest of the body. Like other fur-bearing mammals (such as foxes, otters, and mink), in winter horses have a short and thick coat for insulation as well as longer guard hairs that help wick water away. If we made coats from our horses, we’d sell the skins as fur. In the winter, my horse’s coat is every bit as useful to him as a dog’s (fur) coat is to him, and there doesn’t seem to be any difference between what a horse wears and what a short-haired dog wears. I’m happy to call the stuff hair or fur, but I am prone to call his mane and tail “hair” and everything else “fur”. But others all seem to call everything “hair” when it is on a horse (or in their sandwich). Stick with the naming convention, and don’t get in trouble.

In much of the world, horses never get blankets in the winter. In the eastern U.S., it is common in some areas to see horses with blankets when the temperature is 50°F or more. What’s the deal?

After a few minutes of reading the scientific literature, we discover that horses are great at producing heat. They have a high metabolic capacity and large mass — which produces heat — and a relatively small surface area — which dissipates heat. When it exercises, 20% of the metabolism in the muscles is used for work, and the remaining 80% becomes heat.1 In one study, clipped horses exercising at outdoor temperatures down to 16°F (-9°C) maintained their rectal temperature, and were able to return to a normal respiration rate more quickly than horses that had not been clipped.2 I can’t imagine running outside at 16° F without clothes, either before or after a haircut. And it wouldn’t be a pretty sight!

What the horse does with this extra heat depends on external factors. If the horse feels cold, it has reduced blood flow to the skin. But if it feels warm, then blood flow to the skin increases — vasodilation. If it gets too warm, it begins to sweat. Sweat cools the horse through evaporation.

Our horse should be able to tell us about his comfort with cold weather. If a horse is shivering, we know he is cold. If he is cold, is he suffering? One test would be to offer the choice of standing in the weather or standing in the run-in shed, assuming that there is plenty of room for him in the shed, and the same attractions — such as hay — are available in the shed as in the adjacent pasture. I have visited my horse on days of deep snow, when I found him standing just outside the run-in shed, his back covered in snow. He could have come into the shed, but chose not to. I stood in the shed and gave him carrots.

Adaptation to the Cold

Your horse may have a different notion about discomfort when it comes to the cold. We seem to prefer temperatures that are 60-75 ° F. Horses housed outdoors in the winter, on the other hand, have a “thermoneutral zone” of about 5°F (−15°C) to 68°F (20°C).3 Below 5°F, a cold-adapted horse must increase its metabolic rate to maintain his core temperature. Above 68°F, a cold-adapted horse must begin to sweat. Some like it hot, but horses like it cool. If we look at the temperatures worldwide where horses are often found, it is clear that horses are most often found in countries with cold winters, or cold climates.

Your horse can adapt to the cold, but it takes a little time. In the fall, as the days grow shorter, your horse will develop his winter coat. Day length, rather than temperature, is the likely trigger for developing a winter coat, so a horse kept in a well-lit barn may not develop as much protection as a horse who has a view of the sky during daylight hours.

The coat comes in gradually, over a period of a few weeks, so if the temperature fluctuates wildly from one day to the next, he will find himself under-dressed or over-dressed. Once his winter coat is in, he’ll be able to tolerate much colder weather, but still may find himself occasionally under-dressed or over-dressed.

When a horse needs more coat than he has, he’ll likely stand with his hindquarters to the wind, huddle with other horses, or use the run-in shed more often. He’ll also eat more, if hay is available. And he’ll shiver when he needs to. Shivering — involuntary muscle contractions — produces heat, and keeps him warm. It also burns calories, and leaves him hungry.

A horse’s physiological response to the cold is like our own. The autonomic nervous system directs blood in extremities away from the skin surface to internal organs. Near his skin, blood vessels constrict, and inside various arterio-venous shunts are closed. Skin temperature falls.4 During periods of exposure to extreme cold, a horse’s metabolic rate rises. If a horse is accustomed to winter temperature of 64° F (18°C), its metabolic rate may rise to 142% of basal metabolic rate values when exposed to -40°F (−40°C).5

Horses adapt to the cold. In fact, they adapt better to the cold than the heat: Horses try to maintain a body temperature of 99.5°F (37.5°C). If their body temperature drops by more than 10°C or rises more than 5°C, death results. Horses can’t stand the heat, and you won’t find them in the kitchen.

Adapting to the cold — acclimatization — involves raising the basal metabolic rate to increase heat production, and adding a coat to reduce heat loss. As with other large mammals, the horse adapts primarily through changes that reduce heat loss (small mammals adapt primarily through increases in heat production)6. This is accomplished with the development of a winter coat, and as needed, vasoconstriction to shunt blood away from the skin.

Shivering allows the horse to make a quick response to the cold, and is very useful until acclimatization takes place.7 Shivering breaks down sugars and fatty acids, doesn’t build up lactic acid, and can produces a 4- or 5-fold increase in the basal metabolic rate.8 Adapting to the cold also involves aerobic “non-shivering thermogenesis”, in which some specialized brown fatty tissues break down ATP, transforming its energy to heat.9

Hot-Blooded, Warm-Blooded, Cold-Blooded

There are two very different meanings to Hot-Blooded, Warm-Blooded, and Cold-Blooded.

  • The first meaning involves the climate they are best suited for: Hot blooded horses are adapted to hot climates; warm blooded horses are adapted to warm climates; cold bloods to cold climates. These adaptations come from degree of stockiness, length of the extremities, coat thickness, and more.
  • The second meaning involves the reactivity of the horse, its temperament. “Hot bloods” refer to horses that are spirited, bold, agile, fast, and smart. They may be considered difficult or passionate, and, it happens, include the same breeds as hot-blooded by our first definition: Thoroughbreds and Arabians.

In conventional English, the term “Warm Blood” usually refers to breed rather than preferred climate, implying Thoroughbreds and Arabians. A Google search finds 5,960 results for “hot blooded horse”, 7,340 results for cold blooded horse”, and 2,860 results for “warm blooded horse”. If nothing else, we know that the three terms are not used with equal frequency.

Here is a summary of cold and warm blood, from Langlois (1994)10:

Cold-Blooded: Heavier, shorter build in general with a high compactness index (chest circumference/height at withers). The extremities (tail, ears, limbs) are relatively short and are protected by hair, mane and fetlock. The skin is thick and conceals the blood vessels, there are substantial subcutaneous fat deposits which are readily built up or mobilized as the need arises. The animal is calm and very sociable with its fellows. While not capable of sudden effort, it has high endurance capacity, i.e. it can sustain low-powered effort for long periods. It has very considerable aerobic capacity and is apt to mobilize its body reserves for muscular effort.11 Examples: Belgian, Clydesdale, Percheron, Shire.

A cold-blooded horse.12

Warm-Blooded: Lighter, longer-limbed, more slender format. Extremities are longer, manes and fetlocks are less abundant, though this trait is less marked than in the kyang or donkey. The coat provides less insulation and is silkier. The blood vessels can be clearly seen through the skin, which is thinner. The animal is lean, more difficult to fatten, is excitable and less sociable with its fellows. It is capable of sudden, high-powered effort employing anaerobic metabolism. It then mobilizes considerable glycogen reserves in its muscles.13 Examples: the Dutch Warmblood, Hanoverian, Oldenburg, Trakehner, Holsteiner, American Warmblood, Belgian Warmblood, Irish Sport horse, Gelderland, Mecklenburder, and Austrian Warmblood.

A hot-blooded horse. Note that he has less diameter in his neck and trunk, and relatively longer legs than the cold-blood in the previous picture. With relatively less volume and more surface area, he will cool faster, be able to run faster and farther, but need blanketing sooner. This handsome boy is G, standing with the author.

Hot-Blooded: Hot blooded horses fit the description above for warm bloods, but do better in even hotter circumstances. They will need more blanketing than Cold Bloods or Warm Bloods. There are only two recognized hot-blood breeds: the Arabian and the Thoroughbred.

A horse — cold blood, warm blood, or hot blood — can adapt to the cold in your neighborhood, unless you live north of Iceland. But several factors may make life more difficult for Mr. Horse:

  • Rain changes the math. If you will blanket, then rain raises the temperature when you should add a blanket. If your plan is to blanket when the temperature reaches 30°F when it is dry, then do so when it is raining and 50°.
  • Clipping also changes the math. A horse exercising with a heavy winter coat may have trouble keeping cool because the coat limits heat transfer to the air, and the coat absorbs sweat rather than letting it evaporate from the skin. A clipped horse has less trouble keeping cool — less insulation, and sweat evaporates from skin, rather than fur. While clipping increases the efficacy of sweating in an exercising horse, he may have trouble maintaining body temperature in the winter when at rest.14 One authority15 recommends that a clipped horse be blanketed if it is raining and below 60° F or if it’s not raining but below 40° F.
  • The waterproofing of the blanket also affects the math. A blanket that is not waterproof will dramatically reduce the benefit of blanketing. On a rainy day, feel under the blanket to ensure that your horse is dry. And waterproof the blanket after every washing.
  • Housing affects the math, too.  In cool-acclimated horses at rest, the lower critical temperature of the environment has been reported to be 5°F (– 15°C16). In horses kept in stables at night and not acclimatized to winter climate the lower critical temperature has been estimated to 41°F (5°C17).

Your horse will need to eat more as it gets colder. In one study, energy intake increased by 1.8, 0.5 and 0.2 % in November, December and January, respectively, per 1 °C decrease in ambient temperature below the lower critical temperature of 12°F (–11 º C18).

There are drawbacks to blanketing.

  • The weight of a blanket mashes the fur down on your horse’s back. As the fur is compressed, it offers fewer pockets of trapped air, and less insulation.
  • Horses synthesize endogenous Vitamin D3 when their skin is exposed to sunlight. When horses are blanketed, it is likely that their production of this vitamin is diminished. (Shearing sheep increases their Vitamin D3 production,19 and blanketing does diminish Vitamin D3 production in cattle,20 but the effects of blanketing on Vitamin D3 production does not seem to have been studied in horses.)
  • Blanketing likely leaves a horse slightly less well-adapted to cold weather (though I have no evidence for this!)
  • Blanketing blocks the wind, which keeps your horse warmer. When the outside temperature is high enough, your horse will begin to sweat under a blanket. But because your horse and his sweat are under a blanket, you won’t know this unless you are making personal inspections or closely monitoring the temperature.
  • If blanketing results in sweating, your horse will lose water and electrolytes. These losses must be offset.
  • On many winter days, the temperature changes considerably. Blanketing which is done when your horse begins to shiver must be undone when your horse thinks about sweating. This will require attention to detail on your part. Even if you decide that your rule is “blanket on when temperature has dropped to 20° F, blanket off when it is above 50° F.”, you may have to blanket and unblanket daily.

There are benefits to blanketing, of course.

  • If it is precipitating, and the temperature is low, moisture will get past the unblanketed horse’s guard hairs and wet the skin, forcing him to shiver to keep warm. Whether or not the horse minds this condition, shivering may burn more calories than the horse can consume, and weight loss may result. So I recommend blanketing under any condition where the horse is shivering. One winter G lost quite a bit of weight when he was pastured 24×7, and the run-in shed was crowded. G did not enter the run-in shed because of other higher ranking horses already there, or from the fear that he would be trapped in the shed by such a horse when it entered the small doorway. A run-in shed benefits high-ranking horses much more than those of low rank in the herd.
  • A blanket on your horse will satisfy your maternal instincts when you are cold.

I believe that blanketing is unnecessary when a horse is in good health, with a normal store of body fat, has adequate hay, has had reasonable time to acclimate to the temperature, has not been clipped, and is not being exposed to rain or freezing rain.

I believe that blanketing is inappropriate if the blanket is not removed when the horse becomes too warm. Many horses in blankets may be found to be sweating when the temperature is 50° F or more. It is inappropriate if the blanket does not raise the body temperature of the horse, as would be the case with a heavy, poorly insulated blanket, or one that lacked waterproofing.

Guidelines

Virginia Cooperative Extension has offered these guidelines on blanketing:21

  • Horses that are clipped or kept under lights to discourage winter coat growth should be blanketed if it’s raining and below 60° F or if it’s dry and below 40° F;
  • Horses with a moderate to heavy coat will be fine without a blanket down to 20° – 30° F. Even then, they may be able to get by with a little more shelter or forage;
  • Horses that have recently moved from a warmer to a colder climate may benefit from blanketing the first winter, particularly if they come in when the weather’s already cooled;
  • Older horses, thin horses, or those that don’t move around much may benefit from blanketing.
  • Remove the blanket on a regular basis to check for rub marks, skin conditions, and body condition score.
  • Make sure the blanket is waterproof. If you reach under the blanket, the horse should feel warm and dry.

I would recommend that after reading this section on blanketing, you decide what temperature (dry and wet) you will blanket, and what temperature you will unblanket. Write it down, and stick to it. And keep your blanket waterproofed. For G and Bud, I’ve adopted this rule:

  1. Blanket at 30° when dry, 50° when wet.22
  2. Remove blankets at night in their stalls, and whenever the temperature climbs above 40° (dry), or 50° (wet).
  3. Wash and waterproof all blankets every year.

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1 Brody, Samuel, and Henry A. Lardy. “Bioenergetics and growth.” The Journal of Physical Chemistry 50.2 (1946): 168-169.

2 Wallsten, Hanna, Kerstin Olsson, and Kristina Dahlborn. “Temperature regulation in horses during exercise and recovery in a cool environment.” Acta Veterinaria Scandinavica 54.1 (2012): 1

3 Morgan, Karin. “Thermoneutral zone and critical temperatures of horses.” Journal of Thermal Biology 23.1 (1998): 59-61.

4 Charkoudian, Nisha. “Mechanisms and modifiers of reflex induced cutaneous vasodilation and vasoconstriction in humans.” Journal of Applied Physiology109.4 (2010): 1221-1228.; Wallsten, Hanna, Kerstin Olsson, and Kristina Dahlborn. “Temperature regulation in horses during exercise and recovery in a cool environment.” Acta Veterinaria Scandinavica 54.1 (2012): 1; Sessler, Daniel I. “Thermoregulatory defense mechanisms.” Critical care medicine 37.7 (2009): S203-S210.

5 McBride, G. E., R. J. Christopherson, and W. Sauer. “Metabolic rate and plasma thyroid hormone concentrations of mature horses in response to changes in ambient temperature.” Canadian Journal of Animal Science 65.2 (1985): 375-382.

6 Langlois, B. “Inter-breed variation in the horse with regard to cold adaptation: a review.” Livestock Production Science 40.1 (1994): 1-7.

7 Heldmaier, G., Klaus, S., Wiesinger, H., Friedrichs, U., & Wenzel, M. (1989). Cold acclimation and thermogenesis. Living in the Cold II, 347-358.

Chicago

8 MacAardle W.D., Katch, F. and Katch, V., 1987. Exercice et stress thermique. In: Physiologie de l’activite physique: Energie, nutrition et performance chap. 25,353-373. Vigot Ed., Paris.

9 Nicholls, David G., and Rebecca M. Locke. “Thermogenic mechanisms in brown fat.” Physiological reviews 64.1 (1984): 1-64.; Cannon, B., and J. Nedergaard. “Biochemical mechanisms of thermogenesis.” Circulation, Respiration, and Metabolism. Springer Berlin Heidelberg, 1985. 502-518.

10 Langlois, B. “Inter-breed variation in the horse with regard to cold adaptation: a review.” Livestock Production Science 40.1 (1994): 1-7.

11 Quotation from Langlois, B. “Inter-breed variation in the horse with regard to cold adaptation: a review.” Livestock Production Science 40.1 (1994): 1-7.

12 Image source: https://pixabay.com/en/photos/cold%20blooded%20animals/

13 Quotation from Langlois, B. “Inter-breed variation in the horse with regard to cold adaptation: a review.” Livestock Production Science 40.1 (1994): 1-7.

14 Wallsten, Hanna, Kerstin Olsson, and Kristina Dahlborn. “Temperature regulation in horses during exercise and recovery in a cool environment.” Acta Veterinaria Scandinavica 54.1 (2012): 1 .;Morgan, Karin. “Effects of short-term changes in ambient air temperature or altered insulation in horses.” Journal of thermal biology 22.3 (1997): 187-194.

15 Porr, Shea “Winter Care for Horses: When to Blanket”. Greiner, Scott Patrick, Mark A. McCann, W. Dee Whittier, C. A. Porr, and Kayleigh Mize. “Livestock Update. January 2012.” Virginia Cooperative Extension (2012).

16 Cymbaluk, N. F., and G. I. Christison. “Environmental effects on thermoregulation and nutrition of horses.” The Veterinary clinics of North America. Equine practice 6.2 (1990): 355-372.

17 Morgan, Karin. “Thermoneutral zone and critical temperatures of horses.” Journal of Thermal Biology 23.1 (1998): 59-61.

18 Autio, Elena. “Loose housing of horses in a cold climate.” PhD theses. Department of Biosciences, University of Kuopio, Finland (2008).

19 Quarterman, J., A. C. Dalgarno, and Agnes Adam. “Some factors affecting the level of vitamin D in the blood of sheep.” British Journal of Nutrition 18.01 (1964): 79-89; Hidiroglou, M., and K. Karpinski. “Providing vitamin D to confined sheep by oral supplementation vs ultraviolet irradiation.” Journal of animal science 67.3 (1989): 794-802.

20 Hymøller, Lone, and Søren Krogh Jensen. “Vitamin D 3 synthesis in the entire skin surface of dairy cows despite hair coverage.” Journal of dairy science 93.5 (2010): 2025-2029.

21 Porr, Shea “Winter Care for Horses: When to Blanket”. Greiner, Scott Patrick, Mark A. McCann, W. Dee Whittier, C. A. Porr, and Kayleigh Mize. “Livestock Update. January 2012.” Virginia Cooperative Extension (2012).

22 In the Washington, DC area, the average low temperature is below 30 degrees only in December, January, and February, and those temperatures are normally only at night, when Bud and G are in their stalls. In those months, the average daily high temperature is just under 50 degrees. So on normal days, any day of the year, blanketing is not going to happen. On days in those months when the temperature gets below freezing or it is raining, I will blanket them during the day.

 

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