The objectives of this study were to investigate the effects of different forage-to-concentrate ratios and sampling times on the genetic diversity of carbohydrate-active enzymes (CAZymes) and the taxonomic profile of rumen microbial communities in dairy cows. Six ruminally cannulated Holstein cows were arbitrarily divided into groups fed high-forage (HF) or low-forage (LF) diets. The results showed that significant differences were more apparent based on dietary forage-to-concentrate ratio than those based on sampling time of GH family. AF4h (after feeding 4 h) group of HF treatment have the most microbial diversity. Genes that encode glycoside hydrolases (GHs) had the highest number of CAZymes, and accounted for 57.33% and 56.48% of all CAZymes in the HF and LF treatments, respectively. The majority of GH family genes encode oligosaccharide-degrading enzymes, and GH2, GH3, and GH43 were synthesized by a variety of different genera. Notably, we found that GH3 is higher in HF diet samples than LF and in both sets of samples is mainly contributed to by Prevotella, Bacteroides and unclassified reads Most predicted cellulase enzymes were encoded by GH5 (BF0h group of HF treatment was highest) and GH95 (BF0h group of LF treatment was highest), and were primarily derived from Bacteroides, Butyrivibrio, and Fibrobacter. Approximately 67.5% (GH28) and 65.5% (GH53) of the putative hemicellulases in LF and HF treatments, respectively. GH28 under LF treatment was higher than that under HF treatment, and were main produced by members of Ruminococcus, Prevotella, and Bacteroides. This study indicated that HF-fed cows had increased microbial diversity of CAZymes producers, which encode enzymes that efficiently degrade oligosaccharides in the cow rumen.
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