Responses to long-term divergent selection for eight-week body weight in chickens

Thirty-six generations of divergent selection for body weight at 8 wk of age resulted in approximately an eightfold difference between the high (HWS) and low (LWS) lines for this trait. In both White Plymouth Rock lines, selection intensities declined over generations, with responses about five times greater during the first 18 than the last 18 generations of selection in Line HWS and about two times greater in Line LWS. Also, responses to selection were more irregular in the later than in earlier generations. Sexual dimorphism of body weight (male:female) was greater for Line LWS than HWS. A randombred control line (AC) maintained concurrently with the selected lines did not exhibit environmental trends across generations. Relaxed lines, established at different generations of selection, were used to evaluate effectiveness of selection and monitor environmental changes over short periods.