Extreme heat can put horses at risk of developing heat stress or stroke, particularly those exercised in hot and humid weather. However, an appropriate post-exercise cooldown can prevent heat-related illness from occurring in your horse. In this article two experts bust common myths about cooling horses in hot and humid weather.

Sweat, Thermoregulation, and Hydration in Horses

“Horses have a ‘poor engine’ since their muscle energy metabolism is not very efficient,” says Lidwien Verdegaal, MVM,DVM, PhD, EBVS, Dipl. RDVS, Dipl. ECVIM, senior lecturer of equine medicine at the University of Adelaide, in Australia. “Approximately 80% of the produced energy is released as metabolic heat.”

When horses generate more metabolic heat during exercise, the body activates heat loss mechanisms to keep core temperature within the thermoneutral zone (the range of ambient temperature in which an animal’s normal metabolism can maintain an essentially constant body temperature without extra effort). Blood transports the extra heat to the skin surface, where sweat helps it evaporate. Factors such as ambient temperature, high humidity, poor barn ventilation, prolonged exposure to direct sunlight, excessive work, transportation, and obesity can challenge this balance of heat loss/gain and cause horses to overheat.

“Sweat is a mixture of water, electrolytes, and proteins—specifically latherin (the substance that makes sweat foam or lather),” says Jane Williams, PhD, associate professor and head of research in the equine department at Hartpury University, in the U.K. “Horses produce sweat in response to homeostatic changes as a thermoregulatory mechanism.”

When horses sweat a lot, they risk becoming dehydrated. “Water balance in the horse’s body is controlled by hormone and electrolyte concentrations, which is why dehydration and electrolyte losses are linked,” says Williams. “Key to this is sodium; when sodium levels are high in the blood, antidiuretic hormone reduces water losses to conserve water and address this balance.”

Supplementing your horse’s diet with electrolytes in hot and humid conditions, particularly in working horses, can help prevent dehydration and support healthy muscle and cell function.

Common Myths When Cooling Horses

Myth 1: Heat illness (e.g., heat stroke, exhaustion) only affects horses in hot and humid weather conditions.

Horses’ body temperatures can reach up to 102 F, even when exercising in cool temperatures, Verdegaal says. Therefore, horse owners should be aware that heat illness, although less likely, can also occur with horses exercised (or transported) in cold climates.

“In climates with high humidity, however, while the horse may sweat, the evaporation rate will be slower and, therefore, cooling is less efficient due to the higher concentration of water molecules in the air,” Williams adds.

Myth 2: Do not apply cold water on hot horses because this can cause shock, colic, or tying up.

“The repeated application of cold water (e.g., every minute) is the best approach when cooling down horses in hot and humid conditions,” Williams says. Water applied to a hot horse’s body will heat rapidly, reducing the water’s positive cooling effect. She adds that removing tack after exercise increases the body’s surface area available for evaporation. Walking the horse prevents exertional rhabdomyolysis while repeatedly applying the cold water.

Myth 3: Always scrape off water because leaving water on the horse will prevent him from cooling down.

Water left on a sweaty horse warms up and consequently evaporates, which cools the horse but, once the water has evaporated, the horse feels hot again. Therefore, Williams says horse owners should continue applying cold water until the horse has cooled down because this increases the evaporation rate. “If we scrape water off, we are shortening the time water is in contact with the skin and can act as a heat sink to transfer heat from the horse to the air via evaporation.”

Myth 4: Leaving a wet towel on the horse aids cooling.

“Horses’ heat loss depends on sweating to release metabolic heat buildup in their muscles,” says Verdegaal. “Evaporation of the sweat is part of this sweating mechanism.” Leaving a wet towel on the horse to cool him down prevents evaporation because it blocks liquid sweat from evaporating. Therefore, horse owners should always avoid placing towels on horses when helping them cool down.

Myth 5: All horses cool down in the same manner.

 “A one-size-fits-all approach in monitoring and cooling horses after field exercise in hot and humid conditions is inappropriate and potentially dangerous due to horses’ variability in thermal response,” says Verdegaal.

She and her fellow researchers monitored endurance horses’ core thermal responses during field exercise using a gastrointestinal temperature pill. The scientists recorded differences between individual horses’ body temperature responses during exercise and recovery.

“Our study provided reliable supporting evidence for the need for industry-wide temperature monitoring guidelines to prevent heat stress in endurance horses and racehorses when exercising in the field,” Verdegaal says. She adds that all horse owners need to know their own horses’ response to heat, including the temperature evolvement during recovery, to safeguard equine welfare.

Myth 6: Horses exercising at high intensity reach their peak core temperatures during peak exercise.

In one of their studies, Verdegaal and her fellow researchers recorded that (on average) endurance horses reached their peak core temperature when completing 75% of a 40 km exercise leg. “The endurance horses’ body temperatures returned to baseline within the mandatory 60 minutes recovery time, with an average 10-15 minutes, cooling down with pouring water buckets over the entire body with or without walking horses for a few minutes,” Verdegaal says.

In contrast, trotters (on average) reached their maximum temperature during their recovery period 34 minutes post-exercise, Verdegaal says. “Several horses had body temperature values above 38.5 C (101.3 F), a heart rate already returned to 60 bpm, and 30% of the trotters still had temperatures above 39C (102.2 F) at the end of recovery in a cool environment.” Therefore, some horses might need longer cooling periods than others.

Take-Home Message

Adapt your cooling practices to consider each horse’s individual response to heat, his discipline, and exercise intensity. Observe your horse to understand how he handles hot and humid weather so you can quickly react if he begins to experience heat stress, and be familiar with and prepared to apply the best and most efficient cooling techniques to reduce his risk of serious heat-related illness.