Additivity of apparent and standardized ileal digestibility of amino acid determined by chromic oxide and titanium dioxide in mixed diets containing wheat and multiple protein sources fed to growing pigs

: The aim of this study was to investigate the additivity of apparent ileal digestibility (AID) and standardized ileal digestibility (SID) of crude protein (CP) and amino acids (AA) in mixed diets containing wheat, canola meal (CM), meat and bone meal (MBM), and sorghum distillers' dried grains with solubles (DDGS) fed to pigs with chromic oxide (Cr2O3) and titanium dioxide (TiO2) as digestibility index markers. Four diets were prepared to contain wheat, CM, MBM, or DDGS as a sole source of nitrogen; three mixed diets were prepared to contain wheat, CM, and MBM; wheat, MBM, and DDGS; or wheat, CM, MBM, and DDGS; also, a nitrogen-free diet was prepared to estimate the basal ileal endogenous losses (BEL) of CP and AA. Both Cr2O3 and TiO2 were incorporated at 5 g/kg into each diet. Sixteen barrows (initial body weight = 34.7 ± 0.6 kg) surgically fitted with T-cannulas at the distal ileum were allotted to a duplicate 8 × 4 Youden square design with eight experimental diets and four periods. During each 7-d period, the ileal digesta was collected for 2 d after a 5-d adaptation, and the AID and SID of CP and AA were determined using Cr2O3 or TiO2 as marker. There was no interaction between diet and marker type for both AID and SID of CP and AA. Similar BEL, AID, and SID of CP and AA were determined by Cr2O3 and TiO2. In the wheat-CM-MBM diet, the measured AID of CP and most AA determined with Cr2O3 or TiO2 were not different from the predicted values, which was determined based on the measured AID values in wheat, CM, and MBM. In the wheat-MBM-DDGS diet, the measured and predicted AID of CP and seven indispensable AA differed (P < 0.05) using Cr2O3 as marker, and the measured and predicted AID of CP and four indispensable AA differed (P < 0.05) using TiO2 as marker. The measured AID of CP and most indispensable AA were greater (P < 0.05) than predicted AID regardless of marker type for the wheat-CM-MBM-DDGS diet. The measured SID of CP and indispensable AA were consistent with the predicted values, except Lys in the wheat-CM-MBM diet using either marker and His in the wheat-MBM-DDGS diet with Cr2O3 as marker. In conclusion, more accurate prediction of ileal digestibility of CP and AA was achieved using SID rather than AID in mixed diets containing wheat, CM, MBM, and DDGS. The determination of endogenous loss, AID, and SID of CP and AA were not affected by marker type in this study. In addition, the additivity of AID and SID of CP and most indispensable AA in mixed diets was not affected by marker type.