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

Increasing metabolizable Met (mMet) through feeding supplemental rumen-protected Met to dairy cows is a common component of balancing rations for Met. Changes in plasma Met (pMet) brought about by this practice may affect the metabolism of other AA, resulting in changes in the concentration of other 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 infusion to increase pMet. There were 41 studies feeding 60 control and 78 Met treatments that met the selection criteria and entered into the final analysis. Responses entering the meta-analysis were calculated as Met-supplemented cow pAA - control cow pAA and analyzed as weighted mean differences or standardized weighted mean differences where the weighting term was the inverse variance and robust variance estimation was conducted to account for the hierarchical structure of the data. Regression was also conducted where pAA were regressed against pMet weighted by the inverse variance and including the random effect of study and experiment-within-study. Mean pMet in control cows was 19.9 µM, which increased with supplementation by 11.3 µM. Sulfur-containing pAA Cys and homocysteine as well as Lys were increased with increasing pMet whereas Gln, Glu, His, Ile, Leu, Tyr, and Val decreased with increasing pMet. The pAA Arg, Gly, Orn, Ser, Tau, and Trp responded curvilinearly to pMet with the predicted response in Arg, Orn, Tau, and Trp reaching a peak and Gly and Ser a nadir at intermediate concentrations of pMet. Milk protein yield increased in association with greater pMet, but responses to supplemental Met appeared to be limited by the reduction of other pAA, as higher responses to pMet were observed at higher levels of dietary CP. These results imply that concentration changes in pAA should be considered during diet formulation when feeding supplemental mMet to support increased milk protein production.