Effect of high-concentrate diets on microbial composition, function, and the VFAs formation process in the rumen of dairy cows

Although numerous studies have revealed the effects of high-concentrate (HC) diets on rumen microbial composition and metabolism, information regarding changes in the volatile fatty acids (VFAs) formation process under HC feeding is still limited. In the present study, we used a metagenomics approach to compare the effects of low-concentrate (LC) diets and high-concentrate (HC) diets on alterations to the rumen microbiota and their functions, particularly the functional genes involved in the metabolic routes of VFAs. The results showed that the HC diets increased (P < 0.05) the relative abundance of Eubacterium spp., Clostridium spp., Ruminococcus spp., Lactobacillus, and Bifidobacterium spp. and decreased (P < 0.05) the relative abundance of Prevotella spp., Fibrobacter succinogenes, and Bacteroides spp. The changes in rumen microbial composition further resulted in corresponding alterations in microbial function. Results regarding the carbohydrate-active enzymes revealed that the HC diets increased (P < 0.05) the abundance of glycoside hydrolases 13 (GH13; a-amylase activity), carbohydrate-binding module 26 (CBM26; starch-binding function), and CBM34 (starch-binding function) and simultaneously decreased (P < 0.05) the abundance of cellulases (GH5, GH9 and GH51). During the process of starch-pyruvate-VFAs, the HC diets increased (P < 0.05) the abundance of genes encoding pyruvate kinase (PK), pyruvate ferredoxin oxidoreductase (PFOR), and L-lactate dehydrogenase (LDH), which may be a reason behind the accumulation of rumen butyrate and propionate in the present study. Overall, these results clarified the mechanism behind how HC diets affect the formation process of rumen VFAs and provided new insight into the effect of HC diets on rumen microbiota.