Understanding the equine gastrointestinal (GI) tract and how to optimally support it is important for your horse’s health and welfare. Currently, knowledge on digestive disorders and how diet influences the GI environment is lacking.

To address this gap, Federica Raspa, DVM, PhD, of the University of Turin, in Italy, investigated variations in gut microbiota along the length of the digestive tract in healthy horses of the same age and breed, all kept under the same management conditions.

Evaluating the Equine Microbiome

Promoting your horse’s GI health helps his well-being. “The study of microbiota is an important area of research because it allows us to emphasize that diet is critical for ensuring the horse’s gut health,” said Raspa. “Moreover, studies on the gastrointestinal microbiota could enable us to gain a deeper understanding of the mechanisms underlying common gastrointestinal disorders that affect horses.”

Raspa and her colleagues randomly assigned 19 young horses to a high-fiber (HF) or high-starch (HS) group. They housed the horses in indoor pens for the duration of the study (129 days) and fed them the same hay, with differing complementary feeds. They fed the HS group of horses a starch-rich pellet at increasing rates until they reached 8 kilograms/day, and they offered HF horses 3.5 kg of a fiber-rich pellet daily.

After horses consumed the diets for 72 days, the researchers collected postmortem samples from the duodenum, jejunum, and ileum (parts of the small intestine); the cecum; the sternal flexure, pelvic flexure, and right dorsal colon (parts of the large colon); and rectum. Then they extracted DNA and sequenced it to evaluate the microbiota present in each sample.

How Diet Influences Equine Microbiota Composition

The researchers is study found horses on a HF diet had higher diversity in the cecum, pelvic flexure, and right dorsal colon than horses on the HS diet. Previously, scientists had established that greater diversity in the intestinal microbiota is associated with greater microbiome stability and resistance against potentially pathogenic (disease-causing) bacteria increases with greater alpha diversity.

Raspa and her colleagues were also able to demonstrate that changes occurring at the lower taxonomic level—a more detailed, specific grouping within the broader classification—cannot be observed by analyzing fecal samples collected from the rectum, which is how veterinarians check hindgut health in live horses. “From a clinical perspective, the analysis of the fecal microbiota can provide important insights into the characteristics of the diet fed to the horse,” she said. “However, as our study shows, caution is needed when interpreting fecal microbiota reports, as changes occurring at the lower taxonomic level may not reflect the condition of the entire digestive tract.”

The researchers noted the study’s limitations included the smaller sample size and the fact horses did not serve as their own controls (the scientists couldn’t compare fecal microbiota before and after the feeding trial). Additionally, if the study period had been longer, they could have documented significant differences in microbiota presence in more of the evaluated areas.

Take-Home Message

Deepening our understanding of the equine GI tract could help improve how we manage horses’ diets. “I would like this area of research to lead to the development of guidelines for the correct interpretation of fecal microbiota reports so that veterinary nutritionists can have an appropriate tool to understand microbial changes,” said Raspa.

Work with a qualified equine nutritionist and your veterinarian to closely evaluate your horse’s diet and ensure it contains enough fiber to prevent dysbiosis (disruption) of the gut microbiota associated with high-starch diets.

The study, “Microbiota characterization throughout the digestive tract of horses fed a high-fiber vs. a high-starch diet,” appeared in Frontiers of Veterinary Science in May 2024.