Sex differences in incubation length and hatching weights of broiler chicks.

Two experiments were run using broiler eggs (Peterson x Arbor Acres) to determine whether sex differences exist in incubation time required to hatch males and females and in various measures of chick weight (CW). Relationships between measures of CW and incubation time and between CW and egg weight (EW) were determined. Chicks that had emerged from eggs were removed from the hatcher at approximately 6- (5 to 8) h intervals between 490.5 and 527.5 h of incubation in Experiment 1 and at about 2-h intervals between 483 and 524 h of incubation in Experiment 2. Chicks were weighed before and after removal of the residual yolk sac immediately upon removal from the hatcher. In both experiments, a preponderance of early-hatching chicks were female. The time at which 50% of the females had hatched was significantly (P = .05) earlier than the 50% hatch time for males (2.68 and 3.32 h in Experiments 1 and 2, respectively). About 71% of the females had hatched by 504 and 509 h in Experiments 1 and 2, whereas only 57 and 52% of the males had hatched at comparable times. In Experiment 2, the yolk-free CW (YFCW) of males tended to be greater than that of females (P = .07) and the YFCW expressed as a percentage of EW was significantly (P = .007) greater. The regression of CW and YFCW on EW were highly significant and positive in both experiments. There was a significant relationship between EW and time required to hatch for males in Experiment 1 only.