Could gut health be influencing your breeding results?
Fertility in horses is often approached through the lens of reproductive anatomy, hormone profiles and semen quality – and rightly so. However, another topic gaining increasing scientific attention for its influence on fertility is the hindgut microbiome.
Far from being limited to fibre digestion, the microbial population within the equine large intestine plays a central role in regulating metabolism, immunity, inflammation and even hormone balance. As research develops, it is becoming clear that hindgut stability may be a foundational, and sometimes overlooked, contributor to reproductive performance in both mares and stallions.
Equine professionals and horse owners are perhaps more familiar with the hindgut’s role in digestion. Here, billions of microbes break down forage to produce beneficial compounds, known as metabolites, that are an important source of fuel and energy for metabolic function. Furthermore, these microbes and metabolites also interact with the immune system, stress pathways and – crucially for fertility – hormone regulation.
The gut and hormone balance
One of the most intriguing areas of research is how gut bacteria help regulate hormones – an example of which is oestrogen metabolism.
The group of microbes involved in regulating oestrogen are referred to as the ‘estrobiome’ and have been shown to directly influence fertility in other species. In horses we suspect similar mechanisms may influence hormone fluctuations and potentially contribute to irregular oestrous cycles, suboptimal follicular development or even early embryonic loss.
Dysbiosis, inflammation and embryo survival
The delicate balance of the hindgut can be disrupted by dietary and management factors. High starch intakes, sudden feed or forage changes, travel, competition schedules or other stressors may reduce beneficial fibre-digesting microbes and encourage lactic acid-producing bacteria. This leads to a decrease in the production of beneficial metabolites and, instead, a rise in lactic acid levels.
As lactic acid accumulates, hindgut pH drops, leading to irritation and breakdown of the gut lining. When this barrier is compromised, inflammatory compounds like endotoxins enter circulation resulting in low-grade, systemic inflammation – also known as ‘leaky gut’.
While this may not always present with obvious digestive signs, systemic inflammation can have subtle but meaningful effects on reproduction. Oocyte (egg) quality, the uterus’s ability to accept an embryo and early embryo survival are all sensitive to systemic inflammatory status. Increasingly, chronic low-grade inflammation is being increasingly linked to subfertility in broodmares.
Metabolic health and ovarian function
Reproductive performance is closely linked to body condition score, insulin sensitivity and overall energy balance – and the hindgut microbiome plays a central role in all three.
A stable, fibre-focused microbial population supports efficient energy extraction and steadier glucose and insulin responses. Conversely, microbial imbalances can contribute to metabolic dysfunction, which affects ovarian activity. Therefore, supporting hindgut stability may indirectly support more consistent cycling and ovulation through improved metabolic regulation.
Immune tolerance and uterine health
Around 70-80% of immune cells are located in the gut and interact constantly with the gut microbiome, which helps regulate and train immune responses.
For breeding mares, immune balance is critical. Successful embryo implantation and post-foaling recovery require a carefully controlled immune tolerance, while excessive inflammatory responses increase susceptibility to endometritis and may compromise early pregnancy.
The gut-brain-ovary axis
Another emerging area of research is the interaction between the gut, stress hormone cortisol and reproductive function, known as the gut-brain-ovary axis. The microbiome influences cortisol regulation and is closely intertwined with the horse’s fight-or-flight response.
Short-term stress is normal and often unavoidable. However, chronic stress can suppress the release of key reproductive hormones like GnRN (gonadotropin-releasing hormone), FSH (follicle-stimulating hormone) and LH (luteinising hormone), which disrupts ovulation. Competition mares transitioning into breeding programmes and embryo transfer donors may be particularly vulnerable due to the drastic change in their daily routine.
Maintaining gut stability may help buffer some of the physiological effects of stress, although good management practices remain essential. (Read more about stress and the microbiome here.)
Practical considerations
While gut health is not a standalone solution to fertility challenges, it represents an important and often overlooked component of reproductive management.
Key principles include:
- Prioritising high-quality, ad lib forage
- Avoiding excessive starch intake where possible
- Introducing dietary and management changes gradually
- Minimising stress during key reproductive stages
- Providing targeted, evidence-based postbiotic or probiotic support, paying close attention to strain, stability and effective dosage. Learn more about Actisaf® live yeast probiotic and postbiotic Safmannan® on our website and in this video.
Mares who are older, subfertile, embryo transfer donors, surrogates and high-performance athletes entering breeding programmes may particularly benefit from proactive hindgut support.
As our understanding of the equine microbiome continues to grow, one message is becoming clear: reproductive success is not entirely down to the ovary or uterus. A stable, well-supported hindgut may be one of the most valuable investments we can make in long-term breeding performance.
Contact us to discuss how you can incorporate good gut health management into your programme!
