Coping with the heat

August 1, 2007. Posted by Catherine Kohn, VMD

During hot weather, heat dissipation is primarily dependent on evaporation of sweat on the skin surface. Horses are capable of producing large quantities of sweat—sweating rates of 10-15 liters/hour¹,² have been reported during exercise in hot conditions. Sweat evaporates efficiently in hot, dry conditions, but not in hot, humid climates that are likely to impose a particularly large heat load because heat dissipation is compromised, and the horse retains heat. In addition, horses have a large muscle mass and a comparatively small body surface area. This configuration results in a limited surface area for evaporative heat loss.

Clinical Signs of Overheating

Lethargy, unwillingness to perform as expected, a persistently high rectal temperature, and moderate to severe dehydration might be observed in overheated horses. They are usually disinterested in the environment, and they might not voluntarily eat or drink. More severe heat stress is associated with exhaustion, increasing body temperature, decreasing blood pressure, and collapse. Heat stress has deleterious effects on many organs and can be life-threatening.

Helping Your Horse Cope with the Heat

1. Consider clipping horses with robust body condition, a heavy hair coat, and few visible veins on the skin before hard exercise.
2. Condition your horse for the intended exercise. Physical fitness is essential for coping with heat during exercise.
3. Allow your horse to acclimate to hot weather for 10 days to two weeks before riding long distances or training hard.
4. Be sure your horse stays well-hydrated. Provide two water buckets and 24-hour access to as much water as he wants.
5. Give a salt supplement if you are working your horse regularly during hot weather. Avoid supplements that have sugar (dextrose) as the first entry on the ingredient list.
6. If possible, allow your horse to drink during lulls in exercise.
7. Stay in the shade as much as possible to reduce radiant heat. Ride early or late in the day, when ambient temperatures and solar radiation tend to be lower, avoiding the hours between 11:00 a.m. and 4:00 p.m.
8. Reduce the workload commensurate with the weather. Ride shorter distances and reduce the intensity of training.
9. Make certain your horse recovers after exercise. Take his rectal temperature when you are finished riding, and monitor him for an hour or until the temperature is normal (less than 101°F). Know your horse’s resting temperature (take it before riding). Observe how much water the horse drinks. Failure to drink might result in dangerous dehydration, particularly in horses that are exercising day after day in the heat.
10. If your horse is very hot, keep him in the shade, hose him with cool water, and immediately scrape the water off the coat. Wash water heats up on contact with the hot skin, and it rapidly loses its ability to conduct heat away from the horse. Avoid leaving wet towels on the horse; they act as insulators, retaining heat rather than facilitating its dissipation. Continue washing and scraping until the horse cools down.
11. If your horse is excessively fatigued or out of breath while you are riding, get off and get in the shade. Bathe with cool water immediately.
12. Seek veterinary advice if your horse remains listless, unwilling to move, develops colic, has a high heart rate, or has a persistently high rectal temperature.

Heat Dissipation

During hot weather, heat dissipation is primarily dependent on evaporation of sweat on the skin surface. Horses are capable of producing large quantities of sweat–sweating rates of 10-15 liters/hour³,4 have been reported during exercise in hot conditions. Sweat evaporates efficiently in hot, dry conditions, but not in hot, humid climates that are likely to impose a particularly large heat load because heat dissipation is compromised and the horse retains (stores) heat. In addition, horses have a large muscle mass and a comparatively small body surface area. This configuration results in a limited surface area for evaporative heat loss.

Sweating requires diversion of blood to the skin, and this portion of the circulating blood volume is not available to supply oxygen to or remove metabolic wastes from working muscle. Thermoregulatory demands, therefore, compete with the demands of working muscle and might limit performance. A large volume of body water–as well as significant quantities of the important electrolytes sodium, potassium, and chloride–are lost in sweat. If fluid and electrolyte losses are extensive and are not replaced expediently, the thermoregulatory response to exercise will be compromised, and heat storage might increase to a dangerous level.

The respiratory system plays an important role in heat dissipation, accounting for loss of more than 25% of the metabolic heat load in one study of horses exercising on a treadmill¹. Increased respiratory heat loss might be associated with an increased resting respiratory rate; a twofold to threefold increase in resting respiratory rates was observed in horses during heat acclimation².

Horses vary in their ability to respond to a heat load. In general, “hot-blooded” types of horses (such as Thoroughbreds and Arabians) might be better able to dissipate heat than warm- and cold-blooded horses. The latter tend to have a larger body mass, a thicker hair coat, and a smaller body surface to mass ratio than horses of hot-blooded ancestry.

References

1. Carlson, G.P. “Hematology and body fluids in the equine athlete: A review.” In Gillespie, J.R.; Robinson, N.E. (eds): Equine Exercise Physiology 2. Davis, CA, ICEEP Publications, 393-425, 1987.
2. Carlson, G.P. “Thermoregulation, fluid ad electrolyte balance.” Snow, D.H.; Persson, S.G.B.; Rose, R.J. (eds): Equine Exercise Physiology. Cambridge, Granta Editions, 291-309, 1983.

The Source: The Horse