Abstract
Objective: This study investigated associations among ruminal total methanogen abundance, rumen microbiota, fermentation characteristics, feed intake, growth performance, and blood biochemical parameters in Japanese Black–Holstein F1 calves during early life.Methods: Twenty-three male Japanese Black–Holstein F1 calves were raised as a single cohort under identical feeding conditions from one to nine weeks old. All calves were fed the same milk replacer until weaning and had ad libitum access to starter and hay throughout the trial. Based on ruminal total methanogen abundance measured by quantitative real-time polymerase chain reaction at the end of the experiment, calves in the upper and lower quartiles were retrospectively classified as high- and low-methanogen groups, respectively (n = 6 per group). Rumen microbiota was characterized using 16S rRNA gene amplicon sequencing. Growth performance, feed intake, rumen fermentation characteristics, blood biochemical parameters, and rumen microbial profiles were compared between groups, and correlations with total methanogen abundance were assessed using all calves.Results: Total methanogen abundance was higher in the high-methanogen group compared to the low-methanogen group (6.56 vs. 5.53 log copy number/ng DNA; pMethanobrevibacter was enriched in the high-methanogen group (pSuccinivibrio (p<0.05), and Selenomonas (p<0.05) were enriched in the low-methanogen group.Conclusion: These findings suggest that ruminal total methanogen abundance during early life is related with distinct rumen fermentation patterns and microbial signatures, highlighting potential microbial and nutritional factors linked to methanogen abundance in calves.