Effect of Feeding Different Protein and Threonine Levels on Immunocompetence of Broiler Chicks
This experiment was designed to evaluate the effects of different levels of dietary crude protein and threonine on immunological responses of broiler chicks. A total of 432 one-day-old male broiler chickens were used in a completely randomized design. Dietary treatments consisted of three levels of threonine (100, 110 and 120% of Ross 308 requirement recommendations) and three dietary crude protein concentrations (90, 95 and 100% of Ross 308 requirement specifications) in a 3×3 factorial arrangement of treatments with 4 replicates of 12 birds each. Feeding different levels of crud protein and threonine had no remarkable effect on Newcastle antibody titer (P>0.05). Reduction in dietary crude protein content tended to decrease primary (P=0.16) and secondary (P=0.13) immune responses against sheep red blood cell (SRBC); however, incremental level of threonine up to 110% of recommended values promoted antibody responses against SRBC antigen (P<0.05). Administration of diets with at least 110% of threonine recommendations caused to significant decrease in albumin to globulin (P<0.01) and heterophil to lymphocyte (P<0.05) ratios. These changes were due to the remarkable increase in the proportions of globulins and lymphocytes (P<0.05). The interaction between crud protein and threonine was significant on lymphatic tissues responses (P<0.05) and the highest relative weights of spleen and bursa of fabricius and also the numbers of jejunal lamina propria lymphatic follicles were obtained when the birds fed high-crud protein high-threonine diets. In conclusion, the present results indicated that although decrease in dietary crude protein content to 90% of recommended values reduced the humoral immune responses and bursa of fabricius relative weight, fortification of diets with threonine up to 110% of Ross 308 recommended values could improve the humoral immune responses and also the proportions of blood globulins and lymphocytes in broiler chicks.