Abstract
Rumen-protected AA are largely used to supply the MP in dairy diets. Rumen-protected methionine (RP-Met) supplements differ in their proportion of the biologically active L-form isomer and in their bioavailability. Based on the plasma free AA dose-response approach, we determined the bioavailability of L-Met60 (LipoAktiv L-methionine 60), a rumen-protected L-Met supplement, and evaluated its effects on plasma AA profile. Ten multiparous Holstein cows in mid lactation were used in a replicated 5 × 5 Latin square design with 7-d experimental periods. All treatments used L-Met60, with doses and routes of administration as follows: 0 (control), 18 (18IL), or 36 (36IL) g/d supplied by continuous abomasal infusion, and 18 (18FL) or 36 (36FL) g/d as a fed supplement mixed in the TMR. Abomasal infusion was delivered continuously and uniformly into the abomasum via rumen cannula using a peristaltic pump, whereas fed L-Met60 was consumed before each meal. Milk and blood samples were collected during the last 3 d of the covariate and the experimental periods. No treatment effects were observed for animal performance, except for a discrete increase of 0.06 percentage units in milk protein for 36IL. Infused and fed L-Met60 supplementation did not affect plasma metabolites, but altered plasma Met, cystine, cystathionine and allocystathionine, glutamine, taurine, and total sulfur AA (TSAA). These changes, especially for taurine and glutamine, might be explained by a higher activation of the transsulfuration pathway. Plasma TSAA increased linearly with both routes of administration; however, the regression analysis showed steeper slope for abomasal infusion than for the fed supplementation. Based on this response, the relative bioavailability of L-Met60 was calculated as 43.4 ± 2.5% (TSAA, % of [total AA - TSAA]).