We investigated the relationships between ruminal pH, gene expression in the rumen epithelium (RE), peripheral blood mononuclear cell subpopulations, and blood metabolites in Holstein calves during weaning transition. Calves (Weaning group, n=7) were assigned to one of two groups, and fed calf starter with forage (Forage group, n=3) or without forage (Starter group, n=4). Ruminal pH was measured continuously. Samples were collected at -1, 0, 1, and 3 weeks (blood and rumen fluid) or 3 weeks (rumen epithelium) after weaning. In the Weaning group, ruminal pH increased, and several blood metabolites increased (blood urea nitrogen [BUN], beta-hydroxybutyrate [BHB], and gamma-glutamyl transferase [GGT]) or decreased (total cholesterol [T-CHO] and phospholipid) after weaning. Ruminal pH was positively correlated with CD8+CD45R- cell populations and blood metabolites (BUN, glucose, and BHB) and negatively correlated with GGT activity. The 24 hr mean ruminal pH was higher in the Forage group during weaning transition, and toll-like receptor 4 mediated signaling pathway was activated in the Starter group at 3 weeks post-weaning. The number of CD8+CD45R- cells tended to be higher, and several blood metabolites (glucose, triglycerides, T-CHO, and phospholipid) were higher in the Forage group after weaning. Calves with higher ruminal pH also showed a greater energy metabolism status simultaneously with lesser hepatic disturbance enzymes in the peripheral blood. The results of our study indicate that serum GGT activity may be a plausible biomarker for predicting ruminal acidosis in Holstein calves during weaning transition.
Keywords: blood metabolites; peripheral blood mononuclear cell subpopulations; ruminal acidosis; toll-like receptor 4; weaning transition.