Assessment of blood sampling time points to determine the relative bioavailability of ruminally protected methionine supplements using the plasma free amino acid dose-response technique

• Published in: JDS communications 2024-11, Vol.5 (6), p.539-542
• Main Authors: Whitehouse, Nancy L., Chirgwin, Devan L., Schwab, Charles G., Luchini, Daniel, Lobos, Nelson, Brito, André F.
• Format: Article
• Language: English
• Subjects: Animal Nutrition and Farm Systems
• ISSN: 2666-9102; 2666-9102
• DOI: 10.3168/jdsc.2023-0508

[Display omitted] •Plasma Met concentration reached steady-state conditions by day 4.•Plasma Met concentrations within an 8-hour sampling period did not differ from each other.•The bioavailabilities for Smartamine M from the different blood sampling times were not different from each other. The calculation of the relative bioavailability (RBV) of rumen-protected AA supplements using the plasma free AA dose-response technique currently relies on blood samples obtained 2, 4, 6, and 8 h after the 0500 h feeding during the last 3 d of each period in Latin square experiments with cows fed every 8 h (0500, 1300, and 2100 h). The objective of this study was to determine if this current blood sampling protocol captures the changes that may occur in plasma Met concentrations within a 24-h day to adequately determine the RBV of Met from Smartamine M (SM). Five multiparous lactating Holstein cows were used in a 5 × 5 Latin square design with 7-d periods. Treatments were (1) control (abomasal infusion of tap water), (2) 12 g/d of abomasally infused dl-Met, (3) 24 g/d of abomasally infused dl-Met, (4) 15 g/d of fed Met (20 g/d of SM), and (5) 30 g/d of fed Met (40 g/d of SM). Blood samples were collected via jugular catheters every 2 h after the 0500 h feeding starting on d 5 and ending on d 7 of each period. Plasma Met analysis was conducted using gas chromatography after chloroformate derivatization. Plasma Met concentration was averaged across days for 2–8 h after the 0500 h feeding, 2–8 h after the 1300 h feeding, 2–8 h after the 2100 h feeding, and 2–24 h after the 0500 h feeding. In addition, plasma Met concentration was regressed on 0, 12, and 24 g of infused dl-Met and 0, 15, and 30 g of fed Met. The calculated RBV of Met from SM averaged 83.8%, 83.6%, 87.4%, and 83.0% for the 2–8 h, 10–16 h, 18–24 h, and 2–24 h sampling periods, respectively. The similarity in the estimations of RBV for the 2–8 h and 2–24 h sampling periods indicates that our original blood sampling protocol seems reliable for determining the RBV of ruminally protected Met products.