Evaluation of the effects of microencapsulated DL-methionine on productive performance, immunity, plasma amino acids, and hepatic growth-related gene expression in broilers

A 42-day feeding trial was conducted to compare a diet supplemented with crystalline DL-methionine ( control ) with diets containing microencapsulated DL-methionine ( MEM ) supplied at 60, 70, 80, 90, or 100% of the commercially recommended level in broiler chickens. A total of 1,260 one-day-old Arian broilers were randomly allotted to six treatments, with seven replicates of 30 birds each. Body weight gain ( BWG ), feed intake, and feed conversion ratio ( FCR ) did not differ between birds fed the control diet and those fed MEM diets containing ≥70% of the recommended level, whereas the 60% MEM diet reduced BWG and increased FCR. Increasing the dietary MEM level resulted in a quadratic increase in leg yield and a linear decrease in abdominal fat, without affecting hematological traits or leukocyte differential counts. The 100% MEM diet increased influenza (H9N2) antibody titers, and all MEM levels enhanced Newcastle disease virus titers compared with the 60% MEM diet; moreover, titers in the 100% MEM group also exceeded those of the control. Plasma methionine, serine, and total amino acid concentrations were higher in birds fed the 60% MEM diet than in those fed the 90% MEM diet, while plasma cysteine showed a quadratic response as MEM level increased. Diets providing 80–100% MEM upregulated hepatic GHR expression, and diets providing 90–100% MEM also increased mTOR expression compared with the 60% MEM diet. Overall, microencapsulated DL-methionine enabled approximately a 30% reduction in supplemental methionine without impairing performance, while higher inclusion levels (≥80% of the recommended level) beneficially modulated immune responses and growth-related gene expression.