Primary and secondary myogenesis are the most important myogenic developmental windows and can be altered by incubation temperature. In chickens, it is known that primary and secondary myogenesis are completed by ED 12 ( Yablonka-Reuveni, 1995). The secondary myofibers are known to be more susceptible to environmental changes compared with primary myofibers. Therefore, the number and size of primary myotubes can influence the total number of myofibers present at hatch ( Zhang and McLennan, 1999). The primary myofibers serve as a scaffold for the formation of the secondary myofibers. Posteriorly, the second wave of myogenesis occurs from ED 8 to 12 of incubation by the fusion of fetal myoblasts and results in the formation of secondary myofibers ( Crow and Stockdale, 1986 Al-Musawi et al., 2011). In chickens, the first wave of myogenesis takes place from embryonic day ( ED) 4 to 7 of incubation by the fusion of embryonic myoblasts and results in the formation of primary myofibers. Cells from the somites will, posteriorly, be committed into the muscle lineage to form primary and secondary myofibers. Once the egg is fertilized, myogenesis begins around the 48th h of incubation with the formation of the somatic cells ( Bellairs and Osmond, 1998). Significant changes in incubation temperature during embryonic phases, generally lead to cumulative negative impacts on the posthatch growth ( Tong et al., 2013).ĭuring embryogenesis, striated skeletal muscle development is achieved mainly by hyperplasia. However, achieving consistent internal egg temperature to cover the embryonic requirements thorough incubation is a challenge in the current broiler industry, and even when machinery is set to provide adequate conditions, most of the time, those conditions are not met ( Gigli et al., 2009). Incubation temperature is the most influential physical factor during chicken embryogenesis because it determines embryonic and posthatch growth, metabolism, and developmental characteristics ( Sozcu and Ipek, 2015). Overall, these data indicate that the inherent differences in environmental factors among incubation LOC can impact broiler carcass and breast meat yields. As expected, male broilers had heavier carcass, breast, tender, wings, drumsticks and thighs weights and were more severely affected by WB than females ( P < 0.05). However, broilers from warmer MID trays had greater carcass yield than those from cooler TOP trays ( P < 0.05). Broilers from the BOT trays had higher breast meat yield as a proportion of carcass weight (25.00%) than warmer MID (24.54%) broilers ( P < 0.05). However, broilers from BOT trays had heavier tender and breast weights than broilers from warmer MID trays ( P < 0.05). Growth performance and incidence and severity of WB and WS were similar among LOC ( P > 0.05). No LOC × Sex interactions were observed ( P > 0.05). At day 41, all birds (n = 720) were processed to determine carcass and carcass part yields and incidence and severity of the meat quality defects wooden breast ( WB) and white striping ( WS). Chicks hatched from the 3 LOC (n = 720 per LOC) were vent sexed, vaccinated, and separate-sex reared with 12 birds per pen in a floor-pen facility and fed a common corn and soybean meal–based diet for 41 d. Broiler hatching eggs were obtained from a 40-week-old commercial broiler breeder flock and incubated in trays placed in the bottom ( BOT), middle ( MID), and top ( TOP) thirds of the racks (n = 4 racks per incubator tray LOC) in a single-stage incubator in a commercial hatchery. The objective was to determine whether small incubation temperature variations owing to incubator tray location ( LOC) could alter posthatch female and male broiler growth performance and carcass characteristics. Large variations in hatching egg incubation temperatures have been previously shown to negatively impact posthatch growth in broiler chickens.
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