Blood methionine concentration in dairy cows supplemented with rumen-protected methionine or two methionine hydroxy analogs.

Bioavailability of methionine (Met) of three commercial supplements was assessed in three lactating dairy cows by a standardized blood plasma test. A spot dose of rumen-protected D,L-Met (SmartamineMTM or SM; 65.4 g/cow/day), Met hydroxy analog (Alimet® or AL; 57.5 g/cow/day) and its isopropyl ester (MetaSmart or MT; 88.5 g/cow/day) were fed daily (9:00h) by oral bolus, providing 50 g/cow/day of D,L-Met, to 3 cows in a Latin square design. Blood samples were obtained by jugular venipunctures the day before (day 0) and five days after (day 5) the treatment at 9:30h, 13:00h and 17:00h and plasma was harvested. On day 0, the Met concentration was similar among treatments. At day 5, the plasma Met was increased with a different response (P<0.05) among treatments: +285.8% for SM (89.67 at day 5 versus 23.79 μmol/L at day 0), +167.2% (55.62 versus 21.60 μmol/L, respectively) for MT and +50.9% for AL (35.07 versus 23.19 μmol/L, respectively).