As we move into the heart of the show season, temperatures and humidity in many parts of the country are on the rise. Replacement
of fluid and electrolyte losses in the exercising horse becomes critical ...
OK, so you have heard the message countless times before.
You already know about the difficulty horses have in dissipating temperature and their poor ability to evaporate in humid
conditions. You have read about the massive electrolyte losses in equine sweat and the need to replace sodium, potassium,
chloride and magnesium. You know all about the correct ways to both prevent electrolyte imbalance and heat stress and to re-hydrate
the equine athlete following exercise in hot, humid conditions. Or do you?
Weighing horses during competitions has allowed researchers to record more than 50 pounds of fluid lost by some athletes
in hot, humid conditions. Some trained horses being supplemented with electrolyte solutions during exercise, however, have
shown an ability to drink enough fluids to maintain body weight and reap the benefit of improved performance.
There may just be a few things about equine sweat and electrolyte balance in the performance horse that might surprise you.
More and more research is being done in this area and the increasing popularity of endurance riding and the ever-growing number
of other equine athletes working and competing in the summer is forcing us to do a better job dealing with heat and humidity.
And that means learning more about equine sweat.
Losing heatExercising horses produce a large amount of both heat and sweat. The heat is the byproduct of muscular work, and the sweat
is a physiologic adaptation that allows the body to dissipate or lose that generated heat. In ideal conditions, sweat is evaporated
off the body surface and cools the horse.
Evaporation accounts for 65 percent of heat dissipation, with the heat transfer in exhaled air from the lungs accounting for
another 25 percent and the last 10 percent taken care of by a host of less effective methods. Horses are not the most efficient
animals at producing energy and getting rid of heat. It is estimated that only about 25 percent of the fuel (food, water and
oxygen taken in) in the horse is transferred to work. A large amount of the remainder (about 80 percent) winds up as heat
that must be eliminated.
Dr. Kerry Ridgway, a noted endurance veterinarian, addressed this tremendous heat production in a presentation he made as
part of "On To Atlanta '96". This publication was the result of the FEI Samsung International Equine Sports Medicine Conference
in Atlanta in 1994 as part of an effort to learn as much as possible about holding equine events in conditions of high heat
and humidity. The Atlanta Olympic Games of 1996 marked the first time in the modern era that a significant competition was
hosted in a location during a season that almost guaranteed weather extremes. The desire to be able to hold an event worthy
of the Olympics while ensuring the safety and well-being of the equine athletes prompted this conference and the large amount
of research that preceded the successful Atlanta games. In his presentation in that publication, Ridgway wrote, "To give you
some idea of the incredible heat production occurring in the exercising horse, a horse racing at 13 meters per second (about
28 miles per hour) produces enough heat to bring 8 liters of water from room temperature to a boil. That is enough heat to
bring more than 2 gallons of water to a boil after just a two-minute race.
Even more astounding, an endurance horse traveling at about 17 km/hr sweats approximately 12.5 liters every hour and produces
enough heat in that hour to bring about 25 gallons of water from room temperature to a boil."
Sweating like a racehorseSweat and evaporation are the horse's main means of losing heat. Because of the horse's low body surface area to weight ratio,
however, this task is a definite challenge. Humans have a three times greater advantage in this ratio compared to horses,
and it turns out that when we say, "I'm sweating like a racehorse," we are probably not being very accurate. Humans sweat
at a rate of 2 to 3 liters per hour, which cannot compare with the equine's rate of 10 to 15 liters per hour. This elevated
sweating rate allows the horse to attempt to compensate for its poorer body weight to surface ratio and to try to dissipate
more heat. Additionally, human sweat is hypotonic — it contains a lesser concentration of electrolytes than in plasma.