Changes in faecal bacteria associated with concentrate and forage‐only diets fed to horses in training

Reasons for performing study: Diets rich in readily fermentable carbohydrates, fed traditionally to meet the increased energy requirements of the performance horse, are associated with a number of gastrointestinal disorders that involve disturbances in the intestinal microbiota, however, these changes are poorly understood. Objectives: With the long‐term objective of improving intestinal health and to increase understanding of the relationship between diet and microbiota, the effect of feeding Standardbred horses a high‐energy forage‐only (F) diet was studied compared to a more traditional forage‐concentrate (C) diet on faecal microbiota. Methods: Diets were fed in a cross‐over design to 6 mature geldings on a scheduled training regime, both periods consisting of 29 days. DNA was extracted from faecal samples collected at 4 time points from each period, bacterial 16S rRNA genes were amplified and community composition assessed by terminal‐restriction fragment length polymorphism, cloning and sequencing. Faecal pH and cultivable lactic acid bacteria (LAB) and enterobacteria were also assessed on the final collection day of each period. Results: Diet F resulted in a microbial composition that was more stable between sampling periods and had lower counts (P<0.05) of cultivable LAB and specifically members of the Streptococcus bovis/equinus complex. Motile and swarming Lactobacillus ruminis was present in all horses on diet C and not in horses on diet F. Diet C also resulted in the increase (P<0.05) in members of Clostridiaceae cluster III and a concomitant reduction (P<0.05) in an unknown group of Bacteroidales. Conclusions and potential relevance: The greater microbial stability and reduction in LAB and members of the Streptococcus bovis/equinus complex on diet F indicate an opportunity to develop feeding strategies that support equine health and welfare. Novel changes identified in the faecal microbiota that resulted from carbohydrate inclusion merit further investigation.