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 wer e 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) a t PS and subsequently allowed restricted (Rm or ad libitum (AL) access to f eed. Of the 30 Shaver Starbro pullets assigned to each BW group at PS, 10 b irds of each size were processed immediately for carcass analysis and 10 bi rds assigned to each of the RF and AL feeding regimens. Remaining birds wer e 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 fa t pad weight, and total carcass lipid content of LOW birds were significant ly lower than those of STD or HIGH birds. Body weight at PS primarily affec ted lipid stores, with absolute carcass lipid content being 103, 180, and 2 41 g in LOW, STD, and. HIGH birds, respectively. The mean AL BW increased b y 85% between PS and SM compared to 46% for RF birds. Although LOW birds we ighed less than HIGH birds at SM, abdominal fat pad weight and carcass lipi d content did not differ. Mean carcass lipid weight was 740 g in AL birds c ompared to 370 g in RF birds at SM. The use of AL feeding accelerated the o nset of lay (25 d from PS) compared to RF birds (39 d), and removed body si ze 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 gr eatly at PS, improving BW and composition uniformity at PS would be benefic ial for a more uniform onset of lay and reduced early production losses fro m 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 product ion in their AL counterparts suggests that the actual internal signal for r eproductive development is more likely a metabolic one.