The author was a former Rutgers Ph.D. student and is now with the University of Wisconsin-River Falls.

As hindgut fermenters, horses utilize forage differently than ruminant animals.

Horses are classified as hindgut fermenters. The main site of digestion and absorption of dietary sugars, starches, fats, and protein is the small intestine. However, unlike ruminant forage consumers, horses do not produce enzymes capable of breaking down more complex carbohydrates such as fructans, cellulose, and hemicellulose. Rather, fructans and dietary fibers pass into the hindgut and are subject to microbial fermentation.

While the primary function of the hindgut microbiota is to allow the horse to extract energy from these more complex plant components, hindgut bacteria are also capable of fermenting any nutrients escaping digestion in the foregut. Given its essential role in fiber degradation and the high-fiber content of pasture forages, the hindgut microbiota is of central importance in the nutrition and health of grazing horses. Grazing horse microbiome research has primarily focused on inter-diet comparisons with concentrates or other forages. Seasonal and environmental associations, as well as relationships with horse metabolism, have also been explored.

Comparing diets

Diet is considered the dominant factor affecting the community structure of the gut microbiota across animal species. In horses, inter-diet comparisons have been conducted to evaluate the hindgut microbiome of horses adapted to variable diets, as well as their responses to transitions between diets.

Differences in microbial community structure, species composition, and function have been documented in horses fed high-concentrate versus high-forage diets. In general, the microbiomes of horses fed high-forage diets are characterized by greater diversity and temporal stability, greater prevalence of fiber-degrading bacteria, lesser prevalence of starch-degrading and lactate-utilizing bacteria, and greater butyrate and acetate concentrations with less propionate and lactate.

While many of these studies have utilized hay or silage, inter-diet comparisons between the microbiome of horses grazing pasture forage and horses fed mixed diets including concentrates have similarly demonstrated greater diversity in the hindgut microbiome of grazing horses.

It has been previously shown that there is a greater gut microbial diversity in horses maintained on pasture compared to horses fed either silage or hay, as well as differences in microbial community composition in horses consuming different forage sources. Microbial community composition was also impacted by forage type, with abundance of family Streptococcaceae lowest in pastured horses. This family of bacteria notably contains bacteria that can cause hindgut acidosis.

Differences in fecal microbial structure and composition have also been identified in horses grazing warm-season versus cool-season grasses, as well as in comparison to a standardized hay diet. As shown in Figure 1, this research conducted in our lab at Rutgers University demonstrated greater microbial diversity in horses grazing warm-season grasses in comparison to when horses were grazing cool-season grasses in either the spring or the fall.

When diets change

Research shows rapid changes in the hindgut microbiome following abrupt changes between concentrates or abrupt inclusion/elimination of supplemental concentrates in horses maintained on hay or pasture. Comparatively, few studies have investigated longitudinal shifts in the hindgut or fecal microbiome of horses during transitions between all-forage diets; however, results suggest more subtle changes compared to diets including concentrate.

Research horses graze warm-season grasses at Rutgers University.

For example, when horses are transitioned between hay and pasture, research has shown no significant changes in hindgut microbial diversity, but broad phylum-level changes for certain bacteria have been documented. Research has also demonstrated the resilience of the equine hindgut microbiota in response to transition between pasture forage and silage, with microbial communities exhibiting stability after just 96 hours.

In our studies of the grazing horse microbiome at Rutgers, we similarly found that equine fecal microbial community structure and composition were largely stable across transitions between warm-season grass and cool-season grass pasture. Species composition did not appear to shift uniformly across days of this transition regardless of if horses were transitioning from cool-season to warm-season grasses or back to cool-season grass pasture. However, research is needed across additional grass species/varieties and to include assessments of horses grazing pasture forage that includes legumes.

Microbiome and metabolism

While the associations between the equine hindgut microbiota and metabolic health have been explored, there is a lack of information on the interplay between forage nutrients, the hindgut microbiome, and metabolism of grazing horses. Previously, correlations have been reported between pasture nutrient composition and several unclassified bacterial genera, with limited to no information about potential functional significance of these relationships.

Our lab demonstrated that nonfiber nutrient fractions including crude protein (CP) and non-structural carbohydrates (NSC) exerted the greatest influence on bacterial species composition in the microbiomes of grazing horses. We also found that bacteria associated with gut health in other animal species were positively correlated with CP and negatively correlated with NSC. Interestingly, C. butyricum was also negatively correlated with peak plasma glucose in response to an oral sugar test.

Equine microbiome researchers have begun to explore the impacts of season and climate on the hindgut microbiome of the grazing horse. Weather variables including temperature and precipitation have been associated with compositional changes in horse microbial community.

Fluctuations in bacterial diversity and relative abundance across seasons and relationships with climatic conditions were also subsequently documented. However, seasonal controls were not implemented in these prior studies, leaving potential implications of seasonal changes confounded by inherent differences in pasture forage quality and availability.

Overall, grazing horse microbiome research is limited. Impacts of management systems and stocking methods, common pasture management practices, pasture quality, and effects of commonly utilized supplemental feeding strategies are unknown. An expansion of existing research is required to determine responses of the hindgut microbiota across pastures with varying botanical composition, as well as to evaluate geographical and climatic influences.