Horse Behavior and the Microbiome: What's the Connection?

Is your horse’s diet and microbiome to blame for bad behavior?

We all know that horse. The anxious, flighty one that’s unpredictable and can be difficult to handle. If it’s your own horse, have you searched for ways to help him relax? Have you used calming supplements or training techniques, with less than stellar success? Based on recent work, researchers suggest we’ve been going about the problem the wrong way. Instead of focusing on calming the brain, they say we should be focusing on the gut and the millions of microbes within it to modulate equine behavior.

“There is anecdotal evidence of a link between diet and behavior in horses, where diets high in starch reportedly lead to behavioral changes, with horses reported as ‘spooky’ and ‘on their toes,’” says Jo-Anne Murray, PhD, MSc, PgDip, PgCert, BSc(Hons), BSHII, RNutr, PFHEA, FRSB, honorary professor at the University of Glasgow School of Veterinary Medicine, in Scotland.

This link between the central nervous system (CNS) and intestinal tract helps explain how diet affects behavior—with horses fed certain diets (e.g., high-starch) sometimes developing vigilant, reactive, and unwanted behaviors—and how the brain (and mental health) affects gastrointestinal (GI) tract function. Researchers have now taken this relationship one step further, adding the intestinal ­microbiome (ecosystem of microbes) to the mix, creating a complex system called the microbiome-gut-brain axis (MGBA). 

In this article we’ll break down the research showing how diet affects equine behavior. We’ll also outline the complex bidirectional relationship between the nervous system (brain), GI tract structures, and population of bacteria, viruses, and protozoa that makes up the intestinal ­microbiome.

Equine Studies on the Microbiome-Gut-Brain Axis (MGBA)

In 2015 French researchers demonstrated that diet appears to influence equine behavior (Destrez et al.). In their study, horses received either a primarily hay diet (90% hay and 10% barley) or a high-starch diet (57% hay and 43% barley), or they transitioned from a high-fiber to a high-starch diet over five days. The research team collected fecal samples from each horse to analyze the microbiome and performed two behavior tests: a sociability test in response to an unfamiliar horse and a neophobia test in response to a novel object. Finally, the researchers assessed the horses’ use of their time when stalled. Here’s what they found:

  1. Horses on the high-starch diet had more anaerobic bacteria (that do not need oxygen to grow), lactate-utilizing bacteria (lactate is produced by rapidly fermented starch in the hindgut—the GI tract beyond the small intestine), and amylolytic bacteria (that break down starch). Murray says these results are not surprising, as previous studies have noted these bacteria in the microbiomes of horses on high-starch diets.   
  2. Vigilant behavior expressed by horses fed high-starch diets tended to increase when the amylolytic bacteria concentrations increased.

“This research group therefore concluded that in horses, changes in behavior appear to be linked with changes in the gut microbiota,” she says.

Murray and colleagues in the U.K. conducted similar types of studies. In one they fed ponies either a high-fiber or high-starch diet for 28 days. At the end of that period, they performed two behavioral tests. The first was a passive human test where an individual the ponies did not know stood motionless in a familiar test barn. The second was a novelty test in which they placed an object unfamiliar to the ponies in front of a bowl full of food to which the ponies were ­accustomed. 

“The behavioral responses of the ponies to the human were recorded during these two tests, including alertness, feeding, interaction, and locomotion,” Murray says. “Behaviors such as glancing at the human/stimulus, sniffing/touching, feeding, and moving at various gaits and pace changes were assessed.”

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