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

## TL;DR

Adding methionine to dairy cow diets changes plasma amino acid levels, which affects milk protein production and should be considered in feed formulation.

## Contribution

This study provides a meta-analysis showing how supplemental methionine alters plasma amino acid concentrations and impacts milk protein synthesis in dairy cows.

## Key findings

- Increased plasma methionine reduced concentrations of several amino acids needed for milk protein synthesis.
- Sulfur-containing amino acids like cysteine and homocysteine increased with higher plasma methionine.
- Milk protein yield increased with higher methionine, but was limited by reductions in other amino acids.

## Abstract

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 in response to altered plasma Met. 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. PPi = diphosphate; Pi = inorganic phosphate; PtdEtn = phosphatidylethanolamine; PtdChol = phosphatidylcholine; THF = tetrahydrofolate; MTHF = methylenetetrahydrofolate.

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 in response to altered plasma Met. 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. PPi = diphosphate; Pi = inorganic phosphate; PtdEtn = phosphatidylethanolamine; PtdChol = phosphatidylcholine; THF = tetrahydrofolate; MTHF = methylenetetrahydrofolate.

•Increased plasma Met (pMet) resulted in changes in several plasma AA (pAA).•Plasma concentrations of several essential AA and nonessential AA were inversely related to pMet.•Sulfur AA in plasma were increased with increasing pMet.•Changes in pAA should be considered when increasing metabolizable Met.

Increased plasma Met (pMet) resulted in changes in several plasma AA (pAA).

Plasma concentrations of several essential AA and nonessential AA were inversely related to pMet.

Sulfur AA in plasma were increased with increasing pMet.

Changes in pAA should be considered when increasing metabolizable Met.

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.