Increasing trypsin inhibitor concentrations in growing pig diets compromises growth performance and protein digestibility

Soybeans contain trypsin inhibitor proteins that are antinutritional and may result in growth performance inhibition through reduced amino acid bioavailability. The objective of this experiment was to determine the level of active trypsin inhibitor units (TIU) per milligram (TIU/mg) of complete feed that leads to reductions in growth performance and nitrogen digestibility. Forty-five grower gilts (39.7 ± 2.1 kg BW) were individually penned and fed 1 of the 5 dietary treatments (n = 9 pigs/trt) in a completely randomized design for the duration of this 27-d experiment. Diets were fed in meal form and formulated utilizing raw ground soybeans and soybean meal to achieve increasing levels of TIU/mg of complete feed of 0.89, 1.77, 3.55, 6.99, and 11.54 TIU/mg. Analyzed complete diets contained 0.99, 2.23, 3.07, 6.49, and 9.38 TIU/mg. Diets were balanced to the same grams of standardized ileal digestible (SID) lysine and SID lysine per megacalorie of metabolizable energy of 2.85 and neutral detergent fiber (NDF) concentration of 7.4% utilizing crystalline amino acids, soybean oil, and soybean hulls, respectively. Pig body weight (BW) and individual feed intake were collected on days 0, 21, and 27 to calculate average daily gain (ADG), average daily feed intake (ADFI), and feed efficiency (G:F). On day 21 of the experiment, pigs were placed into metabolism stalls for 6 d to determine nitrogen balance and apparent total tract digestibility (ATTD) of nitrogen. All data were analyzed with pig as the experimental unit and TIU/mg level as a fixed effect by ANOVA, including linear and quadratic effect contrasts. Average daily gain decreased as dietary concentrations TIU/mg increased (1.07, 1.03, 1.02, 0.91, and 0.83 kg/d, respectively, linear P < 0.001). No differences in ADFI (P > 0.10) were detected, but G:F decreased (0.48, 0.45, 0.43, 0.41, 0.36, respectively, linear P < 0.001) was observed as TIU/mg increased from 0.99 to 9.38 TIU/mg. Final BW of pigs fed 9.38 TIU/mg was reduced by 9.32% compared with 0.99 TIU/mg (62.3 vs. 68.7 kg, linear P < 0.001). Nitrogen ATTD decreased when pigs were fed 9.38 TIU/mg compared to 0.99 and 2.23 TIU/mg fed pigs (83.26% vs. 90.29% and 89.57%, respectively, P < 0.001). In conclusion, as TIU from soybeans increased in grower pig diets, linear reductions in protein digestibility, growth rates, and feed efficiency were observed.