The effect of duration of thermal manipulation during broiler chick embryogenesis on body weight and body temperature of post-hatched chicks

The significant developments in the genetic selection of fast-growing meat-type broiler chickens, coupled with insufficient development of their visceral systems, have enhanced the interest for thermal manipulations (TM) during susceptible periods of broiler embryogenesis, in order to improve thermotolerance acquisition. The duration of TM may affect both body weight (BW) and body temperature (Tb) of the chicks. This study was aimed at elucidating the effect of different duration periods of TM during broiler embryogenesis on the hatching rate, BW and Tb at hatch and following thermal challenge (41 °C for 6 hours) at the age of 3 days (Challenged C or Naïve N, i.e. non-challenged chicks). Control embryos were incubated at 37.8 °C, whereas the TM-embryos were treated for 3 (D1), 6 (D2), 12 (D3) or 24 (D4) hours per day at 39.5 °C during late embryogenesis from E16 to E18. Different durations of TM did not affect BW of the hatched chicks, but significantly affected hatchability, which was higher in the D3 and D4 treatments compared to the D1 treatment. It further affected the Tb of the treated chicks, which was significantly lower in all treatments than in the controls. During the challenge (C), all 4 treatments (D1C to D4C) exhibited a significantly lower Tb compared to the controls. Eighteen hours post-challenge, D1C chicks maintained significantly lower Tb than D2C, D3C and D4C chicks. The BW of the naïve chicks continued to be similar, whereas that of the challenged ones demonstrated a significantly higher value for D2C and D3C chicks compared to the Controls and D1C's. It can be concluded that out of the four TM durations, the best one to initiate improvement of thermotolerance acquisition requires 3 hours of TM per day during E16 to E18, whereas 6 and 12 hours per day may be the best to reach higher hatchability and initiate growth. However, further research is required to follow both responses during the whole life span of the chicks. Effet de la durée d'un traitement thermique des embryons de poulets de chair sur le poids vif et la température corporelle des poussins après éclosion. La sélection génétique des poulets de chair à croissance rapide s'est accompagnée d'un développement insuffisant de leurs organes internes qui limite leur capacité à résister à la chaleur. Les manipulations thermiques (MT) de l'embryon visent à améliorer l'acquisition de la thermotolérance du poulet. Leur durée pourrait affecter le poids vif (BW) et la température corporel