Tryptophan (Trp) is the third or fourth limiting amino acid for the most poultry species fed on corn-soybean meal-diets, playing an important role in the profile of the blood biochemical variables, humoral immunity, and antioxidant capacity. A dose-response assay was carried out to investigate the effects of graded levels of dietary Trp on blood variables, immunity and meat quality in quail chicks during the last two weeks of growing period. A total of 420, 21-day-old quail chicks were randomly distributed across the seven experimental groups (i.e., 2.12, 2.25, 2.38, 2.51, 2.64, 2.77, and 2.90 g Trp /kg of diet) with 4 pen replicates of 15 birds each. Blood variables including uric acid (UA), albumin (ALB), and triglycerides (TG) responded inversely to increasing dietary Trp (P < 0.001). Blood concentration of aspartate aminotransferase (AST), relative weight of bursa of Fabricius (BF), immunoglobulin G (IgG), water holding capacity (WHC), and antigen production against the sheep red blood cells (SRBC) increased with increases in dietary Trp (P < 0.001). In contrast, the concentration of malondialdehyde (MDA) and drip loss in meat samples decreased with increasing dietary Trp (P < 0.001). The best models for determination of optimal dietary Trp were selected based on a statistical merit called model accuracy index (δ). The optimal amounts of dietary Trp for optimizing ALP, UA, total protein (TP), TG, SRBC, IgG, BF, drip loss, WHC, and MDA were estimated at 2.347, 2.371, 2,372, 2.485, 2,691, 2.738, 2.306, 2.359, 2.247, and 2.500 g/kg of diet, respectively. Principal component analysis showed that UA, TG, IgG, and drip loss had the higher association with dietary Trp compared to the rest responses. Taking into account of high δ and eigenvalues for fitted bird responses, the more reliable estimation of optimal Trp for optimization of studied responses in quail chicks would be 2.738 g Trp/kg of diet, which was significantly higher than that recommended for the quail performance.
