Effects of dietary net energy, crude protein, and standardized ileal digestible lysine on growth performance, nitrogen utilization, and hepatic metabolism in 22-35 day-old broilers.

This study investigated the effects of dietary net energy (NE), crude protein (CP), and standardized ileal digestible lysine (SID Lys) densities on growth performance, nitrogen retention efficiency (NRE), and hepatic amino acid (AA) metabolism in male broilers from 22 to 35 days. A total of 936 Cobb 500 male broilers were assigned to 13 dietary treatments with 6 replicates of 12 birds using a Box-Behnken design. The design comprised three dietary NE levels (2200, 2290, and 2380 kcal/kg), three CP levels (18%, 19.5%, and 21%), and three SID Lys levels (1.08%, 1.14%, and 1.20%). The data showed that reducing dietary CP from 21% to 18% did not significantly affect body weight gain (BWG) but did significantly improve NRE (P < 0.05). Dietary CP reduction significantly down-regulated hepatic LKR and BCKDHa mRNA expression and tricarboxylic acid (TCA) cycle-related metabolites, while upregulating glycolysis-related enzymes. Increasing NE did not affect BWG but improved NRE (P < 0.05). High energy levels significantly down-regulated hepatic AMP and TCA cycle-related metabolites, while up-regulating mTOR and S6K1 mRNA expression in breast muscle. BWG responded linearly to increasing SID Lys levels (P < 0.05). However, dietary SID Lys levels showed a quadratic effect for NRE with an estimated optimal SID Lys level of 1.146% for maximum NRE. Increasing dietary SID Lys first increased, then reduced mTOR and S6K1 mRNA expression (P < 0.05). Our results revealed that reducing dietary CP from 21% to 18% did not compromise growth performance. A dietary combination of 1.20% SID Lys and 2380 kcal/kg NE (NE/SID Lys ratio of 198.3 kcal/g) appeared favorable for growth performance. For optimal nitrogen retention efficiency, a moderate SID Lys level of 1.146% with a SID Lys/CP ratio of 0.063 is recommended. These findings suggest that a low-CP diet can be effectively formulated by appropriately increasing SID Lys levels and maintaining adequate dietary energy, thereby achieving protein-sparing effects without sacrificing broiler growth.