Background: In the broiler’s diets based on corn soya bean meal, methionine (Met) and cystine (Cys), known as Sulphur amino acids (SAAs), are the first limiting indispensable amino acids because of their limited presence, which are supplemented with different synthetic sources. Evaluation biological effectiveness of these sources can be important in their correct replacement, especially in the starter and growth diets. Objectives: The current study was done to assess the relative biological efficacy (RBE) of liquid methionine hydroxy analogue free acid (MHA-FA) in compared with DL-methionine (DL-Met) based on broiler performance traits at different levels of digestible SAA in the 1-11 (starter) and 11-25 (grower) days of age periods. Methods: Two experiments were developed with treatments consisted of a basal diet without Met addition, that met the nutrient and energy requirements of broilers with the exception of SAAs (Met + Cys), and 5 increasing Met doses for both sources (DL-Met and or MHA-FA) resulting in digestible SAA concentration from 0.62 to 1.02% of diet in the starter period (Trial 1), and 0.59 to 0.94% of diet in the grower period (Trial 2). The multi-linear regression model and slope ratio method were employed to calculate the RBE of MHA-FA compared with DL-Met for measured variables. Results: In both experiments, the results obtained during the starter and grower periods with the different methionine supplementations show significant growth responses to digestible SAAs levels. By increasing dietary DL-Met and or MHA-FA levels, the growth performance traits, and immune responses were improved (quadratic; P <0.05). The RBE of MHA-FA compared to DL-Met on an equimolar basis were estimated 66-89% (59-79% on a weight-to-weight basis). Conclusions: It is concluded, the RBE of MHA-FA in comparison with DL-Met depended on broiler chicken age and what attribute is being evaluated. 10.1002/vms3.1460

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