Gut Health and Horse Behavior: What’s the Connection?

You’ve probably heard the idea that “a horse is what he eats.” There is some truth in that.
A healthy gut microbiome can have a positive impact on your horse’s behavior. | iStock

You’ve probably heard quite a bit lately about the intestinal microbiome, and the idea that “a horse is what he eats.” There is some truth in that, including the microbiome’s effect on the behavioral characteristics of a horse. In May 2022, Jyme Nichols, PhD, and Tania Cubitt, PhD, presented their research on the topic.

The Gut is the Second Brain of the Horse

Nichols refers to the intestinal microbiome as the second brain of a horse. The gut microbiome links nutrition and health as intestinal microbes harvest nutrients and extract energy from feedstuffs. Intestinal microbial communities—bacteria, fungi, viruses, and parasites—tailor themselves to what a horse is eating. This is where you, as a horse caretaker, come in. Daily management has everything to do with your horse’s overall health, performance, and behavior.

A healthy gut relies on a balance between beneficial and pathogenic microbes. Not only does a healthy microbiome promote efficient digestion and nutrient absorption but it also resists colonization by pathogens, generates antimicrobial products, and creates a barrier against toxins. It also forms a complex immune system responsible for 70% of a horse’s overall immune responses.

A link between the intestinal tract and the brain, referred to as the gut-brain axis, describes the connection between the enteric (intestinal) nerve complex and the central nervous system. Gastrointestinal activity and microbes influence brain neurochemistry to affect a horse’s mood and behavior. Your horse’s mental comprehension, emotions, and coping mechanisms, and appetite are all connected through the gut-brain axis.

The microbiome plays a role as an endocrine system by releasing hormones such as neurotransmitters (epinephrine and norepinephrine). Biochemical signals are passed via hormones, the immune system, and body fluids. Importantly, intestinal microbes are sensitive to change and to stress, taking 3–4 weeks to adapt to even just a new load of hay, says Nichols. Travel for short periods, like 1–2 hours, may affect the intestinal microbial community, and there are reports that horses left at home can experience similar microbiome changes when their equine companions leave their presence. In addition, medications like non-steroidal anti-inflammatory drugs, antimicrobial drugs, and dewormers have the capacity to adversely affect the intestinal microbial community.

What Disrupts the Gut-Brain Axis?

A state of stress or anxiety slows digestion. This is a protective measure that allows the sympathetic nervous system to fulfill survival functions like fight or flight and to direct blood flow to critical organs. Bacteria in the gut are critical to the development and maturation of both the enteric and central nervous systems. Major neurotransmitters also result in the synthesis of serotonin (a happy hormone), dopamine, and opioids. Stress increases epinephrine and norepinephrine release into circulation. Norepinephrine directly promotes the survival of pathogenic bacteria, increases blood pressure and heart rate, and causes a horse to become hyperactive, anxious, irritable, and to sleep poorly.

Intensity and type of exercise can also affect the microbial gut population. In addition, there is a link to behavioral oral stereotypies, such as cribbing, which are indicative of mental distress—cribbing is associated with ghrelin hormone produced by the gastrointestinal tract.

The brain receives signals of hunger or fullness that further control intestinal motility, fluid exchange between the bowel lining and the lumen (intestinal cavity), as well as local blood flow. The parasympathetic nervous system works to balance the sympathetic nervous system by controlling digestion, lowering blood pressure, slowing the pulse, saving energy, and allowing for rest and digestion. Stress changes colonic motility by decreasing blood flow to the gut and decreasing digestion and motility. It can also lead to a “leaky gut” that results from leakage of bacteria past weakened “tight junctions” between intestinal cells. The result is a pronounced proinflammatory response and chronic systemic inflammation as well as behavioral changes of mood and anxiety.

In cases of intestinal dysbiosis (aberrations in normal function due to disruption in the balance of the microbiome), gastrointestinal disorders may develop due to an increase in inflammatory cytokines, neuromodulators, microbial metabolites, and changes in dietary fermentation. Such dysbiotic changes increase the risk for colic, colitis, or diarrhea, as well as influencing behavioral changes that affect performance and a horse’s social dynamics with people or other horses.

Cubitt points out that firmicutes are the predominant microorganisms in a healthy gut, comprising 39.5%–72.6% of bacteria. They produce short-chain fatty acids (SCFA) like butyrate that are important for preventing inflammation while also providing nutrition and fuel for intestinal lining cells (enterocytes or colonocytes). A high-forage diet significantly increases levels of one type of firmicutes organisms —lachnospiraceae—whereas a horse with colitis experiences notable decreases in these beneficial microbes.

Many studies have examined behavioral changes in association with stressful events. One study noted that starch digestion and anaerobes increase in a high-grain diet, causing a horse to become over-reactive through changes in the gut-brain axis. Another study separated 45 elite horses into two groups: 18 were stall-confined and trained 6 days/week. The other 27 horses were pastured for 41 days and then stalled. Forage intake between the two groups was comparable yet the temporary period in pasture increased butyrate-producing bacteria with positive effects lasting one month after the horses returned to the stall. Butyrate production by intestinal microbes is associated with favorable effects on mood and behavior—butyrate improves tight junctions between intestinal cells to reduce leaky gut effects by keeping toxins and bacteria out of the bloodstream.

Strategies for an Improved Microbiome and Cooperative Behavior

Management and diet have everything to do with how a horse behaves and how durable its health. Keeping in mind that everything you do with your horse has an effect on this outcome will help steer you to good management practices.

A number of practical strategies help keep your horse “healthy as a horse.”

• Provide high-quality forage. Know the forage you feed, suggests Nichols: Have it tested and use feed supplements to fill gaps in the diet. When possible, feed at least 2% of a horse’s body weight per day in forage. (For example, in general, an 1100-pound horse needs 22#/day of hay.) Buy enough hay so that once tested, the diet stays consistent.

• Decrease or eliminate concentrate feed that adds too much starch to the diet. Starch such as found in oats, corn, or barley has a dramatic and adverse impact on intestinal microbes.

• Digestive aids may be useful during stressful situations like travel, competition, with the use of NSAIDs or antibiotics, or with changes in routine, ownership, and environment. Intestinal aids include probiotics, prebiotics, and postbiotics—all have an impact on equine intestines.

Prebiotics are non-digestible feed ingredient that serves as food for beneficial intestinal organisms (probiotics)—examples include quality forage, beet pulp, fructooligosaccharides from plants, and psyllium.

Probiotics are live organisms with beneficial properties for intestinal health and digestion. Examples include Lactobacillus, Bifidobacterium, and Enterococcus which are lactic-acid-producing bacteria. Yeast (Saccharomyces) also provides probiotic functions, particularly for fiber digestion.

Postbiotics are compounds that are released from the intestines after gut microbes feed on fiber.

It is noteworthy that not all commercial probiotic products contain the ingredients listed on the bucket, and not all are similarly bioavailable once ingested by the horse. Consult with your veterinarian as to which products are efficacious.

• Decrease stress of living arrangements, feeding protocols, training, competition, and travel.

• Minimize fasting periods. Horses evolved to be trickle feeders, so their intestines flourish best when there is access to free-choice hay or pasture, when possible.

• Provide ample turnout to limit the duration of confinement and to provide free-choice exercise, mental stimulation, herd socialization, and grazing if pasture is available.

• Provide bedding to encourage horses to lie down more to decrease aggressiveness.

• If stalled, provide window openings to the external environment for mental stimulation.

• Consult about dietary strategies with your veterinarian and/or an equine nutritionist with confirmed credentials who is well versed in scientific theory.

The Bottom Line

A “Keep It Simple” strategy reaps great benefits in horse-keeping and horse health. Be cognizant of what you add to your horse’s diet and if it is not an essential nutrient or intestinal protectant, consider leaving it out. If your horse’s normal demeanor and/or performance deteriorates, consider the intestinal tract as a source of the problem and take steps to simplify back to basic dietary management.

The microbiome may be invisible, but the outward effects become obvious when horses begin to act out or experience intestinal distress. Contact your vet at the first signs of any suspect problems. Consult with your veterinarian to put together a feeding program that heads off any unwanted issues.






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