Differences in Urine

Q. My two horses' urine output is very different. For instance, the ammonia smell of my Thoroughbred's urine is a lot stronger than that of my Quarter Horse. I read that protein affects ammonia levels--could that be why? And the color of my Quarter Horse's urine (in the snow) has always been of a medium to dark orange, while my Thoroughbred's is, well, yellow. What could be causing this?

Pascale, Ontario, Canada

A. The kidneys produce urine, which contains waste products that need to be eliminated from the body. These waste products include urea and ammonium ion, two breakdown products of protein. The more protein a horse has in his diet, the more urea and ammonium his body will produce and excrete in the urine. Urea is composed of two ammonia molecules and can be broken down to ammonia after being passed in urine. So the answer to your first question is that the more protein your horses consume, the more urea and ammonium they excrete. This contributes to the ammonia smell in their stalls.

However, if your Thoroughbred and Quarter Horse are eating the same diet, that does not explain the difference between the ammonia smell in their stalls. An additional factor that can play a role can be differences is the normal bacterial flora present in the lower urinary tract. Some horses simply have more bacteria with an enzyme called urease that splits urea back into ammonia molecules. If your Thoroughbred has more bacteria with urease in his lower urinary tract, his body will produce more ammonia and, thus, you will notice a stronger ammonia smell in his stall.

Finally, horses also have varying amounts of compounds called urocatechins in their urine. These can be oxidized by light after they are passed and turn orange to red in color, thereby discoloring shavings. In the winter, they also discolor snow and are often confused with bloody urine. The color change only occurs in urine passed by some horses and not others--the reason for this difference is not fully understood.


About the Author

Harold Schott II, DVM, PhD, Dipl. ACVIM

Harold Schott II, DVM, PhD, Dipl. ACVIM, is a professor of equine medicine at Michigan State University.

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