The impact of soybean derived trypsin inhibitor proteins and soybean hulls on nursery and grow-finish pig nutrient digestibility, growth performance, and carcass composition
Soybean meal and soybean hulls are co-products of soybean oil production and have a nutrient profile that makes them a favorable ingredient in swine diets. However, components of these ingredients have anti-nutritive effects that hinder efficiency at high amounts in growing pig diets. Therefore, it is important to understand how to incorporate these ingredients into swine diets to optimize performance. Interestingly, our understanding of using soybean hulls in growing pig swine diets is not fully understood when one considers dietary energy formulation strategy, feed intake capacity, and their impact on carcass dressing percentage. Further, although it is well known that antinutritional protease inhibitor proteins are innate in soybean ingredients, the specific activity of these proteins in the complete feed that attenuates pig performance and health is limited. Therefore, the overall objective of this dissertation was to evaluate optimizing growth performance in nursery and grow-finish pigs using diet formulations that include soybean hulls and soybean antinutritional proteins. To accomplish this overarching objective, a series of studies were conducted and outlined across four research chapters (Chapter 2, 3, 4, and 5). In Chapter 2, an experiment was conducted to determine the effect of soybean-derived trypsin inhibitor protein on nursery pig performance and health. Likewise, Chapter 3 investigated the effects of trypsin inhibitor protein concentrations on growing pig performance and nitrogen digestibility. Chapter 4 presents an experiment conducted to determine the impact of soybean hull inclusion level and dietary formulation strategy on nursery pig performance. Finally, Chapter 5 investigates the effects of including soybean hulls into grow-finish pig diets and withdrawing the soybean hulls before marketing on pig growth performance and carcass composition. The body of research presented herein confirms the negative effects associated with varying anti-nutritive components of soybean ingredients. Research results from Chapter 2 and 3 confirm that trypsin inhibitor proteins (expressed as trypsin inhibitor units per milligram; TIU/mg) found in soybean protein ingredients hinder growth performance. This work reported that body weight gain, feed intake, and feed efficiency begin to reduce at dietary levels above 1.22 TIU/mg in nursery pigs but were most severely impacted at our highest level of 3.51 TIU/mg (Chapter 2). Additionally, in this experiment the intestinal epithelium was negatively impacted, as villus height and crypt depth in the ileum shortened linearly as TIU concentrations increased. Interestingly, fecal consistency was unaltered in this study which goes against the hypothesis of undigested proteins inducing diarrhea in nursery pigs. In a similar study, Chapter 3 utilized grower pigs and observed reductions in body weight gain and poor feed efficiency in pigs fed diets above 3 TIU/mg. This study also confirmed trypsin inhibitor proteins are negatively associated with protein digestion as hypothesized, as pigs fed increasing TIU diets had poor apparent total tract digestibility of nitrogen and increased nitrogen excretion. Results from these two studies confirm the negative relationship between soybean-derived trypsin inhibitor proteins and pig efficiency. Two studies were conducted to optimize soybean hull utilization in swine diets (Chapter 4 and 5). Results of Chapter 4 indicate soybean hulls can be formulated into diets of nursery pigs up to 8% before performance is hindered. Additionally, the standardized ileal digestible (SID) lysine per megacalorie of metabolizable energy (ME) ratio of these nursery diets containing soybean hulls may be formulated to be either fixed or allowed to float without negative effects to performance. In grow-finish pigs (Chapter 5), we showed that soybean hulls can be formulated up to 15% of the diet before reductions to feed efficiency and digestibility of nutrients occurs. However, as expected, carcass yield is negatively impacted by the inclusion of soybean hulls. If soybean hulls are included at higher levels (up to 22.5% of the diet), growth performance and carcass yield may be partially restored if pigs are transitioned to a traditional corn-soybean meal diet without added fiber at least 13 days prior to marketing. Taken together, this dissertation highlights strategies and implications for utilizing soybean ingredients (soy protein and soybean hulls) in nursery and grow-finish pig diets. Additionally, this body of work demonstrates the negative relationship of soybean antinutritional components, both through soybean hulls and trypsin inhibitor proteins, and performance of growing pigs. Consistently across each study herein, both the addition of soybean hulls and trypsin inhibitor proteins hinder optimal pig efficiency. However, as soybean-based ingredients are highly accessible and desired in formulation for their nutrient composition, knowledge of the implications of the anti-nutritive effects of these ingredients is imperative. The results from this dissertation will allow nutritionists to incorporate soy fibers (i.e., hulls) and proteins into swine diets with greater confidence to optimize pig performance.
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