The purpose of this study was to compare estimates of ileal endogenous amino acid (IEAA) losses determined in broiler chicks with a nitrogen-free diet (NFD), diets containing highly digestible proteins as intact casein or enzymatically hydrolyzed casein (EHC), and the regression method (RM). Male Ross 308 broiler chicks were fed a commercial starter diet from d 1 to 15 of age and the following test diets from d 15 to 21: NFD and diets containing 5, 10, or 15% casein or EHC as the sole protein source. All diets contained chromic oxide as a digestibility marker. Each diet was assigned to 6 replicate cages, with 10 birds each. On d 21, birds were euthanized to sample ileal digesta. Feeding increasing levels of EHC or casein linearly (P < 0.0001) increased IEAA flow. Nonlinearity (P < 0.05) was detected for Ile, Val, Glu, and Serwith EHC and for Met with casein. Compared with the NFD method, IEAA values xtrapolated to zero EHC or casein intake were similar for all amino acids except Ile (200 vs. 321 mg/kg of DM intake), Val (270 vs. 341 mg/ kg of DM intake), Ala (217 vs. 262 mg/kg of DM intake), and Ser (343 vs. 577 mg/kg of DM intake), whose flows were higher with EHC (P < 0.05). The present results show that IEAA values determined with the NFD, EHC, and casein diets, at different inclusion levels, were different for most amino acids and that, for all amino acids, the values obtained with NFD and RM involving feeding graded levels of casein or EHC were comparable. Thus, using IEAA values obtained with either the NFD or the RMto calculate standardized ileal amino acid digestibility coefficients will give similar values.

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