| The purpose of this experiment was to study the effects of different dietary protein and digestible tryptophan levels on the growth performance, blood biochemical parameters (TG, UA, TP, ALB, ALOB, 5～HT), nutrient availability and mortality of 0～21, 22～35, 36～42 days broilers. the relationship of serum 5～HT and feed intake was also studied in this paper.Experiment 1: 960 one～day aged AA broilers were randomly allocated to 5 dietary treatments, each including 180 chicks which were grouped into 6 replicates. Dietary treatments were formulated on basis of dietary protein level and digestible tryptophan. The 5 treatments of crude vs. Dtrp in all three phases(0～3 , 4～5 and 6 week) were 21.0% vs. 0.19%, 19.5% vs. 0.19%, 19.5% vs. 0.17%, 18.5% vs. 0.19%, 18.5% vs. 0.17% in first phase; 18.3% vs. 0.15%, 17.3% vs. 0.15%, 17.3% vs. 0.12%, 16.3% vs. 0.15%, 16.3% vs. 0.12% in second phase; 16.8% vs. 0.13%, 15.8% vs. 0.13%, 15.8% vs. 0.11%, 14.8% vs. 0.13%, 14.8% vs. 0.11% in last phase respectively. ADG, FI and F/G were determined in 21st day, 35th day, 42nd day respectively. Serum biochemical indexes were assayed in 21st day and 42nd day.Experiment 2: Using identical dietary treatment with experiment 1, 60 AA broilers were randomly allocated to 5 dietary treatments, each including 12 chicks which were grouped into 3 replicates. We examined DM, OM, N excretion and DM, OM, CP apparent metabolizability in different phase using the whole-feces-collected method.The results were elucidated as followings:The phase of 0～21 days: the effects of the change in the dietary protein level from 21% to 19.5 on average daily gain and feed/gain were not significant(P>0.05). It had the same effects as the digestible tryptophan level changed. Changes in dietary Dtrp from 0.17% to 0.19% increased average daily gain, feed intake and gain/feed when dietary protein was in 19.5% level(P>0.05). The negative effect of treatment of CP(19.5%) VS. Dtrp (0.19%) on 0～21 day broiler growth performance was the lowest in all the five treatments.The phase of 22～35 days: the effect of the reducing in the dietary protein level from 18.3% to 17.3% on average daily gain, feed intake and feed/gain were not significant(P>0.05). The effects of the change in the digestible tryptophan level on average daily gain, feed intake and feed/gain were not also significant(P>0.05). Changes in dietary Dtrp from 0.12% to 0.15% increased average daily gain, feed intake and gain/feed when dietary protein was in 17.3% level(P>0.05). The negative effect of treatment of CP(17.3%) VS. Dtrp (0.15%) on 22～35～day broiler growth performance was the lowest in the all five treatments.The phase of 36～42 days: the effect of the reducing in the dietary protein level from 16.8% to 15.8% on average daily gain, feed intake and feed/gain were not significant(P>0.05). The effects of the change in the digestible tryptophan level on average daily gain, feed intake and feed/gain were not also significant(P>0.05). Changes in dietary Dtrp from 0.11% to 0.13% elevated average daily gain, feed intake and feed/gain when dietary protein was in 15.8% level(P>0.05). The negative effect of treatment of CP(16.8%) vs. Dtrp (0.11) on 22～35～day broiler growth performance was the lowest in all the five treatment. Uncertain Relationship between broiler feed intake and serum 5～HT was observed in the experiment.The broiler diarrhea rate in phase of 0～21～days was the lowest in treatment of CP vs. Dtrp(19.5% vs. 0.19%). The effect of dietary protein and digestible tryptophan level on mortality of the broiler in 0～3 week, 4～5 week, 6 week was not significant(P>0.05). The mortality of the broiler reached the lowest points in 18.5%, 17.3%, 15.8% protein level in all three phases respectively.The N excretion trend was decreased (P>0.05) in 21st day, 35th day, 42nd day when dietary protein level reduced from 21.0% to 19.5%, 18.3% to 17.3%, 16.8 to 15.8% respectively. The apparent metabolizability of crude protein tended to increase when dietary protein level decreased (P>0.05). On 21st day, N Excretion decreased by 14.8% and CP apparent metabolizability increased by 9.4% as dietary protein level reduced from 21.0% to 18.5%. On 35th day, N Excretion decreased by 6.4% and CP apparent metabolizability increased by 10.1% as dietary protein level reduced from 18.3% to 16.3%. On 42nd day, N Excretion decreased by 14.4% and CP apparent metabolizability increased by 7.5% as dietary protein level reduced from 16.8% to 14.8%.
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