An updated meta-analysis of influences of animal, dietary, and operational factors on the effects of supplemental methionine on lactational performances in dairy cows

= 39.1%-64.6%). Subgroup analyses revealed that responses of lactational performance to Met addition were affected by animal, dietary, and operational factors including parity, lactation stage, cow breed, basal diet type, and MP level, as well as type, dosage, and duration of supplementation. Regression analyses revealed that the associations between milk yield (r = 0.37), milk protein yield (r = 0.42), and milk fat yield (r = 0.33) with digestible Met intake were significant for MP-deficient diets, but not for the MP-adequate diets, and the optimal dosage and duration for rumen-protected Met addition were 13.2 to 16.4 g/d and 68 to 72 d, respectively. These findings provide practical guidelines for Met supplementation to improve the lactation performance in dairy cows.