عنوان مقاله [English]
Introduction: The ideal performance of poultry is achieved by adequately feeding of birds while reducing dietary costs and nutrients excretion. On the other hand, the amount of dietary nutrients can have a significant effect on feed intake (Kidd et al., 2002). Studies have shown that reduced growth and yield due to decreased dietary protein can be compensated by supplementing the synthetic amino acids (Hussein et al., 2001). Diets based on total and digestible amino acids can improve the performance (Rostagno et al, 1995). Some properties of valine and tryptophan in relation to non-protein function and metabolism distinguish them from other amino acids. Valine is the fourth limited amino acid to the growth of broilers. Tryptophan involved in all proteins structure and is a precursor for serotonin and melatonin, which in addition to improving food intake (Silber and Schmitt, 2010). Valine affects the ghrelin and neuropeptide Y production (Coto et al, 2006) while tryptophan affects the serotonin secretion (Henry, 1985) that both affects the feed intake. The functions of the two amino acids can improve the performance and immune system of birds.
Material and methods: The current experiment was performed to evaluate the effects of different levels of valine and tryptophan amino acids in low-protein diets on carcass characteristics and immune response of broiler chickens. A total of 450 day-old Ross 308 broiler chickens were randomly divided into nine treatment groups. The chickens were arranged in a 3×3 factorial experiment based a completely randomized design including 3 levels of valine and three levels tryptophan. Each treatment considered of five replicates and 10 chickens per experimental unit. The birds were reared on the litter pen for 21 days with ad-libitum access to feed and water. Diets were formulated based on linear programming by using of the UFFDA software. Experimental treatments were adjusted based on Brazilian tables and included: 1- recommended level of valine + recommended level of tryptophan, 2- recommended level of valine + 5% more than recommended tryptophan, 3-recommended valine level + 10% more than recommended tryptophan, 4-10% more than recommended valine level + recommended level of tryptophan, 5-10% more than recommended valine + 5% more than recommended tryptophan, 6- 10% more than recommended tryptophan level + 10% more than recommended tryptophan, 7- 20% more than recommended valine level + recommended level of tryptophan, 8- 20% more than recommended valine + 5% more than recommended tryptophan, 9-20% more than recommended valine level+ 10% more than recommended tryptophan. On day 21 of experiment, two birds per pen were randomly selected and slaughtered for whole carcass analysis after 12 hours of fasting. After slaughter, the organs such as breast, thigh, abdominal fat, gizzard, liver, heart, spleen, thymus and burs fabricius were weighed. In order to determination of white blood cells, blood sampling drawn from the wing vein at 21 days of age and the number of lymphocytes, heterophils, monocytes, basophils and eosinophils were determined. Blood samples were taken at 21 days of age as well to determination of the antibody titer of Newcastle virus. To measure the immune response to sheep erythrocytes, 2 ml of blood was taken from the chicken vein on 14 and 21 days of the experiment and used to determination of total immunoglobulin and IgM and IgG titers. The antibody titers were performed by direct hemagglutination (Isaco et al. 2005). The CBH test was used to evaluate the cellular immunity. The experiment was conducted using completely randomized design with factorial structure. Data were subjected to ANOVA using the GLM procedure (SAS, version 9.1) as a 3×3 factorial. Significant means among the variables were separated by Duncan's multiple range tests at 5% level of significance.
Results and discussion: The results of current experiment showed that the addition of valine and tryptophan in low protein diets had no significant effect on carcass yield, thigh, gizzard, heart and abdominal fat (p<0.05). The consumption of 20% and 10% levels of valine significantly increased the breast weight as compared to the recommended level (p<0.05). No interaction effects was observed between the valine and tryptophan amino acids for the weights of carcass parts and internal organs (p>0.05). In a study, consumption of 10 and 20% tryptophan levels had no significant effect on breast and thigh of broilers at 22 days of age (Duarte et al, 2013). Moreover, addition of valine to poultry diets did not have a significant effect on abdominal fat weight (Corzo et al, 2004). Different levels of valine and tryptophan had no affected the liver and spleen weight (p<0.05). Furthermore, addition of 20% valine caused an increase in thymus and bursa weights (p<0.05). The diets containing tryptophan had a significant effect on the number of blood lymphocytes, so that the addition of 10% tryptophan increased the blood lymphocytes (p<0.05). Valine and tryptophan addition did not affected the other blood leukocytes (p<0.05). Addition of 20% valine increased the humoral immunity (HI antibody titer) (p<0.05). Also, addition of 10% tryptophan level increased the immunity of chickens against HI antibody (p<0.05). The results showed that the initial response of total antibody was not affected by experimental treatments (p<0.05) but 10% level of valine significantly increased the IgM (p<0.05). Dietary addition of 20% valine had a significant effect on IgM secondary response SRBC (p<0.05) but had no significant effect on the PHA-P injection response (p<0.05). It has been reported that protein deficiency have caused the unsuitable growth of lymphoid organs (Corzo et al., 2007).
Conclusions: The result of current experiment showed that consumption of high valine and tryptophan levels in low-protein diets improved the carcass characteristics and immune system of broilers.