The racehorse's extraordinary peeing power.

Previously published Slate articles made new.
May 1 2008 4:55 PM

How Much Do Racehorses Pee?

Horses really do possess great powers of urination.

Twenty racehorses will take the track at this Saturday's Kentucky Derby, providing an occasion to wonder: Does a racehorse actually "pee like a racehorse"? David Sessions investigated the matter in a 2007 "Explainer." His findings are reprinted below.

Illustration by Mark Alan Stamaty.

The third leg of horse racing's Triple Crown takes place on Saturday, with the running of the Belmont Stakes. Around 60,000 fans will be watching in Elmont, N.Y., as they put down beer and the track's signature cocktails. Needless to say, they'll probably be peeing as much as the racehorses. Wait, how much does a racehorse pee?

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A lot. Horses typically produce several quarts of urine every four hours, for a total of about 1.5 to 2 gallons per day. (By contrast, an adult male human pees 1 or 2 quarts per day.) The stream, usually one-third to a half-inch in diameter, can last up to 30 seconds. In general, the larger the animal, the more it pees. A Clydesdale, for example, weighs twice as much as a Thoroughbred and produces urine in greater volume (and with a more pungent smell). An average pasture horse that spends its day grazing might also beat a racehorse in a peeing match: Pasture grass contains a lot more water than the carefully prepared grains and pellets fed to racehorses.

The popular notion of incontinent racehorses seems to have roots in the late 1970s, when trainers began the widespread use of diuretics like Lasix (furosemide). Lasix inhibits the absorption of sodium and draws water into the bladder. This causes the horse to excrete more fluids, which could, in theory, make a horse lighter on its feet and faster on the track. Depending on the dose, a Lasix treatment could cause a horse to move several gallons of urine within an hour, which could translate to a quick drop of 10 pounds from a horse's body weight before a race.

It's not against the rules to dose a racehorse with Lasix, but its use is carefully regulated and abuse will result in a penalty. (In general, you're only allowed to use the drug to prevent internal bleeding during a race. You're not supposed to use it strictly as a diuretic.) Racing officials have run drug tests on competitors since 1903, and today they take blood and urine samples before every race. At the Kentucky Derby and most other major races, competitors using Lasix are allowed to compete, but they're marked with an L on the programs.

Got a question about today's news? Ask the Explainer.

Explainer thanks Sue McDonnell at the University of Pennsylvania School of Veterinary Medicine, Hal Schott at Michigan State University's Department of Large Animal Clinical Sciences, and Thomas Tobin at the University of Kentucky's Gluck Equine Research Center.

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