How more soybean meal affects immunity in growing pigs

What's the impact on immune status, antioxidant capacity and acute phase proteins in 30 to 70 kg growing pigs?

By Margaret Putnam and Amy Petry, Division of Animal Sciences, University of Missouri

Corn and soybean meal have long served as the foundation of grow-finish swine diets in the United States. This is due to their reliable availability in major swine-producing regions and their complementary amino acid profiles. Consistently, solvent-extracted SBM contains approximately 47.7% crude protein, 8.2% neutral detergent fiber, 1.5% ether extract, 6.3% ash and 11% moisture on an as-fed basis (NRC, 2012). The remaining approximate 25-40% of SBM that is not accounted for by these categories is partly composed of an array of functional compounds, including oligosaccharides, phenolic acids, isoflavones, saponins and bioactive peptides (White et al., 2024; Choct et al., 2010).

These functional compounds classify SBM as a "functional food," which is described as a food that provides health benefits beyond its basic nutritional value, such as improving health, reducing disease risk or enhancing physiological function (Messina et al., 2022). In human nutrition, functional foods are valued for their role in gut health, longevity and weight management. In terminal swine production, however, the objective is different, placing emphasis on growth rather than longevity. Considering this, the functional compounds innate to SBM have been linked to improved antioxidant status, modulation of immune responses, enhancement of intestinal barrier function, and shifts in the gut microbiome (Choct, 2010; Smith & Dilger, 2018). This influence on health observed with functional feeds may reduce physiological burden, allowing more energy to be directed toward production, resulting in improved performance.

Work in recent years has highlighted performance improvements with increasing SBM, even when formulated net energy is held constant. To explain these results, factors such as limitations in methodologies for net energy estimation, modern genetics of animals and seed varieties, conditionally essential amino acids, and extra-nutrient effects of SBM functional compounds have been discussed. To further explore the functional effects of SBM in swine, this work set out to survey biomarker responses and performance outcomes in a commercial production setting in response to increasing SBM inclusion.

To effectively evaluate SBM's biological role, the effects of SBM inclusion may be compared to alternative amino acid sources. As such, distillers dried grains with solubles often replace part of the SBM in grow-finish diets, altering both nutrient composition and intake of soy bioactives. Therefore, this study examined the effects of increasing SBM levels, with or without DDGS, on immune, oxidative, digestive, and metabolic indicators to determine whether SBM's nutraceutical effects are influenced by DDGS inclusion and how varying levels of SBM alter responses under commercial conditions.

This was a large project in collaboration with The Maschhoffs, Kansas State University and the University of Missouri. The growth performance responses from this study have been previously reported by Giacomini et al. (2025) in a K-State Swine Report as a research objective of a related project. Briefly, pigs fed diets with DDGS had decreased average daily gain, average daily feed intake and gain to feed ratio compared to pigs fed diets without DDGS, while increasing SBM decreased ADFI and improved G:F.

Additionally, in the present study, increasing SBM levels elevated circulating serum IL-8, which acts as a chemokine and a primer for immune responses, independent of DDGS inclusion. Regardless of DDGS inclusion, increasing SBM elevated serum IL-10, an immune regulator, preventing excessive immune activation and excessive inflammation. Further, SBM tended to decrease circulating granulocyte macrophage-colony stimulating factor, an immune stimulator, indicating a decrease in immune cell proliferation in pigs fed diets containing higher SBM levels.

Inclusion of 30% DDGS altered acute phase protein responses, including decreased serum haptoglobin concentrations but increased C-reactive protein. Irrespective of DDGS inclusion, increasing SBM linearly elevated haptoglobin concentrations. Haptoglobin is an indicator of acute phase response and systemic inflammation, binding free heme during inflammation to prevent iron loss and limit oxidative damage, and CRP is an indicator of systemic inflammation. Therefore, these acute phase protein responses indicate increased inflammation with diets containing DDGS, and a potential increased capacity to limit oxidative damage with increased SBM.

Further, total antioxidant capacity, or the ability of a sample to neutralize free radicals tended to increase with increasing SBM when DDGS were absent; however, it tended to decrease with increasing SBM when DDGS were included, supporting SBM's ability to reduce oxidative damage.

Though these responses are complex, the cytokine patterns observed with increasing SBM in the present study suggest coordinated modulation of pro- and anti-inflammatory signaling rather than excessive immune activation. This is supported by the observed immune marker responses, where increasing SBM was associated with elevations in IL-10 and reductions in GM-CSF, suggesting enhanced immune regulation and restraint rather than immune escalation. Additionally, increases in IL-8 with SBM, may reflect immune priming rather than pathological inflammation, indicating preparedness without excessive energetic cost. As such, this balance may allow immune preparedness without diverting energy away from growth, but further research is needed (Huntley et al., 2018).

Because immune activation and oxidative stress increase maintenance energy requirements and divert energy away from productive processes, the coordinated modulation of cytokines and acute phase proteins observed with increasing SBM may contribute to differences in performance outcomes under commercial conditions. Ultimately, increasing SBM influenced immune and oxidative status in growing pigs, with responses modified by DDGS inclusion. These results support the concept that SBM provides functional effects beyond its nutritional value, influencing physiological processes that are relevant to production outcomes, but further research is warranted to truly understand if they have a direct impact on maintenance energy cost.

Disclaimer: We express our appreciation to the United Soybean Board for their support of this project. Project #24-106-D-A-4-A/2421-106-0401, 24-107-D-E-1-B/2415-107-0102. This is unpublished data that has yet to be peer reviewed, and this article was not reviewed by USB.

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