Unfamiliar Handler-Induced Hypercortisolism, Abnormal Oral Behaviors, and Delayed Inflammatory Response in Pre-Weaned Zebu Cross Calves.

The physiological impact of climatic stress is well documented in ruminants, but the acute stress resulting from personnel rotation and its health and welfare consequences are poorly understood. The objective of this observational prospective longitudinal study was to assess the endocrine, inflammatory, behavioral, and cardiorespiratory responses of pre-weaned Zebu-cross calves (¾ Zebu x ¼ Holstein) to handler substitution and environmental variables in the Mexican humid tropics during the rainy season. Fourteen calves were monitored over a 68-day period. The initial behavioral (n = 14) and physiological (n = 5) assessments (S1) were conducted after a 30-day stabilization period with the familiar handler to ensure that animals had reached a physiological steady state, free from the influence of previous personnel rotations and from familiarization with two external observers, prior to the first assessment. Subsequently, the handler was replaced for 17 days (S2). The third (S3) and fourth (S4) samplings were conducted after the familiar handler was reinstated (21-day). There was a significant increase in plasma cortisol and abnormal suckling behaviors at S2 compared to S1 and S4, and a C-reactive protein increase in S3. Furthermore, strong correlations were found between calf handler and cortisol, heart rate, respiratory rate, latency to suckle, and cross-sucking, suggesting an acute stress response with a delayed inflammatory response secondary to the handler's substitution. Environmentally, ambient temperature (AT, °C) showed no correlation with cortisol levels, indicating that the primary driver of the neuroendocrine and behavioral changes was handler substitution rather than climatic variations.