The effects of supplemental threonine on performance, carcass characteristics, immune response and gut health of broilers in subtropics during pre‐starter and starter period

Three hundred thirty‐day‐old unsexed commercial broiler chicks (Vencobb‐400) with initial average body weight of 44.04 ± 0.42 g were allocated into five experimental groups, in a completely randomized design (CRD) with 21‐day experiment. Groups were formed according to dose of supplemental L‐threonine in various rations i.e., 100% NRC specification, 100% threonine of Vencobb‐400 strain specification, 110% threonine of Vencobb‐400 strain specification, 120% of threonine of Vencobb‐400 strain specification and 130% threonine of Vencobb‐400 strain specification. Average daily feed intake (ADFI), average daily body weight gain (ADG), cumulative feed conversion ratio (CFCR), carcass characteristics, immune response, intestinal morphometry and biochemical profile were studied. The ADFI and ADG increased linearly and quadratically as dietary threonine levels were increased. However, the CFCR did not differ (p ˃ 0.05) among the groups. Though the carcass weight and drumstick yield did not differ (p ˃ 0.05) among the groups, the relative breast yield increased linearly (p = 0.007). The relative dressing yield and relative thigh weight increased linearly (p = 0.05 and p = 0.03, respectively). The relative weight of immune organs like bursa and thymus increased linearly. The mean total serum immunoglobulin, ND‐ELISA titre and the mean lymphocyte proliferation response index increased linearly, whereas mean phagocytic activity index of neutrophil increased linearly (p < 0.001) and quadratically (p = 0.001). The mean villus height (VH), crypt depth (CD), villus surface area and mean goblet cell number/villus increased linearly and quadratically, whereas the villus width (VW) and goblet cell density increased quadratically. The serum glucose increased linearly (p = 0.001), whereas serum total protein concentration and serum globulin level increased both linearly and quadratically. The albumin: globulin ratio tended to decrease linearly. There was a significant decrease (p < 0.05) in serum cholesterol and VLDL cholesterol level. However, a linear increment (p = 0.04) in the blood serum HDL cholesterol level with a linear reduction (p = 0.01) in the blood serum LDL cholesterol was noticed.