When the mercury rises or drops dramatically, will your horse be prepared?

In October 2013 South Dakota livestock and farmers were experiencing balmy 70- and 80-degree temperatures when a storm moved in from the Rockies and a cold front from Canada. The collision of the air masses created heavy rain, winds up to 70 mph, and a dangerous blizzard. Many cattle drifted with the storm, piling up against fences, getting covered with snow, and freezing to death because they were soaked with rain before the snow and cold temperatures set in. Though there were some equine losses, outdoor horses generally fared better than cattle because they’re more adept at finding windbreak and shelter. But horses with no reprieve from the elements likely suffered cold stress and frostbite.

Similarly, albeit not so drastically, horses might have a tough time adjusting to the elements when moving from a cold climate to a hot one (or vice versa) or when body-clipped during a serious cold snap.

When horses have a chance to adjust gradually to seasons changing, they typically tolerate heat and cold well, says Bob Coleman, PhD, PAS, associate professor and equine extension specialist at the University of Kentucky, in Lexington. It’s the combination of temperature extremes and swings that can be stressful. Here’s how to help your horse handle these conditions and battle cold or heat stress.

When the Going Gets Cold

Temperature fluctuations in early fall or late spring—when the horse hasn’t grown his winter hair coat yet or has already shed out—can be harder on horses than prolonged summer heat or winter cold.

To protect horses from severe weather changes in winter, “we just need to provide a good windbreak or open-faced shed they can go into if they choose,” says Bruce Connally, DVM, owner of Wyoming Equine, in Berthoud, Colorado. “These precautions can moderate temperature swings, as long as the horse is healthy and has adequate nutrition. The best ‘blanket’ we can put on them is a little fat, for ­insulation. With a good hair coat and a layer of fat under the skin, they don’t lose much body heat.”

A thin, undernourished, sick, or stressed horse can’t handle sudden cold, especially if he doesn’t have or isn’t consuming enough forage; the fermentation of fiber helps him generate body heat. But as for healthy horses, “they know how to regulate their own comfort,” Connally says. “When we restrict them and either lock them in or out, we make it harder for them to do their own thermoregulation. They generally prefer to be outside.”

Kent Allen, DVM, FEI veterinary delegate and owner of Virginia Equine Imaging, in Middleburg, agrees, explaining that horses evolved to handle cold. “You see horses at pasture on a very cold day, with their 2-inch hair coat sticking straight out, and they are happy as clams,” he says. “Horse owners tend to anthropomorphize. If we are chilled and uncomfortable, we think the horses are, too.”

A healthy horse with a good winter coat can handle even a blizzard well. “A spring storm can be worse, at warmer temperatures, because the rain or snow is so wet,” Connally says.

He explains that hair’s natural oil has a waterproofing effect, causing moisture to slide off before it reaches the skin. Prolonged rain or wet snow, however, eventually soaks through, causing the hair to lose its insulating quality.

But if your horse is fully clipped, he will need appropriate shelter and blankets well into early spring, says Carey Williams, PhD, associate professor and extension specialist in equine management at Rutgers, the State University of New Jersey, in New Brunswick. She also recommends riding him with a quarter sheet during warmup and cool-down.

Your horse’s ability to adapt to cold snaps might depend on his breed. “A Shetland pony or a Norwegian Fjord is more comfortable in cold weather than a Thoroughbred with thinner skin and less hair,” Connally says.

If you anticipate a temperature swing toward freezing, a few simple diet changes can help your horse stay warmer.

“He needs as much high-fiber hay as he wants,” says Connally. “He also needs plenty of water that’s not too cold. If he doesn’t drink enough because the water is cold or frozen, he won’t eat enough and won’t have the calories needed to keep warm.”

In these cases you might need to fill water buckets more frequently to keep the water from becoming too cold to drink, says Coleman.

The temperature below which a horse starts to expend additional energy to warm himself is called critical temperature. “As a general rule, a 1% increase in energy … is needed (to replace what’s lost from the cold) for each degree the temperature falls below a horse’s critical temperature,” says ­Williams.

Critical temperature for individual horses varies based on differences in fat cover, hair thickness, acclimatization to cold, hair coat wetness, and windchill. “Clipped horses have a much higher critical temperature and must be blanketed,” she says, adding that shivering can be an indication the horse is too cold and needs shelter or blanketing.

To help horses lower their critical temperature (so it won’t take as much energy to stay warm) and help them adapt to colder temperatures, owners need to provide extra fat for energy and insulation heading into winter.

“If seasons are changing you can make slight adjustments, making sure the horse gets a little extra hay as you go into winter,” says Coleman. “It’s not wise to make major dietary changes just because the temperature changed today. We create problems for horses when we make changes faster than we should or need to. It helps to plan ahead so we can make some gradual changes.”

Sudden Heat Stresses

During periods of extreme heat horses need shade and airflow to stay cool and promote sweat evaporation, says ­Coleman.

“Often the horse just wants a little shade for his head,” he says. “Run-in sheds are great, but horses don’t like to go clear into them for shade because a shed is too sealed up for much air flow and it’s hot inside.” They prefer a breeze, even if that means standing out in full sun to get it.

Take care when exercising horses in hot conditions. “During exercise, heat production in the horse’s body will increase up to 50%,” says Williams. “In response to heat, the horse will sweat more, move a large portion of the blood flow to capillaries under the skin, and increase respiration rate to help the cooling process. Always cool horses properly after exercise, allowing plenty of time and ventilation, and avoid riding in extreme heat and humidity.”

A high resting heart rate can be a sign of heat stress. “At rest, normal range of heartbeats per minute is between 24 and 40. In a heat-stressed horse it can be over 50,” says Williams. “Internal rectal temperature will also be elevated to 104 degrees F or higher.”

One way to help prevent heat stress in hot weather is to make sure horses drink enough and to provide electrolytes if horses are training hard and sweating profusely. You might also want to make dietary adjustments, such as feeding more fat and less protein. 

Connally says horse owners shouldn’t feed a lot of fiber in hot climates because of the heat it generates. “You walk a fine line,” though, he says, “because if you get the fiber too low you create other problems, including colic, because the digestive tract needs a certain amount of fiber to function normally.”

Rich alfalfa hay or any high-protein diet can also be counterproductive for hard-working horses in hot conditions because the protein-metabolizing process likewise produces heat, along with increasing water requirements. Adjust the diet to fit the conditions your horse is working in.

If a horse does suffer from heat stress or exhaustion, he probably needs a veterinarian to administer intravenous fluids, says Connally. “This is the quickest way to replace what he’s lost and prevent severe dehydration,” he says. “If you are out on the trail and this is not an option, simply stop, take off all tack, and cool him with water over his body as best you can. If you have access to (rubbing) alcohol, pouring it over the body (or adding some to the water you put on the horse) will aid evaporation and speed the cooling effect.”

Handling Temperature Swings 

Again, left to their own devices, horses can usually handle temperature changes. It’s when we alter their natural condition and confine them that they tend to have trouble.

“If you are bringing a horse from Florida to Mid-Atlantic or northeastern states in winter, he will need help to stay warm,” says Williams. “When taking a horse the other direction, it will be hard to handle the heat, and he may need to be clipped. Then if there’s a cold snap, he must be blanketed.”

​For this reason Coleman says he’d rather transport horses from hot to cold climates than from cold to hot. “It takes the horse longer to adjust and acclimate to heat,” he says. “When you go south, make sure the horse can sweat adequately. Some don’t acclimate very well and may not sweat enough, and some may develop anhidrosis (the inability to sweat). If your horse is not sweating normally, suffering heat stress, consult your veterinarian for help.”

You might have to keep the horse that’s adapting to sudden heat indoors and out of the sun. “Some of the well-built barns in hot climates can keep horses cool—they’re designed to take advantage of all the natural ventilation possible,” says Coleman.

Fans, misting fans, or air-conditioning can also help.

“Fans create enough air movement to help promote a little more evaporation—to cool the body,” says Connally. Another bonus is they can deter insects, which can be both a nuisance and a disease threat, exposing horses to insect-borne pathogens they might not have encountered previously. While you should already be using core vaccines for mosquito-borne diseases such as West Nile virus and Eastern and Western encephalomyelitis, work with your veterinarian to find out if he or she recommends a booster before going to a warm climate.

“In a cold climate a horse may only need boosters once a year, but there may be mosquitoes year-round in the warm climate, and his immunities may be inadequate,” says Allen.

As for riding horses that have moved from cold to hot weather, Connally says to pay close attention during work and recognize they might not be able to exercise during the heat of the day.

“Exercise adaptation (coming from a hot climate to a cold one) is actually pretty easy for the horse,” adds Allen. “The reverse is more complex—taking a horse from a cool climate to a hot one. It’s much harder for the horse to adjust.”

In addition to the heat and insects, he says the horse must acclimate to other changes, as well, such as ­different-tasting water that might get warmer than the horse likes and new bedding.

Your biggest concern when going from south to north in winter, on the other hand, is whether your horse has an adequate hair coat.

“His body has been coping with heat (with the blood vessels beneath the skin dissipating body heat rather than conserving it), and it takes a while to adjust,” says Connally, adding that these horses probably won’t grow hefty hair coats and truly thrive until they’ve experienced the full transition from fall to winter.

Extreme temperature fluctuations (50 degrees or more between daytime and nighttime extremes) any time of year, however, can be hard on horses. “Temperature swings are a stress, especially for foals,” says Connally. “It can lead to scours (diarrhea) or pneumonia in young foals, but is not as hard on adults,” he says. Multiple stresses combined—such as if a horse is undernourished, traveling, and working hard—can add to the burden of weather changes.

Take-Home Message

Horses are very adaptable and typically can handle significant temperature swings. It’s when we alter their natural condition and confine them (or don’t provide them with a windbreak) or haul them from one climate to another that they tend to struggle. It’s up to us as responsible horse owners to help them adjust.