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

Authors: MILLER, KAYLA - Iowa State University; SPENCER, JOEL - United Animal Health; MENDOZA, OMARH - Maschhoff Pork Farm; Krishnan, Hari; NISLEY, MITCHELL - Iowa State University; GABLER, NICHOLAS - Iowa State University

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2025
Publication Date: 8/11/2025
Citation: Miller, K.A., Spencer, J.D., Mendoza, O.F., Krishnan, H.B., Nisley, M.J., Gabler, N.K. 2025. Increasing trypsin inhibitor concentrations in growing pig diets compromises growth performance and protein digestibility. Journal of Animal Science. 103. https://doi.org/10.1093/jas/skaf256.
DOI: https://doi.org/10.1093/jas/skaf256

Interpretive Summary: Due to its superior nutritional quality, wide availability, and relatively low cost, soybean meal is the most widely used protein source in animal diets. However, soybeans contain anti-nutritional factors such as trypsin inhibitors that can restrict the growth of pigs if not properly heated. Few studies have systematically investigated the level of trypsin inhibitors in the diet that inhibits maximal growth and nitrogen digestibility in growing pigs. Understanding the influence of trypsin inhibitor content and other antinutritional compounds in soybeans is imperative to promote higher soybean meal (SBM) inclusion in animal feed. Therefore, the objective of this study was to determine the level at which trypsin inhibitor units (TIU) from soybeans in dietary formulations impact grower pig growth performance, digestibility, and nitrogen retention. Results from this study indicate that trypsin inhibitor concentrations greater than 3 TIU per milligram of complete feed negatively impact grower pig performance and protein retention. Our research findings will benefit U.S. farmers by promoting the wider use of SBM in the animal industry and eventually leading to the inclusion of full-fat soybeans in growing pig diets.

Technical Abstract: 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.