A horse covering 100 miles on an endurance ride through the California foothills. A pony trailered for two hours to a mid-June gymkhana in Georgia. A retired gelding with Cushing’s disease standing in his field on the first warm day of a Midwestern spring.

What could all of these horses possibly have in common? Sweat, in copious amounts, of course. But, more important, all would likely benefit from electrolyte supplements to replenish the salts they lost in cooling themselves.

Electrolyte supplements are often thought to be needed only by high-level athletes, especially those competing in endurance events, but in reality any horse who sweats for a prolonged period of time can deplete these vital minerals to critical levels. And the consequences in severe cases can range from fatigue and muscle tremors to potentially deadly heat stress and physiological exhaustion.

“There’s no argument that electrolytes are essential at some level to every horse who sweats for long periods of time in hot weather,” says Jeanette Mero, DVM, a veterinarian and endurance competitor from Mariposa, California.

The good news is that forages and commercial feeds are high in electrolytes, and the majority of horses are able to easily replenish their routine losses through their regular diets plus access to a mineralized salt block. But in situations where a horse continues sweating for several hours, he may need the added boost of supplemental electrolyte powders, pastes or gels to speed his return to normal.

By understanding what electrolytes are, and the conditions that might deplete them, you’ll be better able to identify the situations where your horse might benefit from a supplement. Then, you’ll be in a position to sort through your options and deliver extra electrolytes safely and efficiently.

What are electrolytes?

Electrolytes are common minerals that in their solid forms bond readily into salts, such as sodium chloride (better known as common table salt), but when dissolved in water, break down easily into their component elements, called ions, which carry either a positive or negative charge. Their names are familiar: The positive ions are calcium, potassium, sodium and magnesium, and the negative ions are chloride, bicarbonate and phosphate.

The charges those minerals carry enable them to conduct electricity, and that in turn enables them to play a role in most of the electrochemical processes that sustain life: Their electrical charges carry signals across cell membranes and along nerve and muscle cells, powering functions from muscle contractions to digestion. Electrolytes also play a key role in moving fluids in and out of cells, nutrient absorption and regulation of the body’s total fluid balance. In other words, electrolytes make sure the water a horse drinks is delivered to the cells that need it.

When a horse’s electrolyte levels are unbalanced or depleted, any or all of these physiological processes may be disrupted, and critical cellular functions in the muscles, gut and heart begin to slow or shut down. This, combined with the resulting drop in blood volume and pressure, may lead to a potentially deadly metabolic crisis.

And it’s not just the quantity of electrolytes in the body that is important—the balances matter, too. Too much of one mineral or not enough of another can cause or contribute to a number of problems, including synchronous diaphragmatic flutter (“thumps”) and tying up. Fortunately, most of the foods a horse eats, including grass and hay, contain the right minerals in the correct proportions. The only exceptions are sodium and chloride, the two major components of his salt block, and your horse will naturally take only as much as he needs from that.

In addition, the kidneys are very efficient at filtering out excess electrolytes and sending them off to the bladder for elimination. If a horse happens to get too much of something one day, the worst that is likely to happen is a slightly deeper wet spot in the stall. In fact, the only time you really need to worry about your horse’s electrolyte levels is when he’s been engaging in the one physiological process that has the most potential to throw his balances out of whack: sweating—a lot, and for a prolonged period of time.

How are electrolytes lost?

A horse’s ability to sweat is essential. As he works, his muscles break down glycogen and fat for energy, a metabolic process that generates heat. At first, excess heat radiates into the air as warmed blood from the interior of his body passes through the capillaries just under his skin. But then, if his internal temperature continues to climb, he goes to the next stage of heat dissipation: The electrolytes “unlock” the sweat glands, and fluid pours out onto the skin, where it evaporates—a process that speeds cooling.

If the ambient humidity allows for fast evaporation, sweating is extremely efficient and effective, enabling a horse to gallop on hot, sunny days without harm. But there is a price: Equine sweat contains high levels of electrolytes, much more than in human sweat. All those electrolytes in the sweat are lost to the body and must be replenished through diet. But if the sweating is prolonged, and the horse loses too much before he has a chance to replace them, he will start down the path of imbalances and depletions.

It’s important to remember that the cause of the sweat is irrelevant, says Todd Holbrook, DVM, Dipl. ACVIM, an equine medicine specialist at Oklahoma State University experienced with endurance horses. “It’s not just elite athletes that are losing electrolytes; it’s the pleasure horses, too,” he says. “A horse who spends all day sweating on the trails of a local park is at the same risk as the horse who sweats the same amount training over an Olympic cross-country course.”

Many studies have explored just how much of each electrolyte a horse loses per gallon of sweat, with the goal of determining the amount that needs to be replaced, how often and by what means. “This is a topic of endless fascination with endurance riders and endurance-minded veterinarians,” says Mero. “We can spend days talking about it.”

But the research hasn’t always provided straightforward answers. Studies have shown, for instance, that during three hours of foxhunting, a horse may lose from 33 to 148 grams of sodium, 12 to 51 grams of potassium, and 63 to 284 grams of chloride. The ranges are so wide, researchers say, because the actual losses are influenced by variables such as the ambient temperature, the type of work and the amount each horse sweats.

“As hard as we’ve tried to come up with one, there is no tidy formula to determine if a horse needs electrolyte supplementation and how much he might need,” says Mero. “People would love a cookbook approach, but it’s just not possible.” Instead, she says, a rider needs to consider a particular horse’s situation and make a judgment call.

What horses need electrolytes?

The most obvious candidate for electrolyte supplementation is a horse who sweats profusely for a prolonged period of time due to exercise—a description that is admittedly imprecise. “Again, there is no formula,” says Mero, “but whenever you have a horse who is going to be actively sweating for three hours or more, without the opportunity to graze or access to a salt block, you might want to start thinking about giving him an electrolyte supplement.” Endurance horses obviously fit this criterion, but so do trail horses who are out all day and show horses with a busy schedule.

Yet people tend to overlook one other good candidate for supplementation, forgetting that a horse doesn’t have to be working to sweat for prolonged periods. “I used to see this all the time in humid summers,” says Mero: “A horse is trailered a few hours to a show and steps off the trailer soaked in sweat. He’s probably lost a lot of electrolytes. If he were allowed an hour to relax with a flake of hay, a salt block and water, he’d have time to recover them naturally before he started competing, but in a show environment that’s not always possible.”

A horse who stands out in a pasture all day, even in the hottest weather, probably isn’t going to sweat enough to upset his electrolyte levels—with one exception: “An older horse with Cushing’s0 disease may still have a heavy winter coat during summer,” says Holbrook. “In addition to a heavy hair coat, some horses with Cushing’s sweat excessively. It’s very possible these horses can lose significant electrolytes.”

If you find a horse in that situation, a dose of electrolytes can be part of your immediate efforts to make him comfortable—along with a cooling bath. But over the long term, says Mero, “What you need to do is control his Cushing’s with medication and give him a body clip if necessary. Then he’ll stop sweating so much and won’t need the electrolyte supplement anymore.”

Horses who are not good candidates for electrolyte supplements are those who are sweating for only short periods, regardless of how hard they work or how hot it is. “I’m always amused by the fact that they’ll give Thoroughbred racehorses electrolytes before a race,” says Mero. “They go out and sweat for maybe 10 minutes. There’s no way, no matter how fast they run, that they are going to need supplemental electrolytes.”

Show horses don’t usually require much supplementation either, says Holbrook, who owns several Western performance horses. “I’m riding them 90 minutes a day at most,” he says. “They definitely work up a sweat but it’s not typically for a length of time that they’ll need electrolyte supplementation beyond what they get in their feed and from a salt block.” That said, though, Holbrook says that he has given some of his harder working horses electrolytes during periods of unusual and prolonged heat: “Last summer we had a 30-day stretch of incredibly hot weather in Oklahoma. I did add electrolytes to the grain of a few of my harder working horses during that time.”

Ways to give a horse electrolytes

When you do decide to give your horse electrolytes, you have several choices for administering them. Which you choose will depend on when your horse needs them and why.

Powdered electrolytes are fed as top dressing on grain. These products are useful to prepare horses for upcoming events that will cause them to sweat. “If I know I have a big ride planned in the summer, I might start adding electrolytes to my horse’s grain two or three days beforehand,” says Holbrook. “The biggest reason for starting so early is to make sure he eats them. It seems to take some horses a few days to get accustomed to the taste, but then they don’t seem to mind at all.”

One common misunderstanding is that the purpose of starting the supplementation ahead of time is to increase the levels of electrolytes a horse is carrying. “You’ll hear people talking about ‘preloading’ electrolytes before a ride, but horses do not store them from one day to the next,” says Mero. “You can’t send a horse into an event with extra electrolytes. All you can do is make sure he’s starting out with what he’s supposed to have. You’re topping off the tank, so to speak.”

How much of the powder to give and how often will be outlined on the label. “Just follow the directions and give the recommended amount based on the duration of expected work,” says Holbrook.

Gels and pastes are delivered directly into the mouth via syringe. These provide quick relief and are convenient to use during sweaty situations, without the need for water or grain. Plus, they pack easily in saddlebags. “Let’s say you’re a pretty dedicated trail rider, and you’re to be hauling out to meet a friend for an all-day ride on an 80 degree day with 90 percent humidity,” says Mero. “You can pack a few syringes of paste: Give a dose when you unload from the trailer, another during a stop on the trail four hours later, and maybe even one more when you’re done before you head home.”

Mero cautions that regularly giving pastes and gels to horses with an empty stomach could potentially lead to gastric irritation: “Some of the products are pretty caustic. You wouldn’t want to be giving them too often on an empty stomach. Once or twice on a given day is fine, but I wouldn’t rely on them as a routine thing.” Having studied the effect of some of these pastes on gastric ulceration in research horses, Holbrook agrees that some can worsen ulcers.

Dissolvable products are added to a horse’s drinking water. These are a safe and easy way to make sure a horse always has access to extra electrolytes. “The single biggest thing I think people can do at home in regards to electrolytes is to provide two water buckets: One with plain water and one with electrolytes added,” says Mero. “Horses will voluntarily drink the electrolyte water when they are exercising heavily enough to need it. Just follow the directions on the packet for how much to add.” Some horses may be initially reluctant to drink the electrolyte solution, but they will eventually.

Once your horse is accustomed to the taste of the electrolyte-enhanced water at home, you can pack the product for road trips to events where he might really need the extra boost.

“Don’t wait until the day of a show or a ride to try to get him to drink water with electrolytes in it,” says Holbrook. “You need to start at least a week before it’s needed, and make sure that you also offer a bucket of plain water at the same time.”

You can stick with the two-bucket system all summer long, if you like. Mero adds that there is no harm in the horse drinking the water with electrolytes even when he doesn’t need the extra minerals. “He’s just going to pee out the extra,” she says.

If you’re not sure whether you ought to add electrolyte supplements to your horse’s routine, consider that if he’s generally alert and active, and he’s eating and drinking well, he’s probably just fine.

“It’s easy to over-think electrolytes,” says Mero. “People think they have to be this complicated thing, but for the majority of pleasure horses it’s very simple: Just provide water, hay and a salt block, and you’re all set.” And for those rare times a horse is going to be very sweaty for several hours, a scoop or squirt of a supplement is a safe and easy insurance policy against electrolyte depletion.

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