Effects of reducing dietary content of crude protein and indispensable amino acids on performance and carcass traits of single-phase- and 2-phase-fed growing-finishing pigs

•Effect of 2 feeding plans, single-phase and 2-phase on pig performance was studied.•Within each feeding plan reduced crude protein and lysine content were investigated.•Single-phase-fed growing-finishing pigs had the capacity for compensatory growth.•Dietary lysine and crude protein could be reduced below current recommendations.•Reducing the dietary crude protein from high to low can lower the N output by 20%. Effects of reducing dietary content of crude protein (CP) and Lys on performance and carcass traits of growing-finishing pigs were studied in 690 crossbred (Swedish Yorkshire dams x Hampshire sires) female and vaccinated entire male pigs with an average initial live weight (LW) of 32.3 kg. The pigs were raised in pens with mixed sexes and were fed restrictedly using 2 different feeding plans, single-phase and 2-phase. Within each feeding plan, 3 CP contents were investigated: 13.5 (low), 14.5 (medium), and 15.5 (high) g standardized ileal digestible (SID) CP/g SID Lys. Each CP comprised 2 Lys contents. For single-phase-fed pigs this implied either 0.76 or 0.85 g SID Lys/MJ net energy (NE). For 2-phase-fed pigs, the contents were either 0.89 in phase 1 and 0.71 in phase 2 or 0.98 in phase 1 and 0.79 in phase 2 g SID Lys/MJ NE. In low and high Lys, the expected total consumption of digestible Lys from start to slaughter was calculated to be equal for single-phase- and 2-phase-fed pigs. The change of diet in the 2-phase feeding plan was at an average LW of 60.8 kg and slaughter was performed at 117.3 kg. Irrespective of dietary CP and Lys content, 2-phase-fed pigs had higher daily weight gain (DWG) and better feed conversion in phase 1 than single-phase-fed pigs. In phase 2, single-phase-fed pigs fully compensated, so that overall DWG and feed conversion did not differ between single-phase- and 2-phase-fed pigs (1014 vs. 1013 g/d; 24.5 vs. 24.5 MJ NE/kg DWG). Dietary CP content had no effect on DWG and feed conversion in phase 1. However, pigs grew faster and had better feed conversion in phase 2 (P = 0.002 and P = 0.018, respectively) and during the entire raising period with the high CP content (P = 0.003 and P = 0.006, respectively). For both single-phase- and 2-phase-fed pigs, high dietary Lys content improved DWG and feed conversion in phase 1 compared with the low content (1002 vs. 968 g/d; P = 0.001 and 20.7 vs. 21.5 MJ NE/kg DWG; P = 0.004), whereas no difference was observed in phase 2. Overall, Lys content did not affect DWG, while feed conv