40 Effect of Soybean Meal Inclusion on Grow-Finish Pig Performance and Nitrogen Balance

Abstract Mixed-sex pigs (n = 240 PIC 337×1050; initial BW = 23 ± 4.7 kg) were used to evaluate the effects of soybean meal (SBM) inclusion on the growth performance and nitrogen balance of grow-finish pigs. Pigs were assigned across 60 split-sex pens, and pens were randomly assigned to one of three dietary treatments (10 pens∙treatments-1∙sex-1). Diets were formulated with increasing SBM for an approximate 3% incremental difference in crude protein (CP) between treatments. The treatments were: 1) low SBM (L-SBM) corn-soy diet supplemented with synthetic amino acids, 2) medium SBM diet (M-SBM) with SBM formulated to replace all synthetic lysine and equal to L-SBM in SID lysine, and 3) high SBM diet (H-SBM) contained excess levels of all amino acids. Diets were fed in four feeding phases ranging from approximately 25-50, 51-75, 76-105, and 106-135 kg for phases 1-4, respectively. From phases 1 to 4, SBM inclusion ranged from approximately 26-48% (SID Lys 1.06-1.32%), 19-41% (SID Lys 0.89-1.15%), 15-35% (SID Lys 0.76-1.01%), and 13-29% (SID Lys 0.68-0.86%), respectively. Additionally, diets within each phase were isocaloric. Pig BW and feed intake were measured for each phase to calculate ADG, ADFI, and G:F. On day 66, 18 gilts (n =6/treatment) were placed into metabolism stalls for total urine and feces collection over a 72 h period to assess CP digestibility and nitrogen balance. Pigs were marketed at approximately 134 kg BW, and carcass weight, backfat depth, and loin depth were measured at the harvest facility. Data were subject to ANOVA in SAS 9.4. Pen was the experimental unit, and the results were significant at P ≤ 0.05. Overall (d 0-98) pig ADFI, ADG, or G:F did not differ across treatments (P ≥ 0.202). However, there was a significant diet-by-phase interaction for ADG (P = 0.045) and G:F (P = 0.008), where H-SBM fed pigs grew faster in phase two compared with the M-SBM (1.07 vs. 1.02 kg/d; P = 0.026) or L-SBM (1.07 vs. 1.02 kg/d; P = 0.028) fed pigs. Consequently, in phase two, H-SBM pigs had improved G:F compared with M-SBM (0.47 vs. 0.44; P = 0.005) or L-SBM pigs (0.47 vs. 0.44; P = 0.001). However, these effects were not evident in the other phases. Apparent total tract digestibility of CP and nitrogen retention were similar between treatments (P ≥ 0.762). However, as expected, nitrogen retention as a proportion of intake, linearly decreased (P < 0.001) as SBM inclusion increased, which translated to an approximate 130% increase in total nitrogen