This study was conducted to determine the effects of grower diet (d11-24) digestible sulphur amino acids (SAAs) levels on performance traits, blood metabolites, and immune responses to Newcastle disease (ND) and avian influenza (AI) vaccination in the broiler chickens. Additionally, digestible SAAs requirements were determined based on nutrient dose-response data. A total of 360, 11‐day‐old male Ross-308 broiler chicks were randomly allocated to six dietary treatments, graded levels of supplementary DL-methionine (+0.7g/kg), but cystine (0.29%) was equal across groups, lead to levels of 0.59, 0.66, 0.73, 0.8, 0.87, and 0.94% dietary digestible SAAs, with six replicates and ten birds each. All birds were vaccinated with inactivated H9N2/ND and live-ND on days 5 and 11, respectively. Increasing dietary digestible SAAs level improved live body weight (LBW), weight gain (WG), feed efficiency (FE) with linear trends, and improved carcass yield, relative breast weight (RBW), and immune responses with quadratic trends. On day 24, a quadratic increase in blood serum albumin concentration was concomitant with decreasing triglyceride, whole cholesterol, low‐density-lipoprotein cholesterol, creatinine, and alkaline phosphatase concentration with increasing dietary digestible SAAs levels. Based on broken‐line regression models, the LBW, WG, FE, RBW, and immunity against ND and AI vaccination were optimised when dietary digestible SAAs levels were 0.73, 0.69, 0.78, 0.86, 0.73, and 0.72%, respectively. It is concluded, the digestible SAAs requirements of broilers vary according to what production parameter is taken considered for optimisation. The best grower diet digestible SAAs concentration achieved 0.86% for support performance, breast yield, and immune function.

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