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.

Two experiments were conducted to test the hypothesis that diet protein concentration does not affect apparent ileal digestibility (AID) of starch, standardized ileal digestibility (SID) of amino acids (AA), daily nitrogen retention, or net energy (NE) in diets fed to growing pigs. In experiment 1, seven diets were used. A corn-SBM diet without synthetic AA was formulated to contain 17% CP. Five additional diets containing 14, 13, 12, 11, or 10% CP were formulated by reducing SBM and increasing corn and synthetic AA. A nitrogen-free diet was also used to determine basal endogenous losses of AA, and Cr2O3 was included in all diets as an indigestible marker. Seven barrows (initial body weight: 38.2 ± 1.5 kg) that had a T-cannula installed in the distal ileum were allotted to a 7 × 7 Latin square design with seven diets and seven periods, and ileal digesta were collected for 2 d of each period. Results demonstrated that AID of starch was not influenced by diet CP concentration, but SID of CP and all indispensable and dispensable AA increased (linear, P < 0.05) as dietary CP was reduced. In experiment 2, the six protein-containing diets from experiment 1 were used, but Cr2O3 was replaced by corn and no nitrogen-free diet was used in this experiment. Twenty-four growing pigs (initial body weight: 29.9 ± 2.4 kg) were placed in six calorimeter chambers with four pigs per chamber. The six chambers were allotted to the six diets using a 6 × 6 Latin square design with six periods of 14 d. Pigs had ad libitum access to diets throughout the experiment except during the final 36 h of each period, when pigs were fasted. Net energy in diets was determined using indirect calorimetry. Results demonstrated that digestible energy, metabolizable energy, and daily nitrogen retention were reduced (linear, P < 0.05) as dietary CP was reduced, but NE in diets was not influenced by dietary CP. In conclusion, the hypothesis that reducing CP of diets for pigs by reducing SBM and increasing corn and synthetic AA does not affect AID of starch or NE of diets was confirmed, but the hypothesis that SID of indispensable AA and daily nitrogen retention are not affected by dietary protein concentration was rejected.