Soybean meal (SBM) is widely used as a major source of amino acids (AA) and energy in swine diets globally. However, efforts to reduce nitrogen excretion and environmental impact have led to attempts to lower dietary protein by reducing SBM and increasing the use of synthetic AA in pig diets. It has, however, been observed that decreasing dietary protein does not consistently yield the same performance in pigs compared with those fed high-protein diets despite meeting the requirements for indispensable AA. In addition, soybean-derived bioactive components, such as soy isoflavones, may function as immunomodulators, potentially enhancing the immune system and consequently leading to improved pig performance. Therefore, the objective of this research was to test the hypothesis that reducing the dietary protein by decreasing the SBM inclusion and increasing synthetic amino acids AA in the diet may negatively impact growth performance, carcass composition, meat quality, and N balance of growing pigs, and that soybean-derived bioactive compounds (i.e., soy isoflavones) in SBM act as immunomodulators, which improve the immune system and thus results in better performance of pigs. We conducted 2 experiments to test this hypothesis and we used 6 experimental diets in both experiments. Three diets contained different levels of protein (i.e., high, medium, and low). Protein levels in the diets were reduced by reducing the inclusion of SBM. There were 4 low-protein diets with 2 levels of isoflavones (i.e., 0 or 0.4% isoflavones) and 2 levels of an additional N-source (i.e., 0 or 2% L-glutamic acid). In Exp. 1, the digestibility and retention of nitrogen (N) were determined. Results from Exp. 1 indicated that the daily N retention (g/d) was decreased by reducing SBM, although N retention (% of intake and absorbed) was increased by reducing dietary protein. Results from Exp. 2 indicated that growth performance and blood cytokines were not affected by dietary treatments, but backfat thickness tended to increase and loin eye area tended to decrease as dietary protein was reduced.

The overall take-away from these experiments is that it appears soybean meal supports protein synthesis to a greater extend than diets formulated with equivalent quantities of digestible indispensable amino acids supplied from individual amino acids. If this conclusion is accepted by feed formulators and nutritionists, this will greatly increase the usage of soybean meal in diets for pigs, which will increase overall demand and thereby improve profitability of producing soybeans.