[Genetics].

A study was carried out to investigate the effects of energy and protein levels on performance of Cobb Avian 48 broilers to 36 d. Chicks were randomly assigned to a 2 × 2 × 3 × 5 factorial arrangement of treatments, with 2 sexes; 2 levels of early nutrition (0 to 11 d); and after 11 d, 3 metabolizable energy (ME) levels and 5 dietary balanced protein (DBP) levels, balanced for 4 limiting amino acids. The 3 ME levels were 94, 97, and 100% of Cobb-Vantress dietary specifications for maximum growth rate and feed conversion ratio (FCR). The 5 DBP levels were 85, 92.5, 100, 107.5, and 115% of these same specifications. From 0 to 11 d, 2 prestarter nutrient densities were used, based on Cobb’s starter recommendations for maximizing growth rate and FCR (HIGH), or for reduced feed cost (LOW). From 12 to 36 d of age, the prestarter treatment was nested within pens, which housed ME × DBP × sex interactions. By 36 d, the HIGH prestarter increased BW (2.14 vs. 2.08 kg; P < 0.0001). Male broilers were heavier than females (P < 0.005) in all levels of ME and DBP. Females had a higher cumulative FCR than males (P < 0.0001). In females, DBP did not affect cumulative FCR at 36 d. Males fed 100% through 115% DBP had a lower cumulative FCR than those fed 85 and 92.5% DBP (P < 0.05). Covariate analysis (adjusted for BW) indicated that breast yield was higher at 107.5 and 115% DBP than at 85% through 100% DBP (range: 389.7–381.4 g vs. 372.6–378.5 g, respectively; P < 0.05). Fat pad decreased with increasing DPB levels (P < 0.0001). Fat pad was higher at 100% ME than at 97% ME and 94% ME. Females had higher fat pad weight and lower eviscerated BW and legs yield. In conclusion, HIGH prestarter improved growth rate, males were more efficient than females, low DBP levels increased fat pad, and breast yield increased by 1 g for every 1.5% increase in DBP.