Gut Microbiota: The Benefits of Hay for Competition Horses

High-performance horses are subjected to living conditions and intensive training that can compromise their well-being, particularly by altering their gut microbiota. However, although spending a short period of time in the pasture is beneficial for these horses’ mental well-being, it does not appear to be sufficient to restore long-term balance to microbial populations, which show a certain resilience to environmental changes.   Like all athletes, competition horses undergo rigorous training that can cause long-term physical and mental stress. Over time, this can compromise their well-being and alter their gut microbiota. In humans, an imbalance in the gut microbiota has been implicated in the pathogenesis of mental illnesses such as anxiety disorders and depression. However, the role the microbiota plays in the behavioral expressions of horses facing physical and mental stress has yet to be explored. The mechanisms that govern the resilience and dynamics of the microbiota in these animals suffering from behavioral disorders remain poorly understood. Although definitive evidence is still lacking in horses, some authors believe that the impact of stress on these equine athletes could be mitigated by brief periods in a more natural environment, such as being turned out in a pasture with other horses, for example. Indeed, these high-performance athletes often live under highly controlled conditions, in individual stalls that prevent them from engaging in species-specific behaviors and alter their gut microbial communities. This short period spent in the pasture could be particularly beneficial for these animals’ gut bacteria (increased bacterial diversity, reduced abundance of pro-inflammatory taxa and pathobionts). To test this hypothesis, researchers evaluated the effect of a period spent in the grazing area on the horse’s welfare and the composition of its gut microbiota. A total of 27 competition horses were monitored before and after a one-and-a-half-month period of grazing. Their fecal microbiota and behavioral profiles were compared to those of 18 horses housed in individual stalls. Ultimately, the results appear less significant than expected. Although the effects of pasture on equine well-being are evident, the overall diversity and composition of the microbiota do not appear to differ as much as expected between horses placed on pasture and those isolated in stalls. Gut microbial populations demonstrate resilience in the face of this environmental change. It should nevertheless be noted that access to the outdoors leads to an increase in Ruminococcus and Coprococcus populations for one month after the horses return to individual stalls. Although this effect is short-lived, these butyrate-producing and functionally redundant bacteria have positive effects on health. A longer period in the pasture could exceed the intestinal ecosystem’s resilience to environmental changes and enhance the beneficial potential of microbiota changes over the long term, thereby optimizing animal welfare. These results particularly support the hypothesis that suffering in horses can influence the gut microbiota via the gut-brain axis. Future studies examining the discordance between the gut microbiota and behavior across multiple time scales should enable the development of microbiota-targeted strategies to help improve the well-being of competition horses.