Standardized ileal amino acid digestibility estimates are not additive in extruded canine diets with an increasing inclusion of whole pulse flour ingredients

The pet food industry currently formulates using crude protein (CP) and gross amino acid (AA) concentrations of individual ingredients as opposed to formulating on an AA standardized ileal digestibility (SID) basis, like in swine and poultry diets. In order to formulate on a SID basis, AA digestibility of individual ingredients must be additive when combined in a complete diet, but this assumption has not been evaluated in pet food. The objective of the present study was to determine if there is AA additivity in 4 extruded experimental diets using the AA SID estimates from the individual ingredients that were either taken from literature or determined using the cecectomized rooster. One diet contained 45% corn and corn gluten meal (Corn45), and the other 3 had increasing inclusions of whole pulse flours, up to 45% (Pulse15, Pulse30, Pulse45), all with chicken as the main protein source. To determine the AA digestibility, 28 cecectomized roosters (4 roosters per test substrate evaluated) were randomly assigned to the 4 test diets or ingredients (chicken meal, whole pinto bean flour, and whole chickpea flour). Endogenous corrections for AAs were made using 5 additional cecectomized roosters that had been fasted for 48 h. In order to determine AA SID additivity, the difference between measured and predicted SID was determined. Differences were tested for difference from zero using a one-sample 2-tailed t-test by the TTEST procedure of SAS (version 9.4). AA SID data in the complete diets were analyzed using the Mixed Models procedure of SAS. Of the AAs that were different among diets, the majority had greater SID estimates in Corn45 compared to the 3 pulse diets (P < 0.05). Only 5 AAs (tryptophan, alanine, cysteine, glutamic acid, glycine) were not different from zero when predicted SID was subtracted from measured SID in Corn45, one AA (tryptophan) in Pulse15, 2 AAs (histidine, glycine) in Pulse30, and 1 AA (glycine) in Pulse45 (P > 0.05). The results from this study suggest that the SID of ingredients is not additive when extruded in a complete diet intended for dogs. This highlights the need for a greater understanding of ingredient interactions in the food matrix during extrusion.