Background: Broiler diets is commonly supplemented by liquid Methionine Hydroxy Analogue Free Acid (MHA-FA) containing as product with 88% of active substance to meet sulphur amino acids (SAAs) requirements. Objectives: Two experiments were conducted to assess the effects of different dietary MHA-FA levels on growth performance, blood metabolites, and immune responses in broilers. Methods: In experiments 1 and 2, 432 and 360 male broilers were fed either a basal corn-soybean meal diet, met the energy and nutrient requirements with exception of SAAs, or the basal diet supplemented with five graded levels of MHA-FA (+0.9 and +0.8 g/kg), for 1-11d and 11-24d ages, respectively. Chicks were vaccinated with inactivated H9N2/ND and live-ND on days 5 and 11, respectively. Results: Responses in both experiments had similar tendency at corresponding dietary MHA-FA levels. By increasing dietary MHA-FA level; weight gain (WG), feed efficiency (FE), relative breast weight (RBW), and immunity against Newcastle disease and avian influenza virus inoculation improved with quadratic trend. Blood serum triglyceride, low-density lipoprotein, alanine aminotransferase and alkaline phosphatase concentration were affected in response to dietary MHA-FA levels with quadratic trends. By using broken‐line regression analysis, the optimum dietary MHA-FA levels for optimized WG, FE, and RBW during 1-10d age were obtained 2.20, 3.31, and 3.33 g/kg, that based on MHA-FA content of Met equivalent, the digestible SAAs requirements estimated 0.81, 0.91, and 0.92%, respectively. Similarly, for the 11-24d age period, the optimum dietary MHA-FA supplementation levels were obtained 1.79, 2.21, 2.41 and 2.53 g/kg, which the digestible SAAs requirements estimated 0.75, 0.79, 0.80 and 0.81% for optimized WG, FE, RBW, and immune responses, respectively. Conclusions: Supplemental levels of MHA-FA that optimize performance, breast meat and immunity in broilers fed corn-soybean meal diet, varied from 0.18 to 0.33% correspond to age and what traits is taken considered for optimistion.

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