Understanding Hindgut Problems in Horses

The equine hindgut breaks down fiber, which makes up most of the horse’s diet. Learn how diet and management can affect this important part of your horse’s GI system.
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thin horse eating hay near trees
Weight loss can be a subtle sign of hindgut issues in horses. | iStock

Horses commonly experience digestive problems, which can manifest in a variety of ways ranging from abnormal behavior to weight loss. Like humans, horses are monogastric, meaning they have one stomach. In contrast to humans, however, horses are hindgut fermenters.

The equine digestive system consists of two main sections. The foregut, which includes the stomach and small intestine, is where enzymatic digestion takes place, and the hindgut (also referred to as the large intestine), consisting of the cecum, large colon, and small colon, is where fermentation occurs.

The Role of the Equine Hindgut  

The horse digests starch and nonstructural carbohydrates in the small intestine. Because horses do not have enzymes in their foregut, fiber passes through undigested to the hindgut. Fiber often makes up most of the horse’s diet, making fermentation a crucial aspect of digestion. “The fermentation process in the hindgut is when all of the fiber that has not been digested in the stomach and small intestine is mixed with the population of microorganisms in the hindgut,” says Jenn MacNicol, PhD, an equine physiology researcher at the University of Guelph, in Ontario, Canada. “The microbes use the fiber as an energy source, and the byproduct of this process (volatile fatty acids) can then be used by the horse for energy.” In other words, the GI tract microbes have a symbiotic relationship with the horse, meaning both parties benefit.

“Each compartment of the hindgut harbors a specific microbiota with different functions,” says Marcio Costa, DVM, PhD, an associate professor in the Department of Veterinary Biomedical Sciences at the University of Montreal, in Quebec, Canada. “Most of the fiber fermentation occurs in the cecum and large colon, while water is absorbed in the small colon.”

Keeping this environment healthy is a critical aspect of horse management. “Many of the issues that arise in the hindgut are related to the health of the bacteria,” says MacNicol.

Common Hindgut Issues in Horses  

Understanding the prevalent issues that stem from disturbances to the hindgut, recognizing the health concerns they present, and taking steps to mitigate them helps promote optimal equine well-being. “The hindgut is really an environment with a bacterial community,” says MacNicol. “Many of the health issues that stem from this area of the body have to do with the environment being shocked.”

The microbes in the hindgut change depending on the horse’s regular diet (e.g., hay vs. concentrate). Abrupt dietary changes shock the delicate system and kill many of the microbes. “One management example that I have previously seen occur is the feeding of a large bran mash on Sundays,” says MacNicol. “This is a huge change from the horse’s daily feed the rest of the week, which is going to shock the microbes and negatively impact the hindgut environment.”

Clinically, a variety of health issues such as weight loss, dull coat, and aggressive behaviors on the ground and under saddle arise from equine hindgut disturbance. “Intestinal diseases are the major cause of death in equine patients and include colic and colitis (inflammation of the large or small colon that can lead to diarrhea),” says Costa. “Very often those diseases affect the composition of the microbiome, or aberrant microbiomes can even predispose them.”

Besides abrupt dietary changes, antimicrobial drugs, lack of movement, and fasting periods can increase a horse’s risk of developing hindgut health issues.

“When antimicrobial drugs are given, they kill the good bacteria that are protective and compete with pathogenic organisms, increasing the susceptibility to disease,” he adds. “Some bacterial populations can have excessive gas production which can cause colic. Others may produce excessive acid which decreases the pH, killing other bacteria.”

Hindgut microbial disruption can be the root cause of problems ranging from colitis, as mentioned, to laminitis, adds MacNicol. 

Reducing the Likelihood of Hindgut Issues

When making management decisions to promote health and well-being, carefully consider your horse’s digestive anatomy and how it has evolved to function. Domestic horses live vastly different lifestyles than their wild counterparts. “These changes in management that horses undergo in comparison with the continuous grazing habits that the species has been selected in nature to have is associated with hindgut issues,” says Costa. “In other words, horses should spend the majority of their time walking and ingesting small amounts of roughage.”

Owners should ensure horses spend most of their time outdoors where they can move freely and forage continuously. Always make management and dietary changes as slowly as possible, whether this includes transitioning horses to pasture or changing their feed. “When we are making management decisions for horses, always keep the health of the hindgut in mind,” says MacNicol. “You want to make sure that the bacteria always have some type of fiber to keep them happy. You also want to make sure you keep water moving through the system regularly.”

In addition to routine management changes, other stressors such as exercise, transportation, or disease can also alter the hindgut environment. Work toward consistency in your daily management routine both at home and on the road. For example, when traveling to shows, bring your horse’s hay with you so his forage stays consistent, which will reduce his likelihood of developing hindgut problems.

“A final factor to consider is the exaggerated use of antibiotics in veterinary (and human) medicine, and a more judicious use of those drugs is warranted,” says Costa. “We often see healthy foals receiving antibiotics prophylactically (to prevent them from becoming sick) or horses with runny noses who are unnecessarily treated with those drugs. Owners could be discussing alternative options with their veterinarian or waiting a day or two to give them time to recover naturally.”

Take-Home Message

Horse owners should understand how their horse’s digestive system has evolved to consume fibrous diets with minimal fasting periods. Adopting consistent dietary and management strategies and avoiding stressors that disrupt the horse’s hindgut environment can help reduce the likelihood of hindgut issues such as colitis, colic, weight loss, or abnormal behaviors.

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Written by:

Madeline Boast, MSc completed her master’s in Equine Nutrition at the University of Guelph and started an independent nutrition company known as Balanced Bay. She has worked with a variety of equids—from Miniature Ponies to competing Thoroughbreds. Boast designs customized balanced nutrition plans that prioritize equine well-being, both for optimal performance and solving complex nutritional issues and everything between. 

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