Impact of soybean meal levels in grow-finisher pig diets for growth and nutrient metabolism

Abstract Two hundred and forty pigs (27.62 ± 4.54 kg; Genus 337 x 1050; PIC, Hendersonville, TN) were allotted to split-sex pens (4 pigs/pen, 52 pens). In a completely randomized design, pigs were randomly assigned to one of four dietary treatments (n = 153 pens/treatment): 1) low soybean meal (SBM) diet (LSBM) supplemented with crystalline amino acids (AA), 2) medium SBM (MSBM) diet with moderate crystalline AA, 3) enhanced SBM (ESBM) diet replacing crystalline lysine with SBM, and 4) elevated soybean meal (ESBM+) diet replacing additional crystalline AA with SBM. Diets were formulated to meet or exceed NRC recommendations and were fed across three 28-day phases (SBM: 20–50%, 15–45%, 10–40%) from days 0–84. Growth performance was assessed by body weight (BW) and feed disappearance to calculate average daily gain (ADG), average daily feed intake (ADFI), and feed efficiency (G: F). The subset of 24 gilts were randomly selected on D0 and placed in metabolism stalls on days 10, 38, and 66 to evaluate nutrient digestibility, retention, and manure composition. Data were analyzed using Proc GLIMMIX MIXED in SAS 9.4 (Statistical Analysis System, Cary, NC) with pen or stall as the experimental unit. Treatment and dietary phase were fixed effects, and significance was set at P ≤0.05 and tendencies at 0.05 P ≤0.10. Orthogonal linear contrasts were used to evaluate response changes in increased dietary CP. No treatment effects were observed for BW, ADG, or ADFI (P ≥0.39). Final BW was unaffected by diet (106.40 ± 1.028 kg; P = 0.59). A treatment effect on G: F in growth performance on the metabolism subset were observed in phase 3, with higher efficiency present in the MSBM-fed pigs compared to ESBM+, 0.42 vs. 0.28, respectively (P = 0.02). No significant differences were found in ATTD of DM, ash, OM, GE, N, or Ca (P 0.10). Phosphorus digestibility was lower in ESBM + compared to LSBM (P = 0.04). Sulfur digestibility tended to be lower in MSBM compared to LSBM (P = 0.07). Nitrogen, sulfur and phosphorus excretion increased linearly with SBM inclusion (P 0.01). Total carbon excretion tended to increase linearly as SBM increased (P = 0.06) while Ca excretion did not differ by SBM levels (P = 0.36). Manure pH and sulfur levels increased linearly with SBM inclusion (P 0.01). In conclusion, replacing crystalline AA with SBM did not negatively affect growth performance. Pigs fed MSBM had similar nutrient digestibility and retention to ESBM and ESBM+.