Effects of Dietary Threonine Levels and Stocking Density on the Performance, Metabolic Status and Immunity of Broiler Chickens

Abstract

ABSTRACT A total of 240 Cobb 500 broiler chic ks, 18 days old, were allocated to 10 treatments groups, eachof which included 4 replicates. Experimental treatments consisted of a 5×2 factorial arrangementwith 5 levels of L-threonine (Thr) supplementation and 2 levels of stocking density (11.90 birds m G 2 as the normal stocking density or 16.66 birds mG 2 as the high stocking density). L-Thr was addedto a basal diet at 0.0 (control group), 0.25, 0.50, 0.75 and 1.00 g kgG 1 diet. Dietary Thr values were0.69, 0.71 (deficient), 0.74 (adequate; 100% NRC), 0.76 and 0. 79% (excess) of the current NRCrecommendations. At 42 days of age, dietary Thr levels had no significant effect on performance,immunity, plasma total protein and glucose (GLU). However, plasma cholesterol (CHO) levelsdecreased significantly (p<0.05) with L-Thr supplementation and the lowest value occurred at0.74% total dietary Thr. In addition, plasma triio dothyronine (T3) and thyroxine (T4) levels werehigher (p<0.05) at 0.76% total dietary Thr comp ared with the control group. The normal stockingdensity resulted in better performance (p<0.05) co mpared with the high stocking density. However,stocking density did not affect plasma total protein, total Ig, IgG, IgM, total lipids, GLU, CHO, T3and T4 levels. Interactions between Thr level and stocking density were observed for plasmalevels of total lipids and cholesterol. In conclusion, total dietary Thr level at 0.69% (93% of NRCrecommendations) is sufficient to improve broiler performance or immunity under high stockingdensity. However, 0.74% total dietary Thr level has a positive effect on decreasing plasma totallipids and cholesterol levels during growing-finishing phase.