Expert Opinion


Department of Medicine, School of Veterinary Medicine
University of California/Davis

Proceedings of the First International Conference
September 22, 1982 

“Vigorous exercise results in a massive metabolically generated heat load.  When exercise is preformed in environments where temperature and/or humidity are high, the competing demands for evaporative cooling and maximal energy output may limit performance and in some instances, lead to serious heat-associated disturbances.”

“Sweating is the principal means of evaporative cooling in exercising horses.  Sweat promotes heat loss only when sweat water is evaporated.  A horse working at moderate intensity could have a heat production of 270 BTU per minute.  If this pace could be maintained for an hour, the total metabolic heat production would be 16,200 BTUs.  To dissipate this amount of heat by evaporative process alone would require the complete evaporation of just over 14 quarts of water.  However, the effectiveness of the evaporative cooing process is much more complicated than this rather simplistic calculation would suggest.  Air temperature, wind velocity and relative humidity influence this (sweating) process, and have led to the concept of ‘effective temperature.’  (Kleiber.1961): Astrand and Rodahl/1970).  At extremely high environmental temperatures, evaporative heat loss may not be able to keep pace with the exercise induced heat load and the heat gain from the environment.  High humidity prevents complete evaporation.  With incomplete evaporation, sweat production results in little or no heat transfer.  Conditions of high environmental temperature and humidity thus pose a serious risk to the equine athlete, particularly when performing protracted submaximal exercise.  There have been few experimental data on the rolls of these factors in the horse.  One simple system whish has been proposed as a measure of effective temperature is the sum of ambient temperature (Fahrenheit) and relative humidity (Macay-Smith and Cohen/1982).  It has been suggested that when this sum is less then 130, heat loss is generally not a problem.  When the sum exceeds 150, especially if humidity contributes more than half the sum, the evaporative cooling by sweating is severely compromised.  When temperature and humidity total more than 180, normal cooling mechanisms are almost totally ineffectual and exercise can be for only relatively short periods before core and muscle temperatures increase to dangerously high levels. 

“The nearly constant core temperature maintained by most animals represents a carefully controlled and critical balance between heat production, heat gain and heat loss … The evaporative route is the most efficient means of heat loss during exercise and may be the only means of heat dissipation in hot environments. “ (Astrand and Rodahl/1970)

“Normal fluid balance refers to the state in which input matches output.  The water intake of a sedentary hay-fed horse is said to be in the range of 20 - 30 litre (18 -  27 quarts) per day” (Tasker/1967)

“During exercise, sweating is the principal route of both fluid and electrolyte loss.  Sweat rates may approach 10 - 12 liters (9-11 quarts) per hour with prolonged exercise in a hot environment (Carlson and Ocen/1979)….. We can reasonably assume that 90% or more of the weight loss during exercise is due to water loss.  A heavily sweating endurance horse with a weight loss of 88 lbs has probably lost 81-88 lbs of body water.”

“The measured change in body weight provides the most accurate estimate of sweat loss in the exercising horse … Sweat loss can be substantial with protracted exercise, even in moderate to cool climates.  In a series of four controlled rides of 37 miles, the mean weight loss of six horses was 59.5 lbs, or 6% of body weight (White et al/1987).  One of these horses lost 105 lbs, or 9.1% of body weight.  This occurred despite mild climatic conditions (average environmental temperature 69 degrees F), slow pace (6 miles per hour) and access to water at rest stops.  In another study conducted in Scotland during overcast, windy and wet weather, the mean weight loss of four horses, measured on two rides of 36 –50 miles, was 74 lbs, or 7% of body weight (Snow et al, 1982).”

“Dehydration of as little as 2-4% of body weight has been shown to adversely affect work performance in human athletes (Saltin/1964).  The extent to which dehydration affects the performance of normal endurance horses is not known, but fluid and electrolyte disorders are major factors in the developments of the exhaustion disease syndrome.”

“In hot and/or humid environments, sweat losses may be massive.  In these situations, replenishment of the accumulated water is of overriding importance.  Electrolyte supplements may be beneficial and have been provided ‘free choice.’

Researched by Carriage Horse Action Committee

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