Increasing dietary soybean-derived trypsin inhibitor protein compromises nursery pig performance, nitrogen digestibility, and retention

Trypsin inhibitor proteins, intrinsic to soybeans and soy products, have been found to impair amino acid bioavailability, thereby attenuating pig growth performance. The objective of this experiment was to examine the effect of varying dietary trypsin inhibitor units (TIU) on the performance, nitrogen digestibility, and retention in nursery pigs. Sixty barrows (5.6 ± 0.65 kg body weight [BW]), weaned at 19 to 21 d of age, were randomly allocated across six dietary treatments (n = 10/treatment). Diets balanced for metabolizable energy (ME), crude protein, neutral detergent fiber (NDF), and standardized ileal digestible (SID) lysine were fed in two phases over the 42-d study, lasting 14 and 28 d, respectively. Dietary treatments were formulated on a soybean TIU protein (TIU/mg) basis, using soybean meal, raw whole soybeans, soybean oil, and soyhulls to target 0.5, 1, 2, 3, 4, and 6 TIU/mg in complete feed for both dietary phases. For phase 1 and 2 diets, analyzed TIU/mg were 0.22/0.54, 0.47/0.61, 1.71/1.98, 2.88/3.44, 3.79/4.01, and 6.15/5.42, respectively, averaging 0.38, 0.54, 1.85, 3.16, 3.9, and 5.79 TIU/mg across both phases. Pigs were individually housed with ad libitum access to water and feed. Pig BW and feed disappearance were measured at the start and end of each phase to calculate pig performance parameters. On day 17, a subgroup of 48 pigs (n = 8/treatment) were moved to metabolism crates for total fecal and urine collection to analyze nitrogen digestibility and retention. Data were analyzed using pig as the experimental unit to assess the effect of analyzed TIU levels, including linear and quadratic contrasts, on all parameters. No quadratic responses were reported overall (P > 0.05). At the end of the experiment, pig BW decreased from 25.3 to 15.5 kg as the diet increased from 0.38 to 5.79 TIU/mg (linear, P < 0.0001). Moreover, as TIU increased, overall average daily gain, average daily feed intake, and feed efficiency were attenuated, resulting in reductions by 49%, 32%, and 26%, respectively (linear, P < 0.01). During phase 2, at the highest levels of dietary TIU/mg, nitrogen digestibility (%) was reduced by 5.3% (linear, P = 0.001), and nitrogen retention (% of intake) decreased by 15.3% (linear, P < 0.0001). In conclusion, there was a constant reduction in growth performance, feed efficiency, and nitrogen retention as soybean TIU increased in the diet of nursery pigs.