The use of synthetic amino acids (AA) in swine diets has increased through the years reducing the inclusion of soybean meal (SBM) in the diets. Synthetic AA are readily absorbed in the small intestine, but effects of synthetic AA on digestibility of AA in mixed diets by growing pigs are unknown. Therefore, the objective was to test the hypothesis that the apparent ileal digestibility (AID) of starch and standardized ileal digestibility (SID) of crude protein (CP) and AA is not changed by reducing SBM and increasing synthetic AA in diets fed to growing pigs. A total of 7 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 7 diets and 7 periods. Two diets with 17% and 14% CP were formulated. One diet was based on corn, SBM, and no synthetic AA and the other diet was based on corn, SBM, and synthetic Lys, Met, and Thr. Four additional diets containing 13, 12, 11, or 10% CP were formulated by reducing SBM and increasing synthetic Lys, Met, Thr, Trp, Val, and Phe. A N-free diet was also used to determine basal endogenous losses of AA. Ileal digesta were collected, frozen, lyophilized, ground, and analyzed for dry matter, Cr, CP, and AA. Data were analyzed using PROC MIXED of SAS (SAS Inst. Inc., Cary, NC, USA); diet was the fixed effect and animal and period were random effects. Contrasts coefficients were used to determine differences between the 17% and 14% protein diets and linear and quadratic effects of reducing CP in diets; the pig was the experimental unit. Results indicated that there was no effect of reducing CP in diets on AID of starch. However, the SID of CP and of some indispensable AA was greater (P < 0.05) in the 14% CP diet compared with the 17% CP diet, and the SID of CP and of all indispensable AA increased (linear, P < 0.001) as CP in diets was reduced from 14% to 10%. The reason for the increased SID of AA in diets with less SBM is likely the greater SID of synthetic AA, which are almost 100% digestible in the small intestine, whereas AA from SBM have SID values of 90 to 94%. In conclusion, reducing CP in diets by reducing SBM while increasing corn and synthetic AA inclusion did not affect AID of starch but increased SID of CP and AA. These results imply that low protein corn-SBM diets supplemented with synthetic AA may have a greater absorption of AA, which may impact N retention of the diets.

Cristobal, M., S. Lee, A. Ortiz, C. Parsons and H. Stein. 2025. Effects of diet protein concentration on apparent ileal digestibility of starch and standardized ileal digestibility of protein and amino acids by growing pigs. 2025 American Society of Animal Science Midwest Section meeting, Abstract 211.