Semi-synthetic reference diets containing crystalline amino acids at 120% of recommendations for adult dogs are not appropriate for estimating the protein quality of ingredients used in extruded diets

Metabolic availability (MA) of amino acids (AA) in feedstuffs has been assessed in pigs and humans using the indicator amino acid oxidation (IAAO) method. Our lab previously reported higher methionine (Met) content but lower MA in chicken meal (ChM) as compared to peas. However, the MA of met in peas or ChM was not quantified due to the absence of a crystalline AA reference diet, assumed to have 100% MA. This study aimed to quantify the MA of Met in peas and ChM compared to a semi-synthetic crystalline AA reference diet. A partially replicated 7 × 7 Latin square design was conducted in nine mixed-breed dogs [2.5 yrs old; 26.7 ± 2.7 kg body weight (BW)] fed to maintain ideal BW. Dogs received all seven dietary treatments: corn starch and barley-based reference diet supplying AA primarily as crystalline AA with Met at 19%, 40%, or 63% of requirement (BAS19, BAS40, BAS63, respectively), pea- (PEA63) and ChM- (ChM63) based diets with Met at 63%, and blends of BAS19 with peas (PEA40) or ChM (ChM40), both with Met at 40% of requirement. This created three graded levels of Met for ChM, peas, and BAS reference diets, permitting a slope ratio approach to quantify MA with the BAS diet as the common first point. Diets were isoenergetic, isonitrogenous, and met ≥ 120% of Association of American Feed Control Officials (AAFCO) AA recommendations for adult dogs (excluding Met). After 2 (D2) and 6 (D6) d of diet adaptation, IAAO was performed using L-[1-13C]-phenylalanine (Phe, 99%). Enrichment of 13CO2 in collected breath samples was measured using isotope-ratio mass spectrometry to calculate oxidation (F13CO2/kg BW/h). Data were analyzed using proc GLIMMIX with dog and period as random effects, and diet, % Met provided, IAAO day, and their interactions as fixed effects. There was no effect of day on MA, indicating the standard 2-d diet adaptation appears sufficient for determining the MA of AA using IAAO. Overall, ChM had the lowest oxidation level, and the MA of peas was 66% and 51% of ChM on D2 and D6, respectively. Additionally, the BAS diet had no response, suggesting another limiting AA in the BAS diet.