Thermal manipulations of birds during embryogenesis

Broilers are subject to adverse environmental conditions, especially temperature, which decreases performance and increases disease vulnerability. Understanding how the mechanisms involved in thermoregulation of the embryo function is essential for improving production. Studies have shown that thermal manipulation should occur late during embryonic development (14(th) to 18(th) day of incubation) and factors, such as hormones, neurotransmitters and cytokines, are involved in programming the thermoregulatory system. Several methodologies have been applied in experimental situations regarding the frequency, intensity and duration of thermal stimuli during the embryonic period. Hatching yield can be improved, despite different handling strategies, however, there are still divergent results regarding the ability of birds to adapt to thermal stimuli. Thermal management can alter vascular development and hormone levels in embryos. In contrast to high-temperature thermal manipulation, using lower incubation temperatures can positively influence the quality of newly hatched chicks. In general, positive results were observed for thermal manipulation by cold or heat when embryos were stressed for 2 to 6 h/d. Higher hatchability and performance under normal conditions was observed in experiments using temperatures from 36.4 to 36.7 degrees C. Better performance was seen under heat stress conditions in the final phase (21-35 days) when using temperatures from 39.2 to 39.5 degrees C. Thermal manipulation modulates the expression of heat shock proteins (HSP), which vary with stimulus type (heat/cold), breed, tissue and bird age. Finally, thermal manipulation can be effective in helping birds adapt to the breeding environment; however, it is necessary to refine certain points. The present review found that thermal manipulation during embryo development was effective at increasing hatchability and performance of broilers.