By Nicholas Gabler, Kayla Miller and Mitch Nisley of Iowa State University
Soybeans and soybean coproducts, namely soybean meal, are highly desired for their amino acid profile. Still, they contain several antinutritional factors that can impede digestion, absorption and bioavailability of nutrients and energy, and can affect animal health and performance.
These naturally occurring antinutritional factors include lectins, saponins, tannins, raffinose, stachyose, isoflavones, β-conglycinin and two protease inhibitors known as Kunitz trypsin
inhibitor and the Bowman-Birk trypsin inhibitor. Trypsin inhibitor proteins inhibit the enzyme trypsin, which is essential for protein digestion in pigs. At high levels in the diet, active trypsin inhibitor protein reduces protein digestibility, leading to decreased growth and poor feed efficiency.
Although many of these antinutritional factors can be mitigated by processing and heat treatment, trypsin inhibitors can pose problems for pigs. Typical range of trypsin inhibitor activity (TIU) in soybean meal ranges can vary from 2 TIU/mg to 12 TIU/mg.
Iowa State University, in collaboration with USDA-ARS at University of Missouri, United Animal Health and The Maschhoffs, have been studying the impact trypsin inhibitors have within complete feed formulation of nursery and grower pigs. Although we know soybean trypsin inhibitor protein consumption can effect pig performance, our goal was to understand the level to which trypsin inhibitor activity in complete feed impacts nursery pig and grower pig performance, protein (nitrogen) digestibility and retention.
Experiment 1 – Nursery Pigs
In the first experiment, 60 barrows averaging 12.3 lb. that were weaned at 19 to 21 days of age, were placed into individual pens assigned to one of six dietary treatments and fed across two phases. The diets were formulated to six increasing levels of active TIU ranging from 0.5 to 6 TIU/mg of complete feed which were achieved using soybean meal and raw ground soybeans. Analyzed diets averaged 0.38, 0.54, 1.85, 3.16, 3.90 and 5.79 TIU/mg of diet across both phases. The diets were balanced for metabolizable energy (ME), neutral detergent fiber (NDF), and amino acids (AA) utilizing soybean oil, soybean hulls and crystalline amino acids, respectively.
Phase 1 was fed from day 0 to day 14 and phase 2 was fed from day 14 to day 42 of the study. During phase 2, 48 pigs were moved into metabolism stalls and a total collection of urine and feces was completed to assess the impact of trypsin inhibitor activity on nitrogen digestibility and balance. All pigs were weighed on days 0, 14 and 42 to calculate average daily gain (ADG), average daily feed intake (ADFI), and feed efficiency (G:F) by phase.
Experiment 2 – Grower Pigs
Using 45 grower gilts with a starting body weight of 87.5 lb., pigs were randomly assigned to individual pens and assigned one of five dietary treatments containing increasing levels of TIU. The analyzed complete diets contained 0.99, 2.23, 3.07, 6.49, and 9.38 TIU/mg using soybean meal and raw ground soybeans to achieve targeted activity levels. The diets were balanced for ME, NDF and AA utilizing soybean oil, soybean hulls and crystalline amino acids.
Pigs were fed for 21 days and then moved into metabolism stalls for the remaining six days to assess the impact of trypsin inhibitor activity on nitrogen digestibility and balance. The pigs were weighed at days 0, 21, and 27 of the study to calculate ADG, ADFI and G:F.
What Did the Studies Reveal?
In both experiments, end body weights were linearly reduced as TIU in the diet increased. Nursery pig body weights started to diminish at TIU levels above 1.85 TIU/mg, with pigs fed the highest level (5.79 TIU/mg) experiencing a 40% reduction in final body weights. In grower pigs, pigs fed TIU levels above 3.07 TIU/mg had declining body weights. Pigs fed at the highest level of 9.38 TIU/mg saw a 23% reduction in body weights after 27 days.
In nursery pigs, ADG was relatively constant for the first three levels but began to drop at TIU levels above 1.85 TIU/mg and were most extreme at the highest level, where a 16.5% reduction from the first level in daily weight gain was observed. In the growing pigs, ADG dropped as TIU level increased. Comparing the lowest and highest TIU level, ADG was reduced by 16.5% over the 27-day feeding study.
Interestingly, responses in feed intake seem to be driven by the age of the pig. In experiment one, increasing TIU level in the diet reduced pig feed intake, especially at the highest level of 5.79 TIU/mg. Generally, at the other four TIU levels investigated, feed intake was not influenced. However, in grower pigs, ADFI was not altered by TIU level. Feed efficiency, expressed as gain-to-feed ratio, was highly impacted in both nursery and grower pigs as TIU activity of the diet increased.
Apparent total tract digestibility (ATTD) of nitrogen was negatively linearly effected by the inclusion of TIU in nursery and grower pig diets. At their highest levels in these studies (5.79 and 9.38 TIU/mg), ATTD of nitrogen was reduced by 5% and 7% for nursery and grower pigs, respectively.
This suggests increasing trypsin inhibitor protein activities from soybean ingredients in complete feed reduces nursery and grower pig performance, protein digestibility and retention. Although nursery pigs might be more sensitive, activity levels above 3 TIU/mg in complete feed for nursery and grower pigs might be of concern to production efficiency. As the soybean crush continues to expand, and soybean meal is predicted to be more available, consider how to best formulate antinutritional compounds innate to soy products in pig diets.
