Why is my Horse's Manure Runny?
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.

The proportions of the various bacteria in the hindgut mirror the diet, which is one reason why you should avoid sudden diet changes for horses. Numbers of various bacteria need to adapt and adjust to diet changes, which takes time. This is just one reason why veterinarians and nutritionists recommend making diet changes slowly over seven to 10 days. As you have noted, diet changes can have a positive or negative impact, and examining these changes in light of what’s in the new diet can give some clues to what is going on. Changes in hay can have a big impact that’s often overlooked. The type of hay might not have changed, but the proportions of the various carbohydrate fractions can differ considerably from one batch to the next, which can cause problems if the hay is introduced suddenly.

The enzymes secreted in the small intestine are also specific in type and quantity to the nutrients a horse consumes. Therefore, if you change the amount or type of grain in the diet suddenly or feed a hay high in nonstructural carbohydrates, undigested starch and simple sugars might reach the hindgut. This is what can happen in the spring on good-quality, high-sugar pasture and why you should introduce horses to pasture slowly.

Forages (hay, beet pulp, alfalfa), also contain fructans, which are sugars too complex for enzymatic digestion but easily fermented by hindgut bacteria. This means a higher-than-normal availability of energy for the bacteria results in rapid growth, declining pH, shifts in the make-up of the population, and gas production. These changes in the hindgut can cause loose manure, colic, and—in severe cases—laminitis.

Introducing hay containing larger quantities of easily fermentable fructans can really upset some horses’ digestive systems. I had a barn that was feeding a hay containing 8% water-soluble carbohydrate and switched to another orchard hay that had more than 20% water-soluble carbohydrates, the difference being fructans. Unfortunately, the barn managers had started feeding the new hay with no gradual introduction before test results came back, and the majority of the horses in the barn experienced issues ranging from loose manure to mild colic. Gradually, the horses recovered, but a couple took longer and needed additional gastrointestinal support.

I tend to see this fairly often. A hay change might upset a number of horses with the majority recovering just fine without intervention, but one or two have lingering issues—or only the one or two ever have an issue. This was the case in a barn that fed wheat hay. Only one horse was having a problem. The hay had some grain on it, and I hypothesized that possibly the horse had a faster rate of passage through his digestive tract than others in the barn. This would mean undigested grain might arrive in the hindgut, causing a hindgut fermentation disruption. Or, possibly, he produced less of the enzyme in the small intestine needed to digest starch. We reduced his wheat hay intake, added alfalfa, and his chronic loose manure cleared up.

Another client’s senior horse developed severe colitis. We took him off hay because hay’s long-stem nature can be an irritant. He was put on beet pulp and a senior feed with some hay pellets, and after improving initially, he began to deteriorate. In the end we took him off beet pulp and senior feed that contained beet pulp and put him on 100% hay pellets, and his signs resolved. We don’t know why beet pulp caused problems for the horse, but he clearly had a sensitivity to it. Possibly it was too digestible for him.

Research has shown that feeding 30 grams of a short chain fructooligosaccharide prebiotic might help reduce colonic microflora changes and the buildup of lactate caused by sudden introduction of barley to the diet.1 This may help in other situations, too, such as hay changes. It would be interesting to know if your new feed contains a prebiotic when perhaps your previous feed did not, as this could be helping your horse.

Stress can also potentially allow bacterial population shifts. Pathogens, such as salmonella, that typically exist at low levels without causing problems can take hold in stressed horses. This can cause infectious diarrhea, which is why you should discuss all loose manure cases with your veterinarian.

Use of antibiotics and overzealous use of non-steroidal anti-inflammatory drugs can also cause digestive upsets and shifts in the microbial population, leading to loose manure. Alfatoxins or mycotoxins, feed contaminants typically occurring at low levels, might also cause problems in some individuals.

As initially mentioned, the possibilities are many. Knowing that your horse is possibly sensitive to feed changes, consider providing a good-quality prebiotic during times of stress or diet change.


1. Respondek, F., A. G. Goachet, and V. Julliand. 2008. Effects of dietary short-chain fructooligosaccharides on the intestinal micro?ora of horses subjected to a sudden change in diet. J. Anim. Sci. 86:316–323.