Effect of Dietary and Age Changes on Ruminal Microbial  Diversity in Holstein Calves
Abstract: Ruminal microorganisms play a crucial role in the energy supply of ruminants and animal performance. We analyzed the variation in rumen bacteria and fungi at 45 d, 75 d, and 105 d by using 16SrRNA and ITS sequencing data and investigated their correlation with rumen fermentation. According to the results, rumen microflora gradually tended to mature with age and bacterial and fungal establishment gradually stabilized. Compared the three periods, the concentration of propionic acid increased significantly (P<0.05) after weaning, and weaning accompanied by a transition in diet remarkably decreased (P<0.05) rumen diversity in the short term and induced a corresponding change in the rumen microbiota composition. Bacteroidota, Actinobacteriota, and Firmicutes were the core bacterial phyla for all age periods. Ruminococcus, NK4A214_group, Sharpea, Rikenellaceae_RC9_gut_group, and norank_f__Butyricicoccaceae were the markedly abundant bacterial genera in pre-weaning. After weaning, the relative abundance of Erysipelotrichaceae_ UCG-002, Eubacterium_ruminantium_group, and Solobacterium significantly increased(P<0.05). The relative abundance of Acetitomaculum increased with age, with the greatest abundance noted at 105 d. The dominant fungal phyla were Ascomycota and Basidiomycota, and Aspergillus and Xeromyces were the most abundant fungal genera after weaning. Trichomonascus, Phialosimplex, and Talaromyces were enriched at 105 d. In addition, correlations were observed between age-related abundances of specific genera and microbiota functions and rumen fermentation-related parameters. This study revealed that rumen microbiota and rumen fermentation capacity are correlated, which contributed to understand better the effects of age and diet on rumen microbiology and fermentation in calves.