Methionine availability in plasma of dairy cows supplemented with methionine hydroxy analog isopropyl ester.

Adequate Met supply is especially important in the dairy cow for milk protein synthesis. Because of insufficient Met contents in the most frequently used feed-stuffs, Met becomes limiting in the diet of the dairy cow. To restore the amino acid balance of the diet and consequently to optimize lactation performance, Met must be supplied in a protected form because of its high degradability as a free amino acid by rumen microorganisms. A new chemical derivative of Met, the isopropyl ester of the 2-hydroxy-4-(methylthio)-butanoic acid (HMBi) was tested for its metabolic fate by following the evolution of plasma concentrations of its metabolites (2-hydroxy-4-(methylthio)-butanoic acid (HMB), Met, isopropyl alcohol, and acetone) after spot-dose supplementation (50 g Met equivalent) to 15 cows. Results indicated that HMBi would be quickly absorbed and hydrolyzed into HMB and isopropyl alcohol, and then converted to Met and acetone, respectively. In our experimental conditions, the Met availability for cows was estimated to be 48.34 +/- 2.05% using a calibration curve established by modeling the area under the curve response to increasing doses of Met supplied as Smartamine M, whose bioavailability (80%) is considered the reference value. Plasma kinetics and bioavailability of Met were compared between HMBi and Smartamine M in the same cows. Comparison of the kinetics suggests that HMBi would be absorbed through the rumen wall providing good protection against rumen microorganisms. It can thus be concluded that HMBi is a new source of Met for ruminants with an acceptable bioavailability.