Impact of duration of cyclic heat stress exposure at different ages on growth performance, recovery, and histopathology in broilers

Heat stress (HS) poses a significant challenge to broiler welfare and growth. While the effects of constant (24 hr/day) HS are well documented, the consequences of HS duration at specific ages and the capacity for subsequent recovery, remain less understood. Therefore, 630 one-day-old Cobb500 broilers were assigned to 7 treatments and reared during a 6 week period to investigate the effects of varying HS duration on broiler growth, and recovery at different ages of exposure. Broilers were housed either in a climate-controlled (CC) barn or a natural cyclic HS open-sided (OS) barn. Treatments included broilers continuously housed for 6 weeks in CC (6CC) or OS (6OS). In addition, three other treatments were transferred from CC to OS for the final 3 weeks (3OS), 2 weeks (2OS), or 1 week (1OS) of the experiment. The last two recovery treatments (REC1, REC2) were placed in the CC house followed by 1 or 2 weeks in the OS house, from day 21 or 28 until 35, before returning to CC for the final week of the experiment. Each treatment included six replicates, with 15 broilers per replicate pen at the start of the experiment. Cloacal temperature, growth performance, mortality, plasma corticosterone, protein carbonyls, and histological morphology of the heart and lung were evaluated. Exposure to HS increased the cloacal temperature by at least 1.5°C at all ages (P < 0.001). Sudden exposure to HS at 36 days of age (1OS) resulted in severe mortality (67%) compared to control (6CC) and broilers exposed to HS at younger age (3OS and 2OS). HS consistently reduced body weight (BW) and average daily feed intake (ADFI); 6OS broilers finished 40.1% lighter than 6CC broilers (P < 0.001). Upon returning to thermoneutral conditions, REC1 and REC2 broilers immediately resumed growth rates comparable to controls but did not exhibit compensatory growth to recover the accumulated BW deficit. Very short-term HS exposure (1OS) triggered a spike in plasma corticosterone (25.42 nmol/L) and protein carbonyls (1.92 nmol/mg) compared to 6CC controls (P < 0.001). Descriptive histological examination revealed that subendocardial erythrocyte accumulations were increased in 6OS hearts and were absent in CC-housed broilers. Taken together, sudden HS exposure at 36 days of age poses an unmanageable threat to survival. While broilers recover from HS quickly, lost performance is not regained through compensatory growth within the 6 week period.