摘要In this study,we examined the variations between the difference method and the enzymehydrolyzed casein method for determining endogenous amino acid loss and the true amino acid digestibility in ducks fed normal protein-containing diets. These methods were compared to the nitrogen-free(N-free)diet method. The difference method was based on soybean meal as the only protein source,with the experimental diets containing crude protein levels at 15%and 20%.The enzyme-hydrolyzed casein method was based on enzyme-hydrolyzed casein meal as the protein source,with the experimental diet containing a crude protein level of 17. 5%.The N-free diet was prepared with starches and paper fibers. In each method,64 Tianfu meat drakes(7-weeks-old) with an average body weight of 2. 77 ± 0. 16 kg were used and divided into four groups,and fed four different diets.Each group contained four replicates of four drakes and they were force fed trial diets according to the Sirbald method for detecting their apparent amino acid digestibility,endogenous amino acid loss and true amino acid digestibility. The results demonstrated that using the difference,enzyme-hydrolyzed casein and N-free diet methods,endogenous amino acid losses were 0. 9946,1. 2243 and 0. 9297 mg/g dry matter intake(DMI) , respectively. The true amino acid digestibility measured by the difference method was 88.93% 14.43%.Using the enzyme-hydrolyzed casein method with two dietary crude protein levels of 15% and 20%,the digestibility was 91 .15% t 4. 33 % and 91. 97%±4.16%,respectively,and by the N-free diet methods with two dietary crude protein levels of 15% and 20% ,it was 88. 55%±4.29% and 88.82%±4.61%,respectively. The results suggested that when the dietary protein level was 15% to 20%,the true amino acid digestibility and endogenous amino acid loss as determined by the difference method was more accurate than the values determined by the enzyme-hydrolyzed casein method.