Additivity of amino acid digestibility in corn and soybean meal for broiler chickens and White Pekin ducks.

A total of 256 broiler chickens and 208 White Pekin ducks were used in a 5-d trial to test additivity of the apparent (AID) and standardized ileal digestibility (SID) of amino acids (AA) in corn and soybean meal (SBM). A nitrogen-free diet was used for standardization of apparent digestibility. Two diets contained corn or SBM as the sole source of CP and AA. Another diet was prepared by mixing corn and SBM. Chromic oxide was included as an indigestible marker. Birds received a standard starter diet from d 1 to 21 and thereafter the 4 experimental diets for 5 d in a randomized complete block design. On d 26, digesta from the distal section of the ileum was collected. To test additivity of digestibility values, the difference between measured values in mixed diet and predicted values calculated with measured values in individual ingredients was examined. The estimated basal endogenous losses of N and AA were higher in ducks compared with broilers (P < 0.05). Regardless of species, the calculated proportion of basal endogenous losses of CP and AA in calculating SID was higher (P < 0.01) in corn than that in SBM. The measured AID of His, Leu, Lys, Met, Phe, Glu, Pro, and Tyr in mixed diets for broilers were higher than predicted values (P < 0.05), whereas measured and predicted SID did not differ except for Cys. For ducks, the differences between the measured and predicted AID were significant (P < 0.05) for CP and all AA except for 8 AA (Arg, Ile, Thr, Val, Asp, Gly, Pro, and Tyr). Similar to the result from broiler study, there was no difference between the predicted and measured SID of CP and all AA except for Cys. The results showed that AID of some AA for corn and SBM were not additive in the mixed diet, but SID values were additive irrespective of the ingredients and species used. Moreover, extent of difference between measured and predicted values in ducks were higher than that in broilers.