Effect of Blood Sampling Time After Colostrum Intake on the Concentration of Metabolites Indicative of the Passive Immunity Transfer in Newborn Dairy Calves

This study aimed to evaluate the impact of time after birth on the concentrations of metabolites relevant to monitoring passive immune transfer in calves. Additionally, we aimed to identify the best measurement devices based on accuracy, speed, cost-effectiveness, and availability on farms. Calves received 6 L of high-quality colostrum within 6 h. Blood samples were taken before (0 h) and 1, 2, 4, 6, 12, 24, 48, 72, 96, and 120 h after colostrum intake. The total serum protein (TSP) concentration increased during the first hours after colostrum intake, reaching stability between 24 and 48 h. The γ-glutamyl transferase and alkaline phosphatase concentrations increased after 4 h, followed by a significant decrease, and stabilized after 24 h. The albumin concentration decreased from 0 to 12 h then increased until 48 h and stabilized. Hematocrit showed higher values one hour after intake and remained constant after 24 h. Total serum protein stabilized at 24–48 h, which was the optimal window for passive transfer assessment. Digital or optical refractometers are recommended for TSP measurement, showing a strong correlation with enzyme methods, though initial calibration is necessary due to brand variations. These devices offer faster results compared to enzyme methods and direct IgG analysis.