The objective of this study was to determine the relationship between in vivo amino acid digestibility and protease inhibitors in commercial soybean meal (CSBM). A total of 12 CSBM samples were analyzed for: (1) trypsin inhibitor activity (TIA) utilizing the new official AOCS method (Ba 12a-2020). (2) chymotrypsin inhibitor activity (CIA) by the latest optimized method of Liu (2022. J. Food Sci. https://doi.org//10.1111/1750- 3841.16141), and (3) in vivo amino acid (AA) digestibility determined utilizing the precision-fed cecectomized rooster bioassay as described by Corray et al. (2018. Poult. Sci. 97:3987-3991). Linear and quadratic regressions of digestible AA coefficients on TIA and the summation of TIA and CIA (TIA+CIA) were conducted using PROC GLM in SAS (2013). The TIA ranged from 3.53 to 12.82 units inhibited (UI)/mg of CSBM. The TIA+CIA ranged from 8.74 to 23.40 UI/mg of CSBM. All 17 determined digestible AA coefficients for each of the 12 CSBM samples were negatively correlated with TIA and TIA+CIA values. For TIA, linear regression R-square values ranged from 0.48 (r= -0.69) for digestible lysine to 0.72 (r= -0.85) for digestible serine (P<0.05). There was no significant quadratic effect of TIA vs. digestible AA for all AA. For TIA+CIA, linear regression R-square values ranged from 0.41 (r= -0.64) for digestible cysteine to 0.77 (r= -0.88) for digestible serine (P<0.05); there was no significant quadratic effect of TIA+CIA vs. digestible AA for all AA.
Given the fact that TIA values above 5.37 UI/mg of CSBM which is approximately equivalent to 3.58 mg trypsin inhibited (TId)/g of CSBM) were associated with rapid feed passage syndrome outbreaks in the field as previously reported in broilers and broiler breeders [2005 Poult. Sci. 84(Suppl. 1):70; 2008 Poult. Sci. 87(Suppl. 1):30], these data suggest that TIA and TIA+CIA contents in CSBM lots are partially responsible for the indigestible AA fraction in CSBM, and that TIA and CIA are relevant in the formulation of CSBM in broiler and broiler breeder feeds.