Soybean Meal Supports Sow Immune Health and Milk Quality
By Thomas D’Alfonso, Ph.D., Worldwide Animal Nutrition Focus Area Director, USSEC
As genetic selection continues to push litter sizes past 20 pigs,¹ the nutritional demands on sows during gestation and lactation have never been higher. That pressure puts a spotlight on dietary crude protein – specifically, how much is needed and whether synthetic amino acids can replace the protein that soybean meal naturally provides. A recent University of Illinois study examined whether reducing dietary crude protein through lower soybean meal inclusion – and replacing it with crystalline amino acids – can adequately support sow health, immunity and reproductive performance, or whether higher soybean meal inclusion remains the better choice.
What the Research Examined
Researchers at the University of Illinois tested two dietary approaches across gestation and lactation. The first was a high-protein diet based on corn and soybean meal. The second reduced soybean meal and added crystalline lysine, methionine, threonine, tryptophan and valine to meet estimated amino acid requirements. A total of 154 gilts and sows were used to evaluate nitrogen balance, reproductive performance and immune indicators.¹
Reproduction Was Equal. Health Was Not.
Both dietary approaches supported comparable reproductive outcomes. Litter size, pigs born alive and pigs weaned were not significantly different between treatments. For producers focused solely on reproductive numbers, the low-protein approach appeared adequate.¹
However, the picture changed when researchers looked beyond piglet counts. Sows receiving the high-protein corn-soybean meal diet showed:
• Lower oxidative stress at and after farrowing.
• Greater antioxidant defense indicators (glutathione peroxidase, white blood cell count).
• Higher immunoglobulin G in both colostrum and milk.
• Greater protein and fat concentrations in milk.¹
Sows fed the low-protein diet had elevated rectal temperatures at farrowing and 24 hours later. Their serum also showed greater malondialdehyde – a marker of oxidative stress. The low-protein diet did reduce nitrogen excretion, which has environmental implications. But those reductions came with tradeoffs in sow welfare and colostrum quality.
Why Soybean Meal Inclusion Matters
The findings reinforce the value of maintaining soybean meal as the primary protein source in sow diets. Crystalline amino acids can help balance dietary profiles and reduce nitrogen output, but they do not appear to replicate the full nutritional environment that intact soybean meal protein provides – particularly when it comes to immune function and milk composition.
When soybean meal is the right choice, origin matters. Research from the University of Illinois found that the standardized ileal digestibility of crude protein and most amino acids is greater in soybean meal derived from U.S. Soy than in soybean meal sourced from Brazil, Argentina or India, giving nutritionists a more precise and reliable foundation for diet formulation.²
Colostrum and milk quality are critical to early piglet development, and differences in immunoglobulin G and milk nutrient density can have downstream effects on pre-weaning survival and piglet health. These findings suggest that producers who reduce soybean meal inclusion below thresholds supported by research may be trading long-term performance for short-term feed cost savings.
The Amino Acid Advantage of U.S. Soy
Soybean meal is recognized as the premier protein source for pigs,³ and for good reason. Lysine, the first limiting amino acid in grain-based swine diets, is abundant in soybean meal and highly digestible. The same is true for methionine and threonine, the amino acids most critical for tissue maintenance and sow productivity during lactation.
Not all soybean meal delivers these amino acids equally. University of Illinois research showed that soybean meal derived from U.S. Soy achieves a standardized ileal digestibility for lysine of 92.9%. This is compared to 90.6% for Brazilian soybean meal and 90.0% for Argentine soybean meal.² That difference translates directly into more available amino acids per kilogram of feed, enabling nutritionists to formulate with greater precision and reduce dependence on costly crystalline amino acid supplementation.
For sow operations where amino acid supply during lactation directly affects colostrum quality, milk composition and piglet development, the consistency and digestibility of soybean meal derived from U.S. Soy provides a reliable nutritional foundation that supports both sow health and the next generation of pigs.
Read the Full Research Summary
The complete summary of this University of Illinois research is available on the U.S. Soy CAN website: Reducing Dietary Crude Protein in Diets for Sows.
Partially funded by the Soy Checkoff.
¹ Reducing Dietary Crude Protein in Diets for Sows, University of Illinois, J. Ibagon, H. Stein and S. Lee, National Hog Farmer, January 29, 2026
² Chemical Composition and Amino Acid Digestibility of Soybean Meal Produced in the United States, Brazil, Argentina, India, or China, University of Illinois, L. V. Lagos and H. H. Stein, Journal of Animal Science 95(4):1626–1636, 2017
³ Nutritional Properties and Feeding Values of Soybeans and Their Coproducts, University of Illinois, H. H. Stein, et al., Soybeans: Chemistry, Production, Processing, and Utilization, AOCS Press, pp. 615–626, 2008