Meta-analysis of supplemental methionine effects on plasma amino acid concentrations in dairy cows

Authors: Zanton, Geoffrey

Submitted to: Journal of Dairy Science Communications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/25/2025
Publication Date: 4/1/2026
Citation: Zanton, G.I. 2026. Meta-analysis of supplemental methionine effects on plasma amino acid concentrations in dairy cows. Journal of Dairy Science Communications. https://doi.org/10.3168/jdsc.2025-0804.
DOI: https://doi.org/10.3168/jdsc.2025-0804

Interpretive Summary: Supplementing rumen protected methionine (Met) is a common practice on dairy farms. The hypothesis of this study was that cows provided supplemental Met would have altered plasma amino acid (AA) concentrations indicative of AA metabolism. This question was addressed through a meta-analysis of peer-reviewed studies that supplemented lactating cows with rumen protected Met and evaluated plasma AA concentrations. Increased plasma Met resulted in reduced plasma concentrations of several AA required for milk protein synthesis and greater reductions in AA involved in methyl-donor reactions. Sulfur-containing AA increased as plasma Met increased. Supplementing Met resulted in increased milk protein secretion which was comparable in magnitude to the reduction of the concentration of several plasma AA. Milk protein production responses to supplemental Met were limited by the reduction of other plasma AA and greater performance response to Met could be achieved by considering this response during diet formulation.

Technical Abstract: Increasing metabolizable Met (mMet) by feeding supplemental rumen protected Met to dairy cows is a common component of balancing rations for AA, but this practice may affect the metabolism of other AA, including the concentration of plasma AA (pAA). The objective of this meta-analysis was to evaluate the changes in pAA when lactating dairy cows were provided supplemental mMet. Literature studies were identified that fed cows a control diet and the control diet supplemented with Met as either rumen protected Met or through post-ruminal infusion. There were 41 publications feeding 60 control and 78 Met treatments that met the selection criteria. Responses were calculated as Met supplemented pAA – control pAA and analyzed as weighted medians where the weighting term was the square root of n. Regression was also conducted where pAA was regressed against pMet weighted by the square root of n and including the random effects of study. Median pMet in control cows was 20.9 uM which increased with supplementation by 12 uM, but the responses to supplementation ranged from -1.34 to 62.3 uM. Relative pAA concentration changes for proteinaceous AA (aside from pMet) ranged from -5.7% (Ser) to +5.8% (Cys) and were generally not different than changes in milk true protein secretion (+1.8%) caused by increasing supplemental mMet. Regression analysis revealed 13 AA associated with changes in pMet. Sulfur-containing AA Cys, Hcy, and Tau were increased and closely related with increasing pMet (R-squared = 0.258 - 0.676) whereas Gly, His, Ile, Leu, Phe, Tyr, and Val decreased with increasing pMet, but did so less consistently (0.111 <= R-squared <= 0. 331). The pAA Arg, Lys, and Ser responded curvilinearly to pMet with the predicted response in pArg and pLys reaching a peak and pSer a nadir at intermediate concentrations of pMet. Concentration declines in pAA caused by feeding supplemental mMet varied depending on the AA and should be considered during diet formulation to support increased milk protein production.