Naturally suckled Japanese Black calves are vulnerable to nutritional decline around weaning due to low maternal milk yield and abrupt maternal separation. While plasma total cholesterol (TCHO) is a reliable indicator of energy balance, its role as a lagging indicator makes early detection of ‘refractory’ calves—those with delayed metabolic recovery—challenging. This study aimed to characterize nutritional, bone metabolic, and stress-related physiological changes during the weaning transition and to identify early predictors of post-weaning metabolic resilience. Twelve naturally suckled Japanese Black calves were monitored from two weeks before weaning to six weeks after weaning for multiple indicators: plasma total cholesterol (TCHO), beta-hydroxybutyrate (BHB), non-esterified fatty acid (NEFA), glucose (GLU), albumin (ALB), blood urea nitrogen (BUN), calcium (Ca), phosphorus (P), alkaline phosphatase 3 (ALP3), tartrate-resistant acid phosphatase 5b (TRAP5b), cortisol, and oxytocin. Calves were grouped into low and high TCHO groups based on plasma TCHO concentrations six weeks after weaning, and group differences and predictive performance were evaluated using mixed models and ROC analysis. Across all calves, plasma TCHO, NEFA, GLU, ALB, P, ALP3, and TRAP5b decreased after weaning, whereas stress-related biomarkers remained within reported physiological ranges and showed no clear weaning-associated surge. Pre-weaning salivary cortisol showed discriminative ability for subsequent refractory metabolic recovery (AUC=1.00). In conclusion, pre-weaning salivary cortisol serves as a sensitive indicator of ‘metabolic readiness’ to identify calves at risk for refractory recovery, facilitating proactive management to optimize growth and resilience in natural suckling systems.
Kim et al. (Thu,) studied this question.