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.