There is a lack of knowledge regarding the lysine (Lys) requirements of mature dogs and whether there are breed differences. The present study aimed to determine the Lys requirement in three breeds of mature dogs using the indicator amino acid oxidation (IAAO) technique. Thirteen adult dogs were used, four Miniature Dachshunds (5.39 ± 0.71 kg; 1.05 ± 0.02 yr old, mean ± SD), four Beagles (8.09 ± 0.40 kg; 5.03 ± 0.09 yr old, mean ± SD), and five Labrador Retrievers (29.42 ± 2.04 kg; 3.30 ± 0.69 yr old, mean ± SD). After 14 d of adaptation to a basal extruded kibble diet, dogs were fed a test diet mildly deficient in Lys (Lys concentration = 0.36%) at 17 (Miniature Dachshunds) or 13 g/kg body weight (BW; Beagles and Labradors) for 2 d. The test diet was supplemented with one of seven isonitrogenous Lys-Ala solutions, resulting in a final dietary Lys concentration of 0.36%, 0.40%, 0.44%, 0.50%, 0.54%, 0.58%, and 0.62% (as-fed basis). Dogs received dietary concentrations of Lys in random order and no dog received the same order. Following 2 d of adaptation to the experimental diets, the dogs underwent IAAO studies. During the IAAO studies, total daily feed was divided in 13 equal meals. At the sixth meal, dogs were fed a bolus of L-[1-¹³C]-Phe (9.40 mg/kg BW); thereafter, L-[1-¹³C]-Phe was supplied with every meal (2.4 mg/kg BW). Total production of ¹³CO₂ (F¹³CO₂) during isotopic steady state was determined by enrichment of ¹³CO₂ of breath samples and total production of CO₂, measured using indirect calorimetry. A two-phase linear regression model was used to derive the mean Lys requirement, defined as the breakpoint, and the upper 95% confidence limit was calculated as the recommended allowance (RA) for Lys intake. For Miniature Dachshunds, the study was repeated with a feed intake of 14 g/kg BW, but Lys requirements could not be determined at either feed intake, suggesting a requirement below the lowest concentration and intake. Mean Lys requirements for Beagles and Labradors were 0.455% (59.16 mg/kg BW) and 0.440% (57.19 mg/kg BW), respectively, on a dry matter basis. Pooling the data for these breeds provides a mean estimate of the Lys requirement at 0.448% (58.21 mg/kg BW) with an upper 95% CL of 0.526% (68.41 mg/kg BW) on a dry matter basis. In conclusion, the Lys requirements of Beagles and Labradors are similar, while the requirement for Miniature Dachshunds is undetermined and likely lower. The estimated Lys requirement for Beagles and Labradors is higher than the National Research Council recommendation.

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