Hydration 101: You can lead a horse to water and make it drink
Aug 01, 2006
Meanwhile largely unnoticed metabolic and physiologic processes are occurring that soon can make this equine athlete an emergency situation that you will have to deal with. Research has shown that during a 50-mile endurance ride in ambient temperature, the average horse will do enough muscular energy and produce enough heat to melt a 150-pound block of ice and then bring it to a boil. At a temperature rise of almost 2 degrees F per hour (without sweating) a horse would soon reach the critical temperature of 42 C, and a crisis would ensue. The need to remove this heat from the performing/working horse is crucial.
Horses lose heat through evaporative cooling, utilizing sweat and through respiration. About 1.5 to 4 gallons of sweat can be lost each hour during strenuous work in hot conditions. Add slowly progressing dehydration to this picture, and the athletic horse begins a dangerous process that, if not altered, can result in heat stress, heat stroke and possible death. The horse working on a hot day in our example is beginning to become dehydrated. Usually dehydration causes the plasma sodium (Na) concentration to increase, and this rise in sodium is a trigger for the thirst response in horses. However if a horse is becoming systemically dehydrated but is also sweating heavily, then the overall loss of Na in the sweat can offset the drop in plasma volume, and the sodium concentration can stay the same or actually increase. Horses in this situation are losing significant electrolytes and are beginning to experience hydration problems, but they might not be thirsty and might not drink when water is offered. It is important to stress to owners that all exercising horses must drink even if they don't appear to be thirsty.
Dr. Harold Schott and researchers at the Veterinary Medical Center at Michigan State University have shown that offering horses slightly hypertonic water (0.9NaCl) during exercise will stimulate the thirst center and will result in that horse drinking more overall water than if non-salty water had been used. Their research would also support offering exercising horses water at ambient temperature because experimental horses drank more water at room temperature than either warmer or cooler water when offered. The consequences of dehydration are far too serious to ignore possible ways to encourage horses to drink.
Once dehydration begins to take place, the horse's body loses the ability to sweat efficiently. Without enough sweat, the horse cannot continue to utilize evaporative cooling to dissipate the heat that it is still producing. As the body dehydrates, there is a loss of plasma volume, so blood becomes less fluid. The cardiovascular system becomes less efficient at transporting oxygen throughout the body, and it must begin to work even harder to support the working horse.