Reducing dietary protein in corn–soybean meal diets by reducing soybean meal and adding synthetic amino acids does not affect net energy or ileal starch digestibility, but increases ileal amino acid digestibility and reduces nitrogen retention

Lay Summary Two experiments were conducted to determine effects of reducing dietary crude protein by replacing soybean meal with corn and synthetic amino acids on ileal digestibility of starch and amino acids, daily nitrogen retention, and net energy in diets fed to growing pigs. In both experiments, pigs were fed diets with 17%, 14%, 13%, 12%, 11%, or 10% crude protein, with synthetic amino acids and corn replacing dietary soybean meal as dietary protein was reduced. Results indicated that reducing protein in the diet did not affect ileal digestibility of starch but increased the digestibility of protein and amino acids. However, digestible energy was reduced as dietary protein level was reduced, but there were no differences in net energy among diets. Results indicated that reducing dietary protein by reducing the level of soybean meal and increasing corn and synthetic amino acids does not influence the net energy of diets, which indicates that the net energy of soybean meal is likely close to the net energy of corn.