Research On The Effects Of Dietary Tryptophan Levels On The Production Performance And Egg Quality In Laying Hens And Approach To The Mechanism

This experiment was conducted to investigate the effects of dietary tryptophan levels on the production performance, egg quality, and lipometabolism in laying hen. Seven hundred and twenty 300-day-old ROM laying hens were randomly divided into five groups each with 3 replicates of 48 layers. Control group fed with a basal corn-soybean meal diet containing 0.16% tryptophan and the treatment groups fed with the similar diets whose tryptophan levels were 0.20%,0.24%,0.28%, and 0.32% under same circumstances for 49 days. The feed intake, laying rate, and egg weight were recorded daily. At the end of feeding trial,3 laying hens from each replicate were slaughtered and then immediately eviscerated. The samples were collected for laboratory analysis. After determination of correlated index and statistically analysis of the data, the main results were obtained as follows:(1) The results of rearing experiment showed that the rate of egg production of the 0.20% group was increased by 2.18% (P< 0.05),3.08% (P< 0.05),3.23% (P< 0.05), and 3.42%(P< 0.05), and its daily egg yield weight was increased by 3.56% (P< 0.05),3.75% (P< 0.05),4.31% (P< 0.05), and 4.87% (P< 0.05) compared with the other treatments and the control, respectively. No significant difference in feed-egg ration, average egg weight, and daily feed intake was observed among the groups (P> 0.05).(2) Analysis of egg quality showed that the albumen height and Haugh unit of 0.20% group significantly were increased by 6.69% (P< 0.05) and 2.20% (P< 0.05) compared with the control, while those of 0.28% and 0.32% group were deceresed by 10.40% (P< 0.05) and 10.97% (P< 0.05). No significant difference in eggshell thickness, eggshell strength, egg yolk color, water content of albumen, water content of egg yolk, egg yolk weight percentage, protein content of albumen, and fat ratio of egg was abserved among the groups (P> 0.05).(3) The results of analysis of amino acids contents in dried albumen and egg yolk showed that there was no significant difference among the groups in trytophan and other amino acids contents of dried albumen and egg yolk (P> 0.05). (4) The results of lipometabolism parameters showed that serum levels of triglyceride (TG) and serum total cholesterol (T-CHO) of 0.20% and 0.24% group were decreased by 35.94% (P< 0.05),30.28% (P< 0.05),21.22% (P< 0.05), and 34.89% (P< 0.05) compared with control, respectively. Serum very low density lipoprotein cholesterol (VLDL) concentrations and cyclic adenosine 3', 5'-monophosphate (cAMP) levels in abdominal fat of treatments were increased significantly (P< 0.05). Abdominal fat weight percentage of 0.20% and 0.24% group were deceresed by 15.07% (P< 0.05) and 11.76% (P< 0.05). The activities of hormone sensitive lipase (HSL) in abdominal fat and of 0.20% and 0.24% group were increased by 36.05% (P< 0.05) and 21.89% (P< 0.05).(5) Analysis of protein metabolism parameters showed that serum levels of urea nitrogen (SUN) in treatments reduced significantly (P< 0.05). Compared with the control and the other treatments, SUN of 0.20% group reduced 36.49% (P< 0.05), 5.65% (P< 0.05),16.22% (P< 0.05), and 20.06% (P< 0.05), respectively. Serum levels of total protein (TP) and albumin (ALB) of 0.20% group were inceresed significantly compared with the others (P< 0.05). Glutamic pyruvic transaminase (GPT) activities in liver of 0.20% and 0.24% group were inceresed by 11.75%(P< 0.05) and 8.96% (P< 0.05) compared with the control..(6) Analysis of antioxidant indexes showed that Serum malondialdehyde (MDA) concentrations of 0.20% and 0.24% group significantly reduced by 33.72% and 18.71%(P< 0.05), and serum glutathione peroxidase (GSH-Px) activities of those groups were increased by 16.52% (P< 0.05) and 15.50% (P< 0.05) compared with the control, respectively. There was no significant difference among the groups in liver MDA concentration and SOD activity (P> 0.05). Liver GSH-Px activities of 0.20% and 0.24% group were inceresed by 20.83% (P< 0.05) and 23.90% (P< 0.05) compared with the control.(7) Analysis of serum physiological parameters showed that serum level of triiodothyronine (T3) of 0.20% group was significantly increased by 36.75%(P< 0.05), 22.86%(P< 0.05),43.87% (P< 0.05), and 40.72% (P< 0.05) compared with the control and the other treatments, respectively. Serum levels of FSH and Estradiol (E2) of 0.20% and 0.24% were increased by 14.69%(P< 0.05),11.88%(P< 0.05),20.74% (P< 0.05), and 15.56%(P< 0.05) compared with the control, respectively.In conclusion, tryptophan could promote production performance and egg quality of laying hens through enhancing gonadotropin-releasing and protein availability. The results also suggested that tryptophan could decrease weight percentage of abdominal fat and liver fat in laying hen after egg laying peak.