Effects of body weight and feed allocation during sexual maturation in broiler breeder hens. 1. Growth and carcass characteristics.

The effects of broiler breeder BW and nutrient intake on carcass traits were examined at photostimulation (PS) (21 wk) and at sexual maturity (SM) in birds of standard (STD) BW or either 20% lighter (LOW), or heavier (HIGH) at PS and subsequently allowed restricted (RF) or ad libitum (AL) access to feed. Of the 30 Shaver Starbro pullets assigned to each BW group at PS, 10 birds of each size were processed immediately for carcass analysis and 10 birds assigned to each of the RF and AL feeding regimens. Remaining birds were processed for assessment of carcass traits following SM. The mean BW of LOW, STD, and HIGH birds processed at PS were 1,639, 1,995, and 2,394 g, respectively. The relative breast muscle weight, abdominal fat pad weight, and total carcass lipid content of LOW birds were significantly lower than those of STD or HIGH birds. Body weight at PS primarily affected lipid stores, with absolute carcass lipid content being 103, 180, and 241 g in LOW, STD, and HIGH birds, respectively. The mean AL BW increased by 85% between PS and SM compared to 46% for RF birds. Although LOW birds weighed less than HIGH birds at SM, abdominal fat pad weight and carcass lipid content did not differ. Mean carcass lipid weight was 740 g in AL birds compared to 370 g in RF birds at SM. The use of AL feeding accelerated the onset of lay (25 d from PS) compared to RF birds (39 d), and removed body size effects on the rate of sexual maturation. Initial BW affected timing of SM in RF birds, with LOW, STD, and HIGH birds reaching SM 51, 38, and 27 d after PS, respectively. As the carcass composition of these birds varied greatly at PS, improving BW and composition uniformity at PS would be beneficial for a more uniform onset of lay and reduced early production losses from small hens. Whereas thresholds for BW, carcass protein, or carcass lipid appeared to affect the onset of lay in RF birds, the rapid onset of production in their AL counterparts suggests that the actual internal signal for reproductive development is more likely a metabolic one.