Chilling Out After Exercise
The mid-summer heat brings added challenges in managing the athletic horse. Regardless of whether your horse is competing in a show, trail ride, or high-level three-day event, hot weather places additional demands on his body. You need to be aware of these demands and take steps to ensure that he does not overheat. The focus of this article is prevention of overheating during exercise in hot conditions. It will include recognizing the clinical signs of overheating, preparing the horse for exercise in the heat (improving fitness and acclimation to heat), and strategies for avoiding dehydration.
We will begin with a refresher on the effects of exercise and the environment on the horse's body temperature.
Exercise in the Heat
The horse produces a tremendous amount of heat during exercise. Within muscle, the conversion of stored energy (for example, glycogen) into mechanical work (muscle contraction) is very inefficient, resulting in heat accumulation. The rate of heat production is related to the work effort--the faster the horse runs, the greater the rate of heat production. Similarly, steep terrain, difficult footing, and weight carried (rider and tack)--all of which increase effort level--will increase the rate of heat production. At any given work level, the total amount of heat produced depends on the duration of the exercise.
The horse, like all mammals, must operate within a fairly narrow range of body temperatures. At rest, rectal temperature is around 99-100ºF (37.2-37.8ºC). During exercise, the heat produced in contracting muscle spreads throughout the body, resulting in an overall increase in body temperature. A moderate increase in body temperature (one to two degrees) is actually beneficial to body function during exercise (see "Warming Up to the Task", in the April 2001 issue of The Horse). However, an excessive increase in temperature can cause tissue damage or death. The question is, how high is too high?
While there are no hard and fast rules, a rectal temperature above 106ºF (41ºC) generally indicates the need for aggressive cooling procedures, while a horse with a rectal temperature of 108ºF (42ºC) or more can be at serious risk for heat stroke.
Under most circumstances, the horse is able to minimize the rise in body temperature by dissipating heat from the body. The evaporation of sweat from the skin surface is the main means for heat dissipation. In fact, evaporation accounts for about 65-70% of the heat lost during and after exercise. There also is evaporative heat loss through the respiratory tract, and some heat is lost by radiation (emission in the forms of waves or particles) and convection (conduction) at the skin surface.
The horse also has a strategy for selectively cooling the brain (maintaining brain temperature lower than the rest of the body). This is important because the central nervous system is much more susceptible to the negative effects of high temperatures.
If you don't already have one, buy a good thermometer, and get in the habit of taking your horse's rectal temperature before and after exercise. These measurements will allow you to develop an appreciation for the effects of exercise duration, effort, ambient temperature, and relative humidity on body temperature. In cool weather, there might be a two- to four-degree increase in temperature after a reasonably long ride. In hot weather there will be a larger increase in body temperature after the same amount of exercise.
Dispelling Excess Heat
The horse uses a couple of different strategies to deal with hotter conditions. First, sweating rates will be higher in an attempt to increase heat loss over the skin surface. Second, there will be an increase in breathing rate to drive evaporative heat loss through the respiratory tract and to keep the brain cool. Under dry heat conditions (hot, but with low relative humidity), evaporative cooling remains an effective means for heat loss. However, there is a price to pay--heavy losses of precious water and electrolytes can lead to dehydration. This can compound the effects of heat on body temperature because as the horse becomes dehydrated, there is a decrease in the efficiency of heat loss.
Heat and high relative humidity are not conducive to top-notch exercise performance. Like humans, horses can struggle under these conditions because the rate of heat loss is greatly slowed. Efficient evaporation, both on the skin surface and through the lungs, occurs when the air is very dry and therefore able to accept moisture as water vapor. As relative humidity increases, there is less potential for evaporation because the air is already saturated with water, and sweating becomes a much less effective means for heat loss.
The implications are twofold. First, during exercise in hot and humid conditions, body temperature will increase at a much faster rate--up to twice as fast. Therefore, that critical upper limit in body temperature will be reached much quicker than would be the case in cool conditions. Second, unless we make attempts to actively cool the horse, recovery following exercise will be delayed.
How well an individual horse copes with hot, humid weather depends on several factors. Body type is important--a big, heavy horse will overheat faster than a smaller, leaner individual. The amount of heat produced and the rate of heat loss depend partly on muscle mass and the ratio of body mass to skin surface area. Heavily muscled horses have a larger body mass to surface area ratio than leaner animals, which favors heat retention. This helps explain why lighter breeds (Arabs, Thoroughbreds, and crosses) are better suited for endurance events.
The length of the hair coat is also important. A long winter coat will trap heat in the body. Horses in heavy training programs might need to be clipped in early spring to minimize this insulation effect and to minimize heat stress as the conditioning program is stepped up during the spring.
Physical conditioning and heat acclimatization also influence tolerance to work in the heat. A poorly conditioned, overweight horse can be at high risk for overheating if worked too hard in hot conditions. For one, body fat acts as an insulator, impeding heat loss. Second, physical conditioning results in a number of adaptations that improve temperature regulation during exercise. With training, muscles work more efficiently and produce slightly less heat. The cardiovascular system is also better able to move heat from the site of production (muscle) to the skin. Training also increases the efficiency of sweating. Therefore, it is imperative that your horse undergo a training program that is appropriate for the work he will be doing.
Studies conducted in advance of the 1996 Summer Olympic Games in Atlanta showed that training in the heat results in further adaptations that aid temperature regulation during exercise. To some extent, training during the transition from the cool weather of winter and early spring to the warmer temperatures of summer will acclimatize the horse to the hotter conditions. However, when a horse is shipped from the cold North to the warm, humid southern states, some lead time is required to ensure adequate acclimatization. You should plan on a minimum of two to three weeks to account for the time needed to recover from transport and to allow for a period of conditioning in the hot climate.
As mentioned, hydration status also influences the efficiency of heat loss. With dehydration, the cardiovascular system is less able to move heat to the skin surface, and less fluid is available for sweating. This means that a dehydrated horse will overheat faster than a well-hydrated horse.
Dehydration can, in particular, be a problem for endurance horses or horses engaged in other long work sessions. In hot conditions, horses can lose 2.6-3.9 gallons (10-15 liters) of sweat fluid per hour of exercise, so during long rides you should let your horse drink as often as possible. For endurance rides, some electrolytes should also be given during the ride (see "Fluids and Electrolytes" in the April 2000 issue of The Horse, online at http://www.thehorse.com/ViewArticle.aspx?ID=214).
By and large, the risk of overheating and developing heat stroke is greatest during the summer months. However, heat stroke can occur at other times. For example, very warm, even hot days can occur in spring. So, it is possible that an unacclimatized horse in the early phase of its conditioning program will be asked to exercise in fairly hot conditions. Under those circumstances, one should reduce the time and/or intensity of the exercise bout to avoid heat stress.
Close monitoring of your horse's vital signs is very important when training and competing in warm conditions. Rectal temperature, pulse rate, and respiration rate should be recorded before and after the ride. For long exercise sessions, these measurements also should be taken at regular intervals during the ride. The facial artery, located below the cheekbone at the halter check-piece, is perhaps the easiest place to check pulse rate. Monitoring pulse rate will take some mastering, but will become second nature with practice.
The pulse rate will be high immediately after exercise (80 to 100 beats per minute), but in fit horses will decline quickly during recovery time. With 15 minutes of recovery, the pulse rate is often 40-50 beats per minute or even near resting values (35-45 beats per minute). The post-exercise decrease in pulse rate is somewhat slower in hot, dehydrated horses. However, respiration rate and rectal temperature will give you the best indications of heat stress.
The normal resting respiratory rate is 10-20 breaths per minute. Even in cool conditions, the respiration rate will be elevated for 30 minutes to an hour after hard exercise. It might remain at 60-80 breaths per minute for the first 10 minutes of recovery.
Hot horses will have much higher breathing rates, as high as 120-140 breaths or more per minute. This makes sense because the horse is using the respiratory system to lose heat. However, you should be concerned when the respiratory rate remains this high after five to 10 minutes of rest. This indicates that the horse is overheated and in need of active cooling.
During recovery, monitor the horse's rectal temperature at regular intervals. In overheated horses, rectal temperature might decline very little or even increase slightly during the initial 10-20 minutes of recovery. This is evidence that aggressive cooling is necessary. As mentioned, you should be concerned when rectal temperature is over 105-106ºF (40.5-41ºC).
You might notice other signs of heat stress, such as lethargy or lacking his normal "zip." Do not ignore these signs; stop exercise right away, move the horse into the shade, and cool him out. There is a fine line between heat stress and full-blown heat stroke, so it is absolutely essential to remain on the side of caution.
Traditionally, tossing cold water on hot horses has been regarded as a "no-no" because of the misbelief that this practice would result in muscle damage and tying-up. Field and laboratory research over the past decade has largely dispelled this myth. Rather, it is probably the very high muscle temperatures (110ºF or 43.3ºC or more) that cause muscle damage. The bottom line is that applying cold water is the most practical and effective means to assist cooling--and it is safe.
You need a plentiful source of cold water (35-40ºF or 1.6-4.4ºC), a means for applying the water (e.g., plastic buckets), and a scraper to remove excess water. You can use sponges dipped in cold water, but this is a much less efficient method compared to hosing or pouring water on the horse. Water applied in this fashion will warm up almost immediately, so the water should be scraped off quickly and fresh, cool water re-applied.
This explains why it is not a good idea to drape wet towels over the horse. Although there will be a brief cooling effect from the towel, unless you keep pouring water on the towel, it will warm up and impede heat loss by insulating the horse.
This dousing process should be continued until the horse's rectal temperature is below 101ºF (38ºC). Importantly, if your horse does not cool out within 40-60 minutes, you should call a veterinarian.
It is also preferable to put the horse in the shade while cooling him. Try not to shelter him from any breeze, as the wind will help with convective cooling. Electric fans can also be placed in front of the horse. It is fine to keep the horse moving during this cooling off process (e.g., walking in a smallish circle). However, it is not necessary for the cool-down to be any more active. More exercise will simply further delay the decrease in body temperature.
Let the Horse Drink
Another important part of the cooling process is drinking. Traditionally, horsemen have not allowed "hot" horses to drink because of a perceived risk of colic and cold-water founder (laminitis). However, with the possible exception of very hard galloping exercise (e.g., Thoroughbred racing), it is safe for horses to drink right after exercise. In fact, the thirst drive decreases with time after exercise, so offer water as soon as is practical. Let the horse drink up to one to two gallons during the first 15 minutes of recovery.
Remember that you are the best advocate for your horse. Ensure that he is thoroughly conditioned, acclimatized, closely monitored during exercise, and actively cooled after the ride or event to keep him from getting heat stress and heat stroke.
Kohn, C.W.; Hinchcliff, K.W.; McKeever, K.H. Evaluation of washing with cold water to facilitate heat dissipation in horses exercised in hot, humid conditions. American Journal of Veterinary Research, 60, 299-305, 1999.
Foreman, J.H. The exhausted horse syndrome. Veterinary Clinics of North America: Equine Practice 14, 205-219, 1998.
Marlin, D.J.; Scott, C.M.; Roberts, C.A.; Casas, I.; Holah, G.; Schroter, R.C. Post exercise changes in compartmental body temperature accompanying intermittent cold water cooling in the hyperthermic horse. Equine Veterinary Journal, 30, 28-34, 1998.
King, M. Beating the Heat with Working Horses. The Horse, July 1999, 54-62. http://www.thehorse.com/ViewArticle.aspx?ID=350.
About the Author
Ray Geor, BVSc, PhD, Dipl. ACVIM, is professor and chairperson of Large Animal Clinical Sciences at the College of Veterinary Medicine at Michigan State University
POLL: Colic Surgery