Effect of Dietary Rumen-Protected Methionine and Lysine on Performance and Plasma Amino Acid of Dairy Cows: A Meta-Analysis

Low-protein diets supplemented with rumen-protected methionine and lysine (RPML) can optimize amino acid supply while maintaining milk performance and reducing environmental impacts. But responses of cows to RPML are inconsistent and modulating factors remain poorly understood. This meta-analysis tested whether RPML supplementation improves lactational performance and alters plasma amino acid profiles of dairy cows. Forty-two eligible studies published up to 1 June 2026 were retrieved from PubMed, Web of Science and ScienceDirect. Meta-analysis, subgroup analysis, correlation analysis and dose–response regression were adopted. Results showed that RPML supplementation significantly increased milk yield by 0.500 kg/d, energy-corrected milk by 1.378 kg/d, protein yield by 0.038 kg/d, protein concentration by 0.052%, fat yield by 0.030 kg/d, fat concentration by 0.043%, 3.5% fat-corrected milk by 0.778 kg/d, milk total nitrogen by 0.010%, and casein nitrogen by 0.007%. Plasma arginine, asparagine, isoleucine, leucine, lysine and methionine increased, while glycine declined. Subgroup analysis showed that cow breed, parity, lactation stage, basal diet and dietary crude protein were key moderators. Digestible methionine dosage correlated with milk fat concentration and plasma leucine, and digestible lysine dosage correlated with milk protein yield. Significant regression models confirmed the combined effects of methionine and lysine on multiple milk traits. These results indicate that supplementation with RPML represents an effective nutritional strategy for enhancing milk yield and milk quality in dairy cows. Such findings may assist producers and nutritionists in refining amino acid feeding programs to optimize dairy cow productive performance.