Effects of temperature intensity and exposure duration on embryonic thermal manipulation responses in broiler chicks

This study aimed to evaluate whether temperature intensity or exposure duration more strongly influences thermal manipulation (TM) responses in broiler chicks. The objective of this study was to compare a previously developed standard incubation temperature of 37.5°C throughout embryonic development (ED 1-18) with intermittent thermal manipulation (TM) protocols at 39.5°C or 40.5°C, applied for either 6 or 12 h per day from ED 7 to ED 16 to evaluate their effects on chick yolk sac weight, cloacal temperature, blood and endocrine parameters, performance, and carcass yield. A total of 4,300 hatching eggs from a 39-week-old Ross 308 breeder flock were randomly distributed over five treatments (860 eggs each): control (37.5°C and 60% relative humidity (RH) until ED 18) and four TM treatments between ED7 and ED16: 39.5°C for 6h/d, 39.5°C for 12h/d, 40.5°C for 6h/d, and 40.5°C for 12h/d, all with 65% relative humidity (RH). After hatching, a total of 660 male chicks were reared under thermoneutral conditions until 21 d of age; from d 22 to 35, chicks were subjected to cyclic heat stress (32°C ± 1.0°C for 8 h/d). Relative yolk sac weight at d 3 was higher in the 40.5°C/12 h treatment than in the other treatments (P ≤ 0.05). Feed conversion ratio at d 35 was worse in the 40.5°C/12 h treatment (1.53 g:g) and better in the 39.5°C/6 h treatment (1.32 g:g) compared to the other treatments (P ≤ 0.05). Mortality and cloacal temperature during heat stress was lower in all TM treatments than in the control (P ≤ 0.05). At the onset of heat stress, T₃ levels were higher in the control (142.5 ng/ml) and 40.5°C/12 h (176.20 ng/ml) treatments than in the other three treatments (132 ng/ml on average). In conclusion, embryonic TM at 40.5°C/12 h negatively affected performance and endocrine responses, whereas TM at 39.5°C/6 h, 39.5°C/12 h and 40.5°C/6 h showed potential benefits for mitigating heat stress in later life, evidenced by reduced mortality. Nevertheless, evaluating these protocols in commercial incubators is highly recommended to confirm these findings under large-scale production dynamics.