Re-evaluating heat stress assessment in broilers

Taking effective actions and precautions to remedy heat stress in broilers requires proper assessment of the thermal comfort and heat load. The thermal comfort zone may be influenced by exogenous and endogenous heat load from the environment and animal, respectively. To assess exogenous heat load, ambient temperature (T_a;°C) and relative humidity (RH; %) are often combined in temperature-humidity indices (THI), yet many formulas are outdated, and thresholds are used interchangeable despite being species-specific. This study aimed to evaluate the accuracy of commonly used heat stress indices, and its interaction with animal characteristics and physiological responses, including respiration rate (RR) and body temperature (T_b) in broilers. Data from multiple summer trials were compiled into one dataset containing 4,314 individual animal observations linked to the prevailing indoor climate. Linear broken-line models were evaluated on their biological relevance, model simplicity and error metrics. Body weight (BW) was the most influential animal characteristic as a biological proxy for metabolic heat. Final models were based on the THI formula 0.85T_db + 0.15T_wb (Tao and Xin, 2003). Weight-dependent thresholds were formulated, with heavier broilers showing earlier breakpoints in RR and T_b. Considering a BW of 2.5 kg as example, breakpoints based on indoor T_a occurred at 26.6 °C (RR) and 23.3 °C (T_b), corresponding to 72 breaths/min (bpm) and 41.0 °C. Based on THI, breakpoints occurred at 24.6 (RR) and 22.6 (T_b), corresponding to 68 bpm and 41.3 °C. Post-threshold increases in RR and T_b were negatively affected by BW, with a standard slope of 15 - 17 bpm per unit and 0.2 °C per unit, respectively. The use of cooling postponed thresholds based on outdoor temperature by 1.4 °C, and post-threshold increases of RR and T_b were reduced if cooling was applied. In conclusion, different breakpoints were found for RR and T_b, indicating the importance of heat stress response in threshold evaluation. Moreover, species-specific indices were better suited to evaluate broiler heat stress, especially in combination with body weight.