Effects of Rumen-Protected Lysine and Methionine Supplementation on Lactation Performance in Holstein Dairy Cows: A Meta-Analysis

Lysine and methionine are commonly regarded as the first limiting amino acids for lactating dairy cows, yet production responses to rumen-protected lysine (RPLys), rumen-protected methionine (RPMet), and methionine analogues vary across studies due to differences in basal diets, lactation stage, product formulation, and amino acid bioavailability. This meta-analysis synthesized evidence from 42 studies conducted in Holstein dairy cows to evaluate the effects of RPLys, RPMet, their combined supplementation, and methionine analogues on milk yield, milk fat, milk protein, milk lactose, and feed conversion-related traits. Random-effects meta-analysis showed that RPLys primarily increased milk yield, RPMet increased milk yield and milk fat percentage, combined supplementation improved milk yield, and methionine analogues increased milk yield and selected milk components. Exploratory analyses of supplementation level and Lys:Met ratio were considered hypothesis-generating only due to study-level aggregation and variation in product bioavailability. Overall, these findings support precision amino acid supplementation rather than universal strategies, and highlight the need for standardized reporting of basal diet amino acid supply and product-specific bioavailability to optimize lactation performance.