After a ride across the mountain to check my horses on winter pasture, the sun had set and the temperature was dropping toward zero. I didn't want to leave my mare wet and chilling; she needed her coat dry and fluffy for it to be effective insulation. My fingers were stiff with cold, but I had to rub her sweaty long hair dry with towels and turn her out in her pen before I could go indoors and soak up the welcome heat of a wood stove.
Our horses handle cold much better than we do, and my ranch horses in Idaho have managed nicely outdoors, even at 40 below zero. They have several unique ways to stay comfortable in severe weather and do well if allowed to adapt to colder temperatures gradually.
Winter Hair Coat
As days get shorter and nights become cooler, horses grow a new, longer hair coat. These winter hairs stand up, trapping tiny air pockets between them. The effect is like that of a thick, down-filled comforter, with tremendous insulating quality.
"When I lived in Wyoming, occasionally someone would bring horses from Texas," says Bruce Connally, DVM, field service veterinarian at Colorado State University. "Their first winter in Wyoming would be traumatic because their bodies weren't programmed to grow that much hair. They'd lose weight and get sick. If you get these horses through the first winter, however, and let them go through a Wyoming summer, then the next winter they are able to adapt and grow hair."
The horse also needs a full mane and tail if he's spending time outside. Standing with his rump to the wind, his tail protects his delicate underparts, and he can lower his head so it's shielded from the wind by the rest of his body.
Oils in the hair coat help it shed moisture. A snowstorm in cold temperatures is comprised of dry snow; moisture freezes on the outer surface of the hair coat and never reaches the horse's skin. The insulating air pockets don't allow body heat to escape, so snow on a horse with a normal winter hair coat won't melt or make him wet.
"A spring storm can be worse (than a winter blizzard), at warmer temperatures, because the snow is so wet. The hair gets soaked and loses its insulating quality," says Connally. If wet snow or rain is prolonged, it eventually soaks through the horse's coat. The hair loses its insulating quality when lying flat, and the skin becomes wet and cold.
In cold, wet weather, horses should have shelter where they can escape the rain or snow. "If it's wet, horses need some trees or an open-sided shed they can get under," says Connally. "If there's no shelter, a waterproof blanket could be very useful."
Newborn foals also need protection from the elements (via shelter and/or blanket), especially when they are still wet after birth. Their circulatory systems are focused on conserving heat and keeping the body core warm (for survival) rather than the extremities. And because a newborn foal has a short hair coat and very little body fat for insulation, he is at high risk for cold stress or frostbite.
Feet and Legs
The horse's feet and lower legs are designed to handle cold without freezing and without chilling the rest of the body. Therefore, a horse can stand in deep snow and not suffer frostbite. Pamela Wilkins, DVM, MS, PhD, professor of equine internal medicine and emergency/critical care at the University of Illinois, says horses do not have muscle masses below the knee or hock. "The lower leg is mostly tendon and bone, which resist the effects of cold much better than muscle. These are not energy-requiring tissues, in comparison with the rest of the body," says Wilkins.
There's an old saying that the horse has an extra heart in each hoof. The frog, digital cushion, and a mass of veins are all part of this elaborate system. Each time the foot takes weight it pumps blood back up the limb.
"The blood flow in the foot is also part of the cushioning effect when the foot hits the ground; it creates a hydraulic, fluid cushion like a gel pad," says Connally. "If there's enough blood to create a hydraulic cushioning effect for a 1,200-pound horse's foot hitting the ground, there's a lot of blood going through there."
Julia Wilson, DVM, Dipl. ACVIM, associate professor of large animal medicine at the University of Minnesota, points out that the shunting mechanisms in horses' feet also benefit them in cold weather. "According to my spouse, Dr. Tracy Turner (DVM, MS, Dipl. ACVS), horses intermittently shunt the blood away from their feet. This may be a coping mechanism that keeps their cold feet from chilling the rest of their body when standing in snow. It also may be one of the reasons we sometimes see laminitis in winter. If you have a horse with chronic or subclinical laminitis (perhaps due to metabolic syndrome), and he needs to intermittently shunt the blood away from his feet in cold weather so they don't get too chilled, this may be what exacerbates laminitis flare-ups," says Wilson.
Frostbite in an adult horse is rare and usually due to an accident such as falling through an ice-covered pond. There are also some plant toxins that hinder blood circulation, such as feeds contaminated with ergot, or endophyte-infected fescue grasses. If a horse eats these, circulation to extremities might be impaired and ears might freeze.
Daniel Kenney, VMD, Dipl. ACVIM, of the University of Guelph's Ontario Veterinary College, says horses generally have good blood cir-culation to help keep extremities from freezing. "If it becomes acutely cold, however, they refocus circulation to vital internal organs, leaving ears, tail, etc., more vulnerable to frostbite," he says.
"We don't see many frostbite cases here (at the Ontario Veterinary College) because we are a referral hospital, but I have seen a few," says Kenney. "One was a youngster not used to the cold that was put outdoors and suffered frostbitten ears. Another case was a horse that had broken through ice on a pond and was immersed in cold water. That horse came in very hypothermic. We had to stabilize his blood circulation and gradually warm him up."
He warns that if you warm a cold animal too fast, blood vessels at the skin surface dilate, bringing more blood to the body surface. "This takes away essential circulation from the body core, and this can be life-threatening," he says.
Dipping temperatures can be especially problematic for certain equine populations. Julia Wilson, DVM, Dipl. ACVIM, associate professor of large animal medicine at the University of Minnesota, says, "Cold weather creates challenges for horses that are hypothyroid (thyroid hormone-deficient); their ability to thermoregulate is compromised. As a veterinarian, I tell owners that hypothyroid foals are severely at risk for hypothermia ... Another group that can have problems (in cold weather) are obese donkeys that develop hypothyroidism," she says.
Wilson also advises owners to make sure animals have a good dietary source of iodine to prevent cold intolerance and hypothermia (low body temperature). This nutrient can be provided in a trace mineral salt or added in feed. "Owners need to be aware that the iodine in a trace mineral salt block can leach out. Ideally, horses should have a covered source of mineral salt so it won't get wet," says Wilson.
Heather Smith Thomas
The horse's respiratory system is designed to warm incoming air. "It is extremely well-adapted for air warming, just as it is for dissipating heat in summer," says Wilson. Cold stress on the lungs, therefore, is minimal. By the time the air gets through the upper airway (and the moderating effect of the guttural pouches, the two air-filled cavities at the base of the horse's skull), the air is warmer and not as abrasive to the lungs.
Horses with adequate nutrition start building a layer of fat under the skin as days get shorter and nights grow colder. Some horses put on fat readily, while others need more nutrients to gain the extra weight and body fat.
"Wild horses, or even horses that run on large pastures on ranches year-round, eat all spring and summer and go into fall with body condition scores of about 6 on a score of 1 to 9--with 1 being extremely thin and 9 being obese," says Connally. "Mother Nature wants them to have extra fat for winter, to serve as insulation as well as calorie reserves. By the time they come out of winter in the spring, they've dropped to a body condition score of 3, but they survived. Then they gain weight again through summer and are fat by the time winter comes around again."
According to Connally, modern-day horsemen have disrupted this natural cycle. "We want to ride horses in summer and grain them because they're working and get them up to a body condition score of 5 or 6," he says. "Then winter comes and we put them in the barn, put a blanket on them, keep the barn warm, and feed them extra because it's winter--and by spring they are too fat to ride, which can usher in a host of other obesity-related problems."
Connally says the natural thin-fat-thin-fat cycle is better for horses than just getting them fat and fatter. The horse's fat layer holds heat in during winter and is gone by spring. The horse can then more readily dissipate heat when working hard.
Monitoring body condition is important during winter to know whether horses are gaining, losing, or holding their weight. At the end of a working season, horses that have been in fit, athletic condition without much fat need time in the fall to gain more body fat. Wilkins recommends letting those horses gain at least 5% in body weight (50-60 pounds) to give them some reserves to draw upon for heat energy.
Wilson also recommends feeding fat to older horses to help provide them with the extra calories needed to maintain their body condition. "Geriatric horses are hard to keep weight on, with the need for extra heat generation combined with an aging digestive tract that's not efficient anymore," she says. "High fat supplements are a wonderful tool to help horses in that stage of their lives."
"The horse is a digestive system on four legs," says Barney Fleming, DVM, of Custer, S.D. "If he's able to eat more food, he can stay warmer. Grain digests faster than forage and creates heat more quickly, but doesn't last as long." Fermentation of roughage such as grass or hay, on the other hand, takes longer and the hindgut creates heat more steadily as a byproduct of that microbial fermentation. Therefore, if a horse grazes most of the day or eats hay several times a day (keeping the digestive tract full of forage), he will have a constant source of heat energy.
"In cold weather (horses) should have fibrous food in front of them at all times and be able to walk around--which also helps with gastrointestinal motility," says Wilson. "Horses have their own internal furnace, due to being hindgut fermenters. You should increase the proportion of hay in the diet as outdoor temperature becomes colder."
"The horse has a huge circulatory system, which moves the heat around," says Fleming. If heat needs to be dissipated, blood vessels are right under the surface of the skin, and heat escapes into cooler air around the horse. In cold weather the physiology of the horse is programmed to retain heat, rather than dissipate it.
The horse's muzzle is richly supplied with blood, so these tissues rarely freeze. "Ears are most prone to freezing because those tissues are thin, but there is a lot of hair around the ears to help protect them and a lot of blood circulating in the skull--which helps keep the ears warm," says Fleming.
We often worry about horses that shiver, but this is just another mechanism to generate warmth, burning fuel in the muscles. "If they only do it for a little while, they'll be fine," says Connally. "They shiver, then maybe run and buck and do whatever it takes to get warmed up. But if they can't stop shivering for hours because they are soaking wet, you need to help them." Otherwise they'll eventually run out of "fuel" to keep up their body temperature.
This combination of physiologic features makes the horse uniquely prepared to handle the chilly months. "As horse owners, we need to respect their adaptations and not try to monkey with Mother Nature too much," says Wilson.
If you have lingering questions about your horses' management during the winter, especially in particularly frigid climates, ask your veterinarian for tailored advice for your situation.
About the Author
Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog, www.heathersmiththomas.blogspot.com, she writes a biweekly blog at http://insidestorey.blogspot.com that comes out on Tuesdays.