J Anim Physiol Anim Nutr. 2018;1–12. wileyonlinelibrary.com/journal/jpn | 1 © 2018 Blackwell Verlag GmbH Received: 30 April 2018 | Revised: 12 August 2018 | Accepted: 21 August 2018 DOI: 10.1111/jpn.12991 ORIGINAL ARTICLE The effects of supplemental threonine on performance, carcass characteristics, immune response and gut health of broilers in subtropics during pre‐starter and starter period Bikas C. Debnath | Purnendu Biswas | Barun Roy Department of Animal Nutrition, VAS, WBUAFS, Kolkata, India Correspondence Bikas C. Debnath, Department of Animal Nutrition, VAS, WBUAFS, Kolkata, India. Email: bikascvsc@gmail.com Abstract 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 experi‐ mental groups, in a completely randomized design (CRD) with 21‐day experiment. Groups were formed according to dose of supplemental L‐threonine in various ra‐ tions i.e., 100% NRC specification, 100% threonine of Vencobb‐400 strain specifica‐ tion, 110% threonine of Vencobb‐400 strain specification, 120% of threonine of Vencobb‐400 strain specification and 130% threonine of Vencobb‐400 strain speci‐ fication. Average daily feed intake (ADFI), average daily body weight gain (ADG), cu‐ mulative 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 re‐ sponse index increased linearly, whereas mean phagocytic activity index of neutro‐ phil 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 choles‐ terol level. However, a linear increment ( p = 0.04) in the blood serum HDL choles‐ terol level with a linear reduction ( p = 0.01) in the blood serum LDL cholesterol was noticed. KEYWORDS carcass, goblet cell, phagocytic activity index of neutrophil, threonine, Vencobb‐400, villus height