By Brian S. Burks, DVM, Diplomate, ABVP, Board-Certified Equine Specialist
Horses handle winter much better than humans, even at 40 below zero, if allowed to gradually adapt and in good body condition. There are some breeds of horse that have adapted for even colder temperatures, including the Yakut, Icelandic, Bashkir, and Finn. I remember an older Arabian stallion that refused to come in from a winter storm at 40 below zero. He did just fine.
Horses grow a long, thick coat in the winter that insulates them very well. The coat begins to grow when the days grow shorter, until winter solstice as the days become shorter. Horses begin to lose their winter coat, forming a summer coat, as the days begin to lengthen after the solstice. The oils on the coat help it to shed moisture (donkeys lack oils). A snowstorm in cold temperatures is comprised of dry snow and moisture freezes on the surface of the coat, never reaching the skin. Horses fluff up their coat by a process called piloerection, which traps air next to the skin, providing an insulating layer that prevents heat escape. Snow on a normal winter coat will not melt or make the horse wet.
Blanketing a horse with a heavy winter coat compresses the hair and reduce its insulating ability, resulting in a reduced ability to stay warm. Horses that are body clipped, thin, do not have a thick coat, or geriatric may need a blanket.
Blanketing a horse is necessary to reduce the effects of cold or inclement weather when:
Wet snow or rain will eventually soak through the horse’s coat, losing its insulating quality and the skin becomes wet and cold. The first year horses move from a warm climate to a cold climate, they will not grow a thick coat, but the next year they are primed to do so.
Horses also need a full mane and tail to be outside. They will stand with the rump to the wind, so the tail protects the underparts, and the head is lowered to protect it from the wind by the rest of the body.
Horses should have shelter to escape cold, wet weather. They can tolerate cold temperatures better if they can get out of the wind. If there is no shelter, a waterproof blanket may be appropriate. Free access to shelter from wind, sleet, and rain via access to a stall or run-in shed works well. Tall trees can serve as a wind break but may not protect against the wet. If horses are not wet, and are out of the wind, they can tolerate temperatures below zero F; with shelter, they can tolerate temperatures 40 below zero F, although most horses are most comfortable at temperatures between 18- and 59-degrees F, depending on their hair coat.
Neonatal foals need protection from the elements via shelter and/or a blanket. Newborns have a short coat and very little body fat for insulation, so they are at high risk for freezing and frostbite. Smaller horses have a greater surface area relative to body weight and lose heat more rapidly than a larger one. Foals and weanlings reach their lower critical temperature before a mature horse.
A 240-square-foot run-in or open-front shed (i.e., 12 x 20 feet) is ideal for two horses. You should add 60 square feet (i.e., an additional 10 x 6 feet) for each additional horse. These sizes are ideal only if the horses housed together get along.
Horse limbs are designed to handle cold without freezing and making the rest of the body cold. Horses can stand in deep snow and not suffer frostbite. There is no muscle below the carpus and tarsus, the lower limb being mostly tendon, ligament, and bone, which require less energy than the rest of the body tissues. Every time a horse takes a step, the frog and digital cushion pump blood up the limb. There are also arteriovenous shunts in the feet that allows intermittent movement of blood away from the feet, leaving them cold, but conserving heat in the rest of the body.
Horses with adequate nutrition build a layer of fat under the skin as daylength shortens. Eating all spring and summer should have a horse go into winter with a body condition score of 6 (1 emaciated, 9 obese). The fat lay provides insulation and calorie reserves. Many horses will drop weight in the winter in the wild when food becomes scarce, which is how they were designed. Humans interfere with that cycle by continuing to feed them well and blanket them during winter so that they become too fat and then suffer from a host of obesity related problems. To avoid losing weight, horses must increase their caloric intake roughly 15-20 percent for every 10-degree drop in temperature below 30 degrees Fahrenheit.
This fat-thin-fat-thin cycle helps horses dissipate heat in the summer when they are working harder since they do not have the fat layer holding in the heat. Monitor your horse’s body condition during the winter- feel them under the hair- so that they do not lose too much weight, as can happen with older horses. In the wild those horses are more likely to get eaten. Geriatric horses may require a high fat diet and blanketing for the winter.
Water is crucial during the winter months. The incidence of colon impaction significantly increases during the coldest months due to the lack of water intake and exercise. Horses do not like to drink cold, icy water, preferring tepid temperatures between 45-65 degrees Fahrenheit. Most 1,000-pound adult horses need at least 10-12 gallons of water daily. During the summer, grass is 6-80% moisture, but hay is less than 15% water. The combination of dry hay, decreased water intake, and lack of exercise leads to colon impaction. Do not let water freeze in the winter as this can lead to salt toxicity; however, adding several tablespoons of table salt to your horse’s feed several times per day can encourage drinking water and prevent these problems.
The respiratory tract warms air as it enters the horse, lessening cold stress on the lungs. The turbinates and guttural pouches help to warm the air before reaching the lower airways.
In cold weather, the circulatory system is designed to conserve heat; superficial vessels constrict, keeping heat deeper in the body. The muzzle is well supplied with blood and rarely freezes; however, ears are thin and more prone to freezing, which is why hair covers them well and there is a good blood supply.
Shivering is a warming mechanism, burning energy in the muscles. Although it indicates that they are cold, they can do this for short periods, but if they do not stop for hours, intervention is necessary before too much energy is used.
The digestive tract is the powerhouse that creates heat by fermentation of grass or hay as a byproduct of microbial digestion. Keeping the digestive tract full of hay provides a constant source of heat. Increase hay to keep the horse warm, not grain, which will not last very long. For every degree below 18° F the horse requires an additional one percent energy in their diet. If a 1000-pound idle horse needs 16 pounds of good-quality hay daily when the temperature is 18° F, its requirement may increase by approximately 2 to 2.5 pounds to 18 to 18.5 pounds if the temperature drops to 0 degrees F. The increased dietary energy requirement is even greater if the horse does not have access to shelter.