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Methods for assessing chronic heat stress in dairy calves in a subtropical environment.

V. Ouellet




Methods for assessing chronic heat stress in dairy calves in a subtropical environment.
V. Ouellet*, B. Dado-Senn, G. E. Dahl, J. Laporta. Department of Animal Sciences, University of Florida Gainesville, FL.

Dairy calves are susceptible to heat stress as demonstrated by elevated physiological responses and reduced feed intake measured under high temperature and humidity. Yet methods to assess chronic heat stress have not been well-characterized in calves. Objectives were to evaluate the relationship between common environmental and animal-based indicators of heat stress in calves exposed to chronic heat stress or continuous cooling in a subtropical climate. Further, segmented regressions were performed to detect temperature-humidity index (THI) thresholds at which a change in physiological response was observed. Holstein calves were exposed to heat stress (HT, shade of barn, n = 24) or cooling (CL, shade of barn plus 2 fans, one at the calf level and one oscillating ~2 m above the ground; n = 24) from 2 to 42 d of age. Environmental (ambient temperature, humidity, THI, and wind speed) and animal-based (respiration and heart rate, RR, HR; rectal and skin temperature, RT, ST) indicators were recorded thrice daily whereas milk replacer and grain intake (MI, GI) were recorded daily from 15 to 42d of age. In both treatments, there was a positive correlation between most of the animal-based indicators and ambient temperature and THI, with the highest correlation obtained with ST (r ≥0.72). Adding environmental indicators to regression equations to model animal-based indicators greatly improved the r2 of the equations. Among all environmental indicators, THI obtained the highest r2 when predicting rump ST (r2 ≥ 0.92). Segmented regression indicated that THI breakpoints at which RT and RR begin to rise and MI started to decrease under chronic heat stress were 67, 65, and 82 for RT, RR and MI respectively. Cooled calves had a threshold of 69 for RR but no breakpoints were identified for RT or MI, and ST variables in either treatment. In summary, our results suggest that ST is the optimal animal-based indicator to quantify heat load and THI is the best environmental indicator of heat stress in calves in a subtropical environment. At a practical level, heat mitigation should be applied when THI reaches 65 if calves are exposed to chronic heat stress.

Keywords: hyperthermia, correlation, threshold.

Biography: Veronique Ouellet completed her PhD at Laval University in Quebec, Canada. Her work focuses on the effects and strategies to mitigate heat stress stress and improve resilience in dairy herds. She is currently a postdoctoral fellow at the University in Florida under Dr Jimena Laporta and Dr Geoffrey Dahl.