Comparison of central composite design and factorial arrangement to evaluate the interaction between net energy, soybean meal, and standardized ileal digestible lysine content of diets fed to pigs from 11 to 25 kilograms

Two studies compared a central composite design (CCD) and a factorial arrangement of treatments to evaluate the effects of dietary net energy (NE), soybean meal (SBM), and standardized ileal digestible (SID) Lys on the growth performance of 11 to 25 kg pigs. Experiment 1 used 4,681 pigs (PIC 337 × 1,050; initially 13.0 kg) in a CCD with approximately 35 pigs per pen and seven blocks, each comprising 19 pens: eight factorial points, six axial points, and a central point replicated five times. Eight diets were formulated to various NE, SBM, and SID Lys concentrations then blended to create the 15 dietary treatments. Net energy ranged from 2,334 to 2,762 kcal/kg, SBM from 25.5% to 35.9%, and SID Lys from 1.08% to 1.52%. Increasing SID Lys quadratically increased (P < 0.05) ADG and G:F. Increasing SBM linearly increased (P < 0.05) ADG and G:F. Increasing NE decreased (linear, P < 0.10) ADG due to a reduction in the Lys:NE ratio as NE increased. An SBM × SID Lys interaction (P = 0.082) was observed for G:F, where SID Lys increased G:F with increasing SBM due to decreasing the Lys:CP ratio. In experiment 2, there were 4,336 pigs (PIC 337 × 1,050; initially 10.6 kg) with approximately 34 pigs per pen and eight pens per treatment arranged in a 2 × 2 × 4 factorial. Eight diets with various NE, SBM, and SID Lys levels were blended to create the 16 dietary treatments. Main effects included NE (2,425 or 2,676 kcal/kg), SBM (25.5% or 33.5% of the diet), and SID Lys (1.08%, 1.20%, 1.31%, or 1.43%). There was a tendency (P = 0.063) for three-way interaction for ADG driven by the linear increase (P < 0.001) in ADG as SID Lys increased in diets containing 2,627 kcal NE/kg and 33.5% SBM compared to diets at lower SBM and NE with a higher Lys:NE ratio. A three-way interaction (linear, P = 0.023) was observed for G:F. Increasing SBM increased G:F to a greater extent in low-energy diets than in high-energy diets. Increasing SID Lys resulted in a greater response in high-energy diets than in low-energy diets. Diets containing low SID Lys and NE but high SBM increased G:F compared with low SID Lys, NE, and SBM also contributing to the interaction. Data from experiment 1 predicted results for almost all variables from experiment 2 within ±3% of the observed values. In conclusion, a CCD can provide similar estimates of pig growth performance as a factorial arrangement. In addition, both experiments showed the impact of not maintaining Lys:NE ratios when increasing NE and the benefit in G:F when increasing SBM in the diet.