Preweaning period is a critical window for rumen microbial regulation of average daily gain in Holstein heifer calves.

BACKGROUND: Rumen bacterial groups can affect growth performance, such as average daily gain (ADG), feed intake, and efficiency. The study aimed to investigate the inter-relationship of rumen bacterial composition, rumen fermentation indicators, serum indicators, and growth performance of Holstein heifer calves with different ADG. Twelve calves were chosen from a trail with 60 calves and divided into higher ADG (HADG, high pre- and post-weaning ADG, n = 6) and lower ADG (LADG, low pre- and post-weaning ADG, n = 6) groups to investigate differences in bacterial composition and functions and host phenotype. RESULTS: During the preweaning period, the relative abundances of propionate producers, including g_norank_f_Butyricicoccaceae, g_Pyramidobacter, and g_norank_f_norank_o_Clostridia_vadinBB60_group, were higher in HADG calves (LDA > 2, P < 0.05). Enrichment of these bacteria resulted in increased levels of propionate, a gluconeogenic precursor, in preweaning HADG calves (adjusted P < 0.05), which consequently raised serum glucose concentrations (adjusted P < 0.05). In contrast, the relative abundances of rumen bacteria in post-weaning HADG calves did not exert this effect. Moreover, no significant differences were observed in rumen fermentation parameters and serum indices between the two groups. CONCLUSIONS: The findings of this study revealed that the preweaning period is the window of opportunity for rumen bacteria to regulate the ADG of calves.