Two experiments were conducted to test the hypothesis that growing pigs have greater protein deposition, better immune status, and improved intestinal integrity if they are fed a diet based on soybean meal (SBM) rather than substantial amounts of crystalline amino acids (AA). Four diets were formulated to contain different protein levels by reducing SBM and increasing crystalline AA and corn. Therefore, as the inclusion of SBM was reduced, dietary protein was also reduced from 20.0% to 13.4%. Diets were fed to growing pigs in both experiments. In Exp. 1, the apparent total tract digestibility (ATTD) and retention of nitrogen (N) were determined by collecting fecal and urine samples from pigs fed the experimental diets. Results from Exp. 1 indicated that the ATTD of N in diets and daily N retention (g/d) were decreased by reducing SBM and increasing crystalline AA and corn in diets although N retention (% of intake and % of absorbed N) as expected was increased by reducing dietary protein. In Exp. 2, growth performance of pigs, deposition of nutrients and energy, and intestinal integrity and immune status of the pigs fed experimental diets were determined. Results from Exp. 2 indicated that growth performance, dressing percentage, deposition of protein, fat, and energy, blood cytokines, and AA transporters in ileal mucosa were not affected by dietary protein. However, energy efficiency tended to decrease by reducing dietary protein. Blood urea N and bacteria protein were also decreased by reducing dietary protein.

Results from these 2 experiments need to be disseminated to the swine feed industry to make sure formulators of diets for pigs are aware that nitrogen retention and energy efficiency are reduced if crystalline AA rather than SBM are used in diets for growing pigs. In terms of follow-up research, it is advisable to repeat the experiments – maybe under different conditions – to confirm the results and further build trust in these conclusions. Specifically, the reduced energy efficiency with lower-protein diets is important to confirm in future research because, although consistent with some recent data, this goes against what many nutritionists have been taught.