Varying trypsin inhibitor activity in differently processed soybean expellers linearly reduces prececal amino acid digestibility in broilers

The present study investigated the effect of varying trypsin inhibitor activity (TIA) in differently processed soybean expellers on apparent prececal amino acid (AA) digestibility in male broiler chickens. Two different raw soybean batches were treated using four different processing techniques (thermal, hydrothermal, pressure, kilning) at varying intensities. In this way, 45 expeller extracted soybean meal (ESBM) variants were created. The processed soybean variants were then merged into a basal diet (160 g/kg crude protein (CP)) at two inclusion levels (15%, 30%) resulting in 91 different diets (1 basal diet plus 90 experimental diets) with TIA ranging from 0.4 mg/g to 8.5 mg/g. All diets contained 0.5% of titanium dioxide (TiO 2 ). During four experimental runs, a total of 5,460 1-day old male broiler chickens (Ross 308) were fed a commercial starter diet (CP 215 g/kg, 14 g/kg Lysine, 12.5 MJ ME/kg) ad libitum from day 1 to day 14. Subsequently, birds were allocated to a total of 546 pens with 10 birds per pen and were fed the 91 experimental diets ad libitum . At day 22, birds were sacrificed and digesta of the terminal ileum was collected for determination of AA digestibility. TIA depressed the prececal digestibility of every single AA significantly in a straight linear fashion (p < 0.001). cystine and methionine expressed the strongest suppression by TIA with cystine showing the lowest apparent prececal digestibility measured (4.94% at 23.6 mg/g TIA in raw ESBM). Correspondingly, live weight (LW) (p < 0.001) and total weight gain (TWG) (p < 0.001) declined in a linear manner with increasing TIA in feed. The present data demonstrate that TIA severely depresses digestibility of essential and non-essential AA and thus growth performance in a straight linear fashion. On the one hand, this questions the usefulness of defined upper limits of TIA in soy products whereas on the other hand, TIA must be considered when testing raw components for their feed protein value in vivo.