The objective of the trial was to determine the effect of different concentrations of trypsin inhibitor (TI) in soybean meal (SBM) on amino acid (AA) digestibility and peptide characterization of jejunal digesta in the presence of graded concentrations of a commercial protease. SBM of different TI concentrations were produced from a batch of soy beans and 5 batches selected with KOH solubilities >80%, containing 0.64, 0.94, 1.67, 2.24, 3.46 mg/g of TI. The trial consisted of 21 treatments (Trt) (4×5 factorial arrangement of 5 TI and 4 protease concentrations (0, 15, 75, 150 PROT/g feed) plus one purified diet). One batch of a purified basal diet was mixed and batches of SBM replaced 40% of the basal in the SBM Trt diets. Each batch of SBM Trt diet was divided into 4 and protease added to achieve desired concentrations. Each Trt was fed to 8 pens of Cobb 500 broilers from 14 to 16d of age. Broilers were euthanized after 48 h and jejunal (for peptide characterization) and distal ileal content (for standard- ized ileal AA digestibility) collected. There were main effects of TI and protease on Ala, Arg, Cys, Glu, Gly, His, Iso, Leu, Lys, Met, Phe, Ser, Thr, Tyr, and Val (P<0.01) but no interaction (P>0.05) on standardized ileal digestibility (SID). Effects of protease were driven by improvements in SID between 0 and 15 PROT/g protease with no further improvements at higher protease inclusions. SID for all AA were lower (P<0.5) when TI was 0.64 and 3.46 mg/g. The effect of TI appears to be confounded with effects of over processing at 0.64 mg/kg TI. Overall, addition of protease at 15 PROT/g improved digestibility of all AA. The level of free AA, di- and tripeptides in jejunum were not affected by an interaction between TI and protease. However, data showed a decrease in the jejunal concentration of free AA, di- and tripeptides with increasing TI going from 0.64 to 3.46 mg/g (P<0.05). And free AA increased 23% with protease concentration going from 0 to 150 PROT/g (P<0.01).
Further work is needed to understand what in processed SBM is impacting digestibility differently and how addition of a protease is helping improve digestibility.