Abstract: This experiment was conducted to study the effect of low-protein diets on production performance,serum biochemical indices and feces nitrogen content in Mule ducks.A total of 450 1-day-old Mule ducks were randomly assigned to 3dietary treatments with five replicates and thirty ducks with each pen(50% males,50% females).The control group received the diet including the normal level of crude protein(CP)(19% CP during 1-21day-old,16% CP during 22-56day-old),and the level of CP in diet of trial groupⅠand trial groupⅡwas decreased by 1%and 2%,respectively,while supplied with compounded amino acids.The feeding trial lasted for 56 days.The results showed that:(1)There was no significant effect of low-protein diet on average daily feed intake(ADFI),average daily gain(ADG)and feed to gain(F/G)of Mule ducks during 1-21,22-56 and 1-56day-old(P>0.05);(2)The content of total protein(TP),urea nitrogen(UN)and nuric acid(UA)in serum of ducks at 21 d and 56 dold showed tendency to descend,however,the levels of TP,UN and UA in serum among the three groups had no obvious differences(P>0.05);(3)During 1-21day-old,the feces nitrogen content of duck in trial groupⅠand trial group Ⅱwere significantly decreased by 8.17%(P<0.05)and 15.75%(P<0.05),respectively,and which were decreased by 5.64%(P>0.05)and 21.71%,respectively during 22-56day-old(P<0.05).These results indicated that dietary protein decreased by 1%-2% had no obvious effects on growth performance and serum biochemical indices of mule ducks,but which could significantly decrease the feces nitrogen content of ducks under the condition that amino acid in diet met the need of birds.