Two-way selection for threshold body weight at first egg in broiler strain females. 3. Reproductive performance under various levels of feed restriction

Two-way selection was carried out for high and low threshold body weight at onset of lay (ThrWtLay). The present study determined the effect of this selection on reproductive performance under conditions of quantitative feed intake restriction. Female chicks of the two lines were raised under a commercial feed restriction program until 22 wk of age (target weight 2,200 g). At this time they were divided into five treatment groups, one of which had free access to feed, the others receiving 120, 130, 140, and 150 g/ day of a commercial layer mash, respectively. The experiment was carried out in two independent replicates, in consecutive years. In all treatments, low-line birds entered lay at a significantly lower body weight and earlier age than the highline birds. During the prepeak laying period, the early entry into lay of the lowline birds resulted in significantly greater egg numbers as compared with the high-line birds (P < .01). This difference was maintained in the postpeak laying period (34 to 58 wk). When averaged over all treatments and both replicates, the low-line birds produced 70% more eggs than the high-line birds in the prepeak period, 14% more during the postpeak period, and 25% more overall. Over an treatments and both replicates, egg weight of the high-line birds averaged 2.7 g greater than that of the low-line birds. At each feeding level, low-line birds at the end of lay had lower carcass weight and breast weight, and shorter shank length, but a greater absolute weight of abdominal fat than high-line birds. Thus, low-line birds were characterized by increased adiposity. It is hypothesized that the selection implemented in these lines acted primarily to decrease photoperiodic drive of the high-line relative to the low-line birds. This resulted in an increase in the number of days of light stimulation required for initiation and completion of ovum maturation and ovulation by the high line as compared to the low-line birds. A similar decrease in photoperiodic drive may be at the root of current broiler breeder reproductive problems. If so, selection for low ThrWtLay may be a means of ameliorating these difficulties.