Why Is My Horse’s Manure Runny?

Nutrition changes, high-carb diets, and more: Discover the nutritional factors that can cause loose stool in horses.

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Why is my Horse
Many scenarios can cause loose manure. A hay change, for example, might upset some horses' digestive systems. | Photo: Anne M. Eberhardt/The Horse
Q: My horse has had what looks like the effects of loose manure on his rear end since I’ve owned him (for about a year). I had him on a senior feed (though he is a 7-year-old athletic horse—I selected the feed with a nutritionist’s guidance based on its nutrient levels, and I had transitioned him to this from a complete feed of the same brand), a grass hay, soaked beet pulp (because we just couldn’t seem to keep the weight on him), and a daily digestive supplement. He spent half his time on pasture. My veterinarian determined he is a low shedder with fecal egg counts. We’ve since moved to a new boarding farm, and he’s on a new feed and no soaked beet pulp (though there is dry beet pulp in his new feed), the same digestive supplement, and, of course, new pasture. I’ve been holding my breath to see if he loses weight. He’s actually gained weight, even without the soaked beet pulp and even though the pasture is beginning to go dormant. The hay source at the former farm was inconsistent (common around here for large farms because of hay availability), so can I assume that it was the feed causing the loose manure, or could it have been the pasture? There were a lot of weeds in his former pasture. Or could it have been a different level of stress at that farm?– S.L, via email

A: Many scenarios can cause loose manure, which is why it can be so frustrating. I have several clients who’ve come to me because their horse either passes loose manure or what looks like brown water after manure. It’s important to work with your veterinarian as you have done to rule out the possibility of intestinal parasites, infectious diseases, and other conditions, such as mild colitis, which can all cause diarrhea or loose stool. Once a vet gives the horse an otherwise clean bill of health, investigating its current diet is a good place to start.

Understanding the various nutrient fractions in feeds and how they’re meant to be digested is helpful in understanding loose manure. In these situations, I pay the most attention to the diet’s various carbohydrates. Carbohydrates are either digested in the small intestine, with enzymatic help or—if their structure is too complex for this process—carbohydrates pass to the hindgut, where the bacterial population ferment the majority of them. Bacteria use the released energy for their own growth and produce byproducts called volatile fatty acids (acetic, butyric, valeric, and lactic acids) that the horse absorbs and uses for energy. Gas is another byproduct of this process. It’s most likely this hindgut environment that has gone awry in cases of diet-related loose manure.

The cellulolytic bacteria that ferment a majority of structural or complex carbohydrates in feeds, such as hay and pasture, require a neutral acidity or pH and a steady supply of substrate. The composition of volatile fatty acids (VFA) in the hindgut play a large role in the pH level. Most structural carbohydrates, such as cellulose, result in the release of acetic acid, which is a fairly neutral VFA. However, nonstructural carbohydrates (starch, simple sugars) are rapidly fermented and result in the release of lactic acid, which is far more acidic. These simple carbohydrates should be removed during the enzymatic digestion in the small intestine and never reach the hindgut. If the small intestine’s ability to digest and absorb them is overwhelmed, they will end up in the hindgut. This is why nutritionists and veterinarians recommend small grain (feed) meals. Initially, lactic-acid-utilizing bacteria “mop up” lactic acid, preventing the overall hindgut environment from becoming too acidic. But, again, if their capacity is overwhelmed by the amount of lactic acid produced, the hindgut will eventually become too acidic. Declining acidity will hamper the cellulolytic bacteria, which might start to die off, releasing toxins. Once the hindgut ecosystem is out of balance it can be difficult to restore

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Written by:

Clair Thunes, PhD, is an equine nutritionist who owns Clarity Equine Nutrition, based in Gilbert, Arizona. She works as a consultant with owners/trainers and veterinarians across the United States and globally to take the guesswork out of feeding horses and provides services to select companies. As a nutritionist she works with all equids, from WEG competitors to Miniature donkeys and everything in between. Born in England, she earned her undergraduate degree at Edinburgh University, in Scotland, and her master’s and doctorate in nutrition at the University of California, Davis. Growing up, she competed in a wide array of disciplines and was an active member of the U.K. Pony Club. Today, she serves as the district commissioner for the Salt River Pony Club.

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