Review ArticleThe Impact of Diet on the Hindgut Microbiome

- Diet has a profound effect on the microbiome of the equine hindgut.
- In horses fed a hay-based diet, the bacterial community shows a high richness and diversity suggesting a strong resilience in the hindgut microbial ecosystems. However, many athletic horses are submitted to diets and feeding practices that can differ largely from those of the horse in a natural environment.
- Studies indicate a clear adverse effect of high-starch diet on microbial populations in the hindgut. This highlights the importance of managing the concentrate feeding to the horse for health and welfare.
- There is a need for further investigations regarding other dietary practices on the hindgut microbiome in order to limit their potential detrimental impact.
- Also, it would be of interest to have a better knowledge of the use of feces which appears to be good indicator of hindgut microbiota variation under dietary changes.

In horses, dietary components that are not digested by host enzymes in the foregut pass unchanged through to the hindgut where they have a profound effect on the microbiome. Epidemiologic studies have given evidence that important equine health issues could result from dysbiosis in the hindgut ecosystem as a result of dietary factors. This review presents an outline of plant carbohydrates degradation by the equine hindgut microbiome. Then, data are summarized from studies assessing the variations of the microbiome occurring in the hindgut under different dietary changes. In the literature, most studies examined the impact of abrupt changes and changes between high-starch versus high-fiber diets, while the effect of feed form and processing or feeding frequency was rarely explored. Finally, the representativeness of the fecal microbiome changes is compared to the hindgut changes when horses are submitted to dietary changes.