Increasing dietary soybean trypsin inhibitor protein attenuates nursery pig performance

Trypsin inhibitor proteins are antinutritional compounds innate to soybeans that reduce protein digestibility, amino acid bioavailability, and growth performance of pigs. The objective of this study was to evaluate the impact of increasing levels of dietary trypsin inhibitor unit activity (TIU/mg) on nursery pig growth performance and health. In a 41-d study, 1,140 newly weaned nursery pigs (5.9 ± 0.34 kg BW) were allotted into split sex pens, blocked by body weight, assigned randomly to one of five dietary treatments (n = 19 pens/treatment) varying in TIU/mg concentration, and fed over three dietary phases. Treatments targeted 0.41, 1.32, 2.20, 3.08, and 3.96 TIU/mg of complete feed averaged over the three phases and were achieved by using a corn-soybean meal basal diet with added soybean flour. Analyzed dietary treatments averaged 0.61, 1.22, 2.19, 3.41, and 3.51 TIU/mg. Pen BW and feed disappearance were recorded at the start and end of each phase to calculate ADG, ADFI, and G:F. Fecal consistency was scored and recorded daily. On d 21 of the study, 10 pigs per treatment were sacrificed for intestinal sample collection. Data were analyzed with pen as the experimental unit, the random effect of block, and the fixed effect of TIU, including polynomial contrasts for linear and quadratic effects of 0.61 to 3.51 TIU/mg treatments. No quadratic responses to dietary TIU/mg activity were reported in any parameters. Overall, as active dietary TIU/mg increased, ADG, ADFI, and G:F linearly decreased (P < 0.001). Pigs fed the highest level (3.51 TIU/mg) exhibited reduced ADG by 25%, ADFI by 17%, and G:F by 8% compared to pigs fed the lowest level (0.61 TIU/mg). Dietary TIU/mg did not affect fecal consistency, mortality, or removals (P > 0.10). Individual and total concentrations of colonic biogenic amines and short chain fatty acids did not differ (P > 0.10). Histological lesions of the ileum and colon did not differ (P > 0.10). Ileum VH tended to decrease (P = 0.078) and CD linearly decreased as TIU/mg increased (P = 0.004), but VH:CD and colonic CD were similar (P > 0.10). Moderate relationships between TIU intake and G:F (R² = 0.393), caloric efficiency (R² = 0.378), and lysine efficiency (R² = 0.376) were observed. In conclusion, soybean-derived active TIU concentrations negatively impact nursery pig performance above 1.22 TIU/mg, with minimal impacts on intestinal and pig health.