Optimizing dietary crude protein, branched-chain amino acids, and energy levels for broiler performance using a Box-Behnken design

This study aimed to investigate the limitations and interactive effects of dietary crude protein (CP; 15%, 17% and 19%), excess branched-chain amino acid (BCAA) inclusions (0%, 20% and 40%), and net energy (NE; 9.0, 9.7 and 10.4 MJ/kg) on performance and nutrient utilization for Cobb 500 mix-sex broilers, using a Box-Behnken design. The study consisted of 2 experiments: Exp. 1 involved 1092 chickens, and their performance was measured from d 19 to 35, and Exp. 2 employed 156 birds for NE measurements from d 25 to 28. Both experiments used the same diets (<i>n</i> = 13), each replicated 7 times for Exp. 1 and 6 times for Exp. 2. On d 35, 4 birds per pen (2 males and 2 females) were sampled to collect ileal digesta and weigh carcass parts. Feed intake (FI), NE intake (NEi), weight gain (WG), feed conversion ratio (FCR) and breast yield were affected by CP × NE (<i>P</i> < 0.001) and CP × BCAA (<i>P</i> = 0.041). Diluting NE in the reduced CP (RP)-diets led to a higher FI (<i>P</i> < 0.001) and breast yield (<i>P</i> < 0.001) than high NE, but the NE levels had no effect (<i>P</i> > 0.05) on FI and breast yield in high CP (HCP)-diets. Breast yield increased (<i>P</i> = 0.041) with BCAA in HCP-diets, whereas in the RP-diets, the yield lowered (<i>P</i> = 0.041) with increased BCAA. A similar trend was observed with fat content: in HCP-diets, fat content increased (<i>P</i> = 0.007) with BCAA, while in RP-diets, fat content decreased (<i>P</i> = 0.007) with higher BCAA levels. In addition, increasing BCAA in the RP-diets decreased (<i>P</i> < 0.001) FI, NEi, WG and increased FCR (<i>P</i> = 0.001) than low BCAA. However, BCAA levels had no effect (<i>P</i> > 0.05) on these measurements in HCP-diets. High NE increased (<i>P</i> < 0.001) NEi and decreased (<i>P</i> < 0.001) FCR compared to low NE in the HCP-diets. However, the NE effect on both measurements did not differ (<i>P</i> > 0.05) in RP-diets. These results indicate that increasing BCAA levels beyond the recommended amounts in RP-diets impairs energy utilization, leading to poor performance.