| Summary: | The objective of this study was to evaluate the relative bioavailability of two 2-hydroxy-4-(methylthio)butanoic isopropyl esters (HMBi) obtained through different production processes and an encapsulated rumen-protected Met using the area under the curve (AUC) method. The new HMBi product (Kessent MF Liquid, Kemin Animal Nutrition and Health) was compared with an existing HMBi product (Metasmart, Adisseo SAS) and a pH-sensitive coated Met (Smartamine, Adisseo SAS). Nine multiparous lactating cows (30 kg of milk/d and 227 d in milk) fed a 45:55 forage:concentrate diet were randomly assigned within square to a triplicate 3 × 3 Latin square design. Each period consisted of a 3-d sampling period and a 3-d washout period. Treatments were dosed on d 1 of each period, and blood samples were collected from the coccygeal vein at 0, 1, 2, 3, 4, 6, 9, 12, 24, 30, and 48 h thereafter. The daily dose was 50 g of Met equivalent of each treatment. The HMBi treatments were administered directly into the cow's mouth, whereas Smartamine was fed mixed with 0.5 kg of concentrate and fully consumed within 15 min. Nonlinear models were fitted to raw data, and the basal concentration at time 0 h, time at peak (Tmax), concentration at peak, and AUC of plasma Met were determined. The Met basal concentration at t = 0 h (26.7 ± 7.67 µM) and concentration at peak (210 ± 22.2 µM) were similar among treatments, but the Tmax (11.3 vs. 1.4 h) was delayed and the AUC was 1.8-fold larger (3,457 vs. 1,868 arbitrary units) in Smartamine compared with HMBi. Results of this study indicate that the 2 HMBi products have similar plasma kinetics and bioavailability. Smartamine had different kinetics compared with HMBi products, with delayed Tmax and larger AUC and relative bioavailability.
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