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A study was conducted to evaluate growth performance, carcass traits, blood serum parameters, and immune responses of Ross 308 male broilers fed diets containing 2 different crude protein (CP) levels (97.5 and 100%) and 4 threonine (Thr) levels (100, 110, 120, and 130% of Ross recommendations for starter and grower periods). A completely randomized block design was adopted and main effects (CP and Thr) were arranged in a 2 × 4 factorial approach. Optimum growth performance was achieved when broiler requirements for CP and Thr were 100% satisfied. The 110% Thr inclusion in 97.5% CP diet increased ADG, ADFI, energy intake, and protein intake (Thr, P < 0.01; quadratic, P = 0.01). The G:F (linear, P = 0.05) and energy efficiency (linear, P = 0.04) tended to decreased (Thr, P = 0.09) by increasing Thr supplementation level, whereas protein efficiency tended to increase (CP, P = 0.06) by reducing CP level. The 110% Thr inclusion in 97.5% CP diet increased eviscerated carcass weight (CP × Thr, P = 0.03) and carcass yield (Thr, P = 0.08; quadratic, P = 0.05). The reduction of CP content promoted fat abdominal deposition (CP, P = 0.05). Incremental Thr raised abdominal fat (Thr, P = 0.01; linear, P = 0.01). The 97.5% CP diets resulted in higher serum concentrations of uric acid (CP, P = 0.02), total and high- and low-density lipoprotein-linked cholesterol (CP, P≤ 0.01), and alanine aminotransferase (CP, P = 0.05) and lower (CP, P = 0.01) concentrations of triglycerides and very low density lipoproteins compared with the 100% CP diets. However, the Thr inclusion improved serum lipid profile. Irrespective of CP content, incremental Thr levels up to 120% increased (Thr, P = 0.01) broiler immune responses against Newcastle disease virus and sheep red blood cells. In order to reduce dietary CP content, strategies to increase synthetic amino acid availability, such as the use of encapsulated amino acids, should be taken into account.