Ruminal methanogen abundance and associated physiological characteristics in Japanese Black–Holstein F1 calves

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 1 to 9 weeks of age.All calves received the same milk replacer-feeding program until weaning and had ad libitum access to starter and hay throughout the experimental period.Based on ruminal total methanogen abundance measured by qPCR 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 than in the low-methanogen group (6.56 vs. 5.53 log copy number/1 ng DNA; p < 0.001).Average daily gain did not differ between groups (0.64 vs. 0.69 kg/d).Starter intake was higher in the highmethanogen group at 6 weeks (p < 0.05) and was positively correlated with total methanogen abundance (r = 0.42; p < 0.05).Ruminal propionate proportion was higher in the lowmethanogen group (28.59% vs. 20.58%;p < 0.05).Methanobrevibacter was enriched in the high-methanogen group (p < 0.05), whereas Prevotella (p < 0.01), Succinivibrio (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 associated with distinct rumen fermentation patterns and microbial signatures, highlighting potential microbial and nutritional factors linked to methanogen abundance in calves.