Selection for high and low threshold body weight at first egg in broiler strain females. 1. Direct response to selection and correlated effects on juvenile growth rate and age at first egg.
A selection program for high and low threshold body weight at first egg was carried out in a broiler line. Selection was on the basis of weight at first egg, following gradual release from feed restriction at a relatively advanced age. After six generations of selection, the lines differed by 862 g in the trait under selection. In addition, 6-wk body weight of high-line (HL) birds was 91 g greater, and age at first egg was 32 days greater than in low-line (LL) birds. When raised under an ad libitum feeding regimen from hatch, HL birds entered lay 20 days later than LL birds. When onset of lay was markedly delayed by maintaining feed restriction until 29 wk of age, body weight at first egg of HL birds was greater by 645 g than that of LL birds. Similarly, when birds of the two lines were subjected to forced molt and brought to a body weight well below that of initial body weight at first egg, and then allowed to gain weight and reenter lay, body weight difference at first egg of HL and LL birds following rehabilitation was similar to that found on original entry into lay. It is proposed that the results may most plausibly be explained as resulting from a primary effect of the selection procedure on the time required to first egg from onset of sexual competence, defined as onset of responsiveness to lay-inducing factors such as light. Alternative explanations involve effects of the selection procedure on threshold weight or threshold age requirements for sexual competence.