Longitudinal Analysis of Physiological Adaptation in Primiparous Dairy Cows During the Transition Period.

The transition period is a physiologically demanding stage in primiparous dairy cows, as the onset of lactation occurs simultaneously with continued growth. This longitudinal study initially enrolled 22 primiparous Holstein dairy cows. Eight cows that developed transition disorders during the monitoring period were excluded according to the predefined study protocol. Therefore, the final analysis included 14 clinically healthy primiparous Holstein dairy cows monitored from four weeks before calving to five weeks postpartum to characterize normal physiological adaptation during the transition period. Cows were monitored using continuous intraruminal sensors and automated milk analysis systems, while blood samples were collected weekly and analysed using linear mixed models with repeated measurements. Milk yield increased from 17.9 kg/day in week +1 to 26.3 kg/day in week +5 (a relative increase of 47%). Significant temporal changes were observed in iron (Fe), phosphorus (P), glucose (GLUC), triglycerides (TRIG), urea (UREA), albumin (ALB), and alanine aminotransferase (ALT). Serum Fe decreased from 30.9 µmol/L at week -2 to 14.5 µmol/L at week +2 (-53%), while P declined from 2.95 mmol/L at week +1 to 1.41 mmol/L at week +2 (-52%). Sensor data showed reduced physical activity (-59%) and increased water intake from 60.6 L/day in week -4 to 99.4 L/day in week +1, whereas rumination time and rumen pH remained relatively stable. Correlation analysis identified significant associations among milk yield, metabolic variables, and behavioural indicators. These findings suggest coordinated physiological adaptation during the transition period and support the use of combined blood and sensor-based monitoring to better understand early-lactation dynamics in primiparous dairy cows.