Is it always appropriate to administer electrolytes to a slightly dehydrated horse? I say that if the horse has become dehydrated from not drinking (vs. sweating) then electrolytes by paste or feed aren’t a good idea. Don’t the electrolytes need appropriate amounts of water within the body to “dissolve”? My counterparts say that the horse will naturally drink more (or start drinking) in response to the salt in the electrolytes and voila, hydrated horse.

Regulating the amount of water and electrolytes in the body is a complicated process, and with such an important event riding on my answer, I turned to my favorite resource, The Veterinary Clinics of North America Equine Practice, for help. The April 1998 issue entitled “Fluids and Electrolytes in Athletic Horses” had the answer to your question.

Turns out: you’re both right! (don’t you hate when that happens?) The idea behind giving slightly dehydrated horses electrolyte paste is to stimulate thirst and encourage them to drink, replacing fluid lost in sweating during exercise and/or hot temperatures. And while some research has shown that giving horses electrolyte paste can cause them to drink more water than horses NOT given electrolyte paste, there are horses who continue to refuse to drink and therefore wind up MORE dehydrated. Here’s what happens when you give a horse electrolyte paste:

Once in the intestine, the electrolytes draw water from the blood into the gut. Taking water out of the blood causes the concentration of electrolytes (especially sodium) in the blood to increase. The increased sodium concentration in the blood stimulates the thirst mechanism, encouraging the horse to drink. However, if water is not available, the horse won’t drink, or blood flow to the intestines is reduced because of exercise, giving concentrated electrolytes may worsen the dehydration by causing water to leave the bloodstream and enter the GI tract.

So what should owners do? Experts suggest giving electrolytes approximately four hours BEFORE the horse begins exercising. This timing gives most horses the opportunity to drink before they begin sweating and become dehydrated. Depending on the sport, it may also be necessary to give electrolytes and fluids during and after competition, to keep horses hydrated and to help them recover more quickly.

To give you an idea of how much water a horse can lose while exercising, I was part of the team of researchers that studied the effect of heat and humidity on three-day event horses in preparation for the 1996 Olympic Games in Atlanta, GA. One of the things we did was weigh horses before and after they competed. We found that some horses lost up to 50 kg after cross-country! (that’s 110 pounds or almost 14 gallons of water!)

It’s not possible to keep up with that much water and electrolyte loss during an event or even in the first 24 hours of recovery. That’s why experts in the field recommend horses that will be sweating profusely be supplemented daily with electrolytes. While you can’t “preload” a horse with electrolytes (they’re readily excreted in urine), having them on board at all times and available for immediate use means it will take longer for supplemented horses to reach dehydration and fatigue.

The best way to get electrolytes into horses is to topdress their feed with either plain salt or a commercial preparation (electrolyte supplements) containing not just sodium and chloride but also potassium, calcium, magnesium and other ions. Regular salt blocks were made for the rougher tongues of cattle and horses won’t lick enough to replace their losses if they’ve been sweating heavily. And while some horses can be taught to drink salt water, if you’re going to put electrolytes in water you should always offer fresh, non-salted water for the horse to drink.