Gut Microbiome & Performance in Horses: What Research Shows

A horse's gut microbiome, the vast community of bacteria and other microbes living mainly in the hindgut, does far more than simply digest fibre. Research now links it to energy supply, immunity, behaviour and even athletic performance. One landmark study suggests the microbiome’s influence starts in the very first weeks of life. If you're new to the topic, our guide to why gut health matters for horses covers the basics. Here, we look at what the latest research shows.

It starts in the first month of life

The most striking evidence so far comes from the University of Surrey's Alborada Well Foal Study. Researchers followed 52 Thoroughbred foals bred for racing, analysing 438 faecal samples taken at nine points during their first year, alongside detailed health and performance records up to three years old.

What they found:

• Respiratory disease: foals with lower gut bacterial diversity at one month old had a significantly higher risk of respiratory disease later in life.
• Orthopaedic and soft-tissue problems: low bacterial diversity early in life was also associated with a greater risk of soft-tissue and orthopaedic issues as the horses grew.
• Digestive illness: gastrointestinal illness was linked to the abundance of a particular bacterial family at 90 days old.
• Performance: athletic performance was positively associated with higher bacterial diversity at one month old.

The researchers describe it as the first study to show a relationship between early-life gut bacteria and later athletic performance, with possible implications for athletes of all species, including humans.

The antibiotic finding

The study also looked at foals given antibiotics in their first month of life. Those foals had significantly lower bacterial diversity at 28 days old, a significantly higher risk of respiratory disease later on and significantly lower average prize money earnings, which the researchers used as a measure of athletic performance.

The lead researcher, Professor Chris Proudman, said the findings reinforce that antibiotics should be used sparingly and appropriately. That is not a reason to avoid them when they are necessary and your vet prescribes them. Instead it's a reason to take gut health seriously during and after any course of treatment. Our article on supporting gut health after worming or antibiotics covers this if you want to learn more.

The gut and behaviour

The link doesn't stop at physical health. In a study of 185 healthy horses, French researchers found that how horses were ridden and managed was strongly associated with differences in their gut microbiota. Behaviours that point to compromised welfare, such as stereotypies, hypervigilance and aggressiveness, were also associated with the gut microbiota, supporting the idea of a microbiota-gut-brain axis.

A follow-up study looked at 27 sport horses given six weeks out at pasture with other horses. Their overall gut diversity stayed similar, showing the microbiome is fairly resilient, but time at pasture increased certain beneficial bacterial groups, and the effect lasted a month after the horses returned to their stables. You can read more about this connection in our article on the gut-brain axis in horses.

What the research doesn't show (yet)

It's worth being clear about the limits of this research. These studies show associations, not direct cause and effect and they involve relatively small groups of horses. Not every study finds a link with performance, either. Research on 52 endurance horses found that their gut microbiota was mainly shaped by diet and didn't predict race ranking or whether a horse would finish.

What the evidence does consistently show is that the gut microbiome matters, that it can be influenced by how horses are fed and managed and that early life appears to be a particularly important window.

What this means for your horse

You can't go back and change your horse's first month of life, but you can support a stable, diverse microbiome from here on:

• Put forage first. Fibre is the microbiome's main fuel, so keep forage available for as many hours as possible.
• Feed variety, not just volume. Different microbes use different plant fibres. Horses on hay from a single grass source get far less variety than horses grazing mixed pasture. Our guide to prebiotics, probiotics and postbiotics explains why this matters, which you need and when.
• Make changes gradually. Introduce new forage, grazing or feeds over a week or two to give the microbiome time to adjust.
• Allow turnout and company where you can. Time at pasture with other horses appears to benefit the gut as well as the mind.
• Use antibiotics only when your vet prescribes them, and ask about supporting gut health during and after treatment.
• For breeders: a foal's first month is a critical window for building the gut microbiome. Discuss management during this period with your vet, particularly if any treatment is needed such as painkillers and antibiotics.

FAQs

Can a foal's gut health affect them later in life?
Research suggests it may. A University of Surrey study found that foals with lower gut bacterial diversity at one month old had a higher risk of respiratory disease later in life, and early diversity was also associated with orthopaedic problems and athletic performance.

Do antibiotics affect a horse's gut microbiome?
Yes. In the same study, foals given antibiotics in their first month had lower gut bacterial diversity and a higher risk of later respiratory disease. Antibiotics are often essential for certain ailments, so always follow your vet's advice and ask about supporting gut health during and after treatment.

Does gut health affect a horse's behaviour?
Studies have found associations between the gut microbiota and behaviours linked to poor welfare, such as stereotypies and hypervigilance, supporting the idea of a gut-brain axis in horses.

How can I support my horse's gut microbiome?
Put forage first, provide a variety of fibre sources, make dietary changes gradually, allow turnout and company where possible, and use antibiotics only when prescribed by your vet.

Sources

1. Leng J. et al. (2024). Early-life gut bacterial community structure predicts disease risk and athletic performance in horses bred for racing. Scientific Reports 14:17124.
2. Mach N. et al. (2020). Priming for welfare: gut microbiota is associated with equitation conditions and behavior in horse athletes. Scientific Reports 10. doi: 10.1038/s41598-020-65444-9.
3. Mach N. et al. (2021). Gut microbiota resilience in horse athletes following holidays out to pasture. Scientific Reports 11.
4. Plancade S. et al. (2019). Unraveling the effects of the gut microbiota composition and function on horse endurance physiology. Scientific Reports 9:9620.