Protein digestibility evaluations of meat and fish substrates using laboratory, avian, and ileally cannulated dog assays

Meat and fish serve as important protein sources in the companion animal diet; however, limited protein digestibility data are available for assessing protein digestibility differences among good-quality protein sources. Beef loin, pork loin, chicken breast, pollock fillet, and salmon fillet were evaluated for composition, protein digestibility, and AA bioavailability using the immobilized digestive enzyme assay, cecectomized rooster assay, and ileally cannulated dog assay. Pollock contained the greatest amount of CP, total essential AA (TEAA), and total nonessential AA (TNEAA; DM basis; 96.9, 38.6, and 50.3%, respectively). Salmon contained the next greatest amounts (92.8, 36.4, and 44.6%), followed by chicken (90.3, 36.1, 43.2%). Beef had the least CP content (82.7%), but had slightly greater TEAA and TNEAA concentrations (33.9, 42.0%) compared with pork (86.2, 33.6, 41.3%). Immobilized digestive enzyme assay values were greatest for pollock fillet (0.71) and least for chicken breast (0.52). Beef loin, pork loin, and salmon fillet were similar (0.63, 0.62, and 0.64, respectively). Standardized TEAA and TNEAA digestibility coefficients, evaluated using the cecectomized rooster assay, were greatest (P < 0.05) for pollock fillet (90.4 and 89.8%, respectively) and least (P < 0.05) for chicken breast (86.6 and 85.9%, respectively) and salmon fillet (87.8 and 86.4%, respectively). Dogs assigned to a 5 x 5 Latin square design were fed 5 diets, with each test substrate as the major protein source. No significant differences (P > 0.05) were found in ileal digestibility of protein. Values ranged from 88.9% for chicken to 90.5% for pork loin and pollock fillet. Ileal TEAA and TNEAA coefficients were not different among test substrates, with values between 91.7 and 92.7%, and 88.8 and 90.4%, respectively. Total tract CP apparent digestibility values ranged from 94.4 to 94.8%, with no differences noted among treatments. Despite marked differences in composition and predicted and standardized digestibility values, when the protein sources were added to diets at a concentration of approximately 30% (25% of total energy intake), no differences in test protein substrates were noted in either ileal or total tract nutrient digestibility.