The methionine precursor 2-hydroxy-4-(methylthio) butanoic acid (HMB) is expected to be more extensively degraded in the rumen than its isopropyl ester (HMBi).

In this study four pulsed treatments were dosed every 8 h simultaneously with 3-times-daily feeding into continuous cultures. The treatments were:

1) control

2) —0.097% dl-methionine

3) 0.11% HMBi (HMBi)

4) 0.055% HMBi plus 0.048% Met (Met + HMBi)

Starting on d 9, for 6 consecutive doses, both [1-13C]-l-Met and [methyl-2H3]-l-Met replaced part of the unlabeled dl-Met, [13C5]-dl-HMBi replaced a portion of the unlabeled dl-HMBi, and [1-13C]-l-Met plus [13C5]-dl-HMBi replaced a portion of the respective unlabeled doses for the Met + HMBi treatment. After the sixth dose (d 11), unlabeled Met or HMBi provided 100% of the doses to follow elimination kinetics of the labels in HMBi, free Met, and bacterial Met compartments.

The free [1-13C]-l-Met recycled more and was recovered in bacterial Met to a lesser extent than was the free [methyl-2H3]-l-Met recycling and that was recovered in bacterial Met. Increasing HMBi inclusion (0, 50, and 100% substitution of the exogenously dosed Met on a molar equivalent basis) tended to increase HMBi escape from 54.7 to 71.3% for the 50 and 100% HMBi treatments, respectively.

Despite HMBi substituting for and decreasing the dosage of Met, increasing HMBi increased accumulation of free Met in fermenter fluid. The HMBi (after de-esterification of the isopropyl group) presumably produces Met through the intermediate α-ketomethylthyiobutyrate with an aminotransferase that also has high affinity for branched-chain AA.

This study provides evidence that the HMBi-derived Met is likely released from bacterial cells and accumulates rather than being degraded, potentially as a result of lagging d-stereoisomer metabolism. More research is needed to evaluate racemization and metabolism of stereoisomers of HMBi, Met, and other AA in ruminal microbes.


Link to JDS abstract

Author: J.L. Firkins, Department of Animal Sciences, The Ohio State University,