Correlated responses to long-term divergent selection for eight-week body weight in chickens: growth, sexual maturity, and egg production.

Thirty-six generations of divergent selection for BW at 8 wk of age (BW8) resulted in approximately an eightfold difference between the high (HWS) and low (LWS) lines for this trait. Correlated traits included BW at 4, 24, and 38 wk of age (BW4, BW24, BW38, respectively), age at first egg (AFE), and percentage hen-day egg production (HDP). Responses of BW4 followed the same pattern as that for the selected trait, with the response about five times greater during the first 18 than the last 18 generations of selection in Line HWS and less than two times greater in Line LWS. For BW24 and BW38, correlated responses were greater for LWS than for HWS females without feed restriction, suggesting changes in growth curves after selection age. Although AFE was delayed in both lines, the delay was greater in Line LWS (some individuals of which were anorexic) than in Line HWS and greater in the second half than the first half of the experiment. For pullets that commenced lay, HDP declined slightly in both lines. Correlations between BW at 4, 8, 24, and 38 wk of age were moderate to high and positive in both lines. When feed intake was restricted in Line HWS, however, there were no correlations of BW4 or BW8 with BW24 or BW38. Correlations between AFE and BW at all ages were negative in Line LWS. In Line HWS there were negative correlations of AFE with BW24 and with BW38. Relaxed lines, established periodically during the experiment, were satisfactory monitors of environmental influences for primary and correlated traits.