Determination of the comparative bioavailability of lysine in two rumen-protected lysine products using the in vivo plasma lysine response method

Abstract Text: The objective of this study was to use the commercially available rumen-protected lysine (RPL) AjiPro^®-L (AJI; Ajinomoto Heartland, Inc.) to estimate relative bioavailability of a second generation RPL product (A2G; Ajinomoto Heartland, Inc.). Ten multiparous lactating Holstein cows (109 ± 8 days in milk (DIM)) housed in a tie-stall facility were used in a replicated 5 x 5 Latin square design with 7-d periods. Cows, blocked by DIM and milk production, were assigned to treatment sequence. A common basal diet formulated to meet lysine (Lys) requirement, prepared once daily, was fed proportionately at three time points (33.4% at 0500 h, 33.3% at 1300 h, and 33.3% at 2100 h). Treatments included 0 g/d Lys, 75 g/d AJI, 75 g/d A2G, 150 g/d AJI, or 150 g/d A2G and were administered 3x/d 1 h prior to each feeding time on d 2 through 7 of each period in amounts proportional to feed offered to simulate inclusion in the diet. Blood samples were obtained from each cow on d 6 and 7 of each period from the tail vein at 2-h intervals starting at 0600 h resulting in 4 samples/cow/d. Resultant plasma was pooled by day and analyzed for amino acid (AA) concentrations. Data were reduced to a period mean and analyzed using the MIXED procedure (SAS, v. 9.2). The REG procedure was used to generate linear regression models for each RPL product using Lys (µmol) and Lys (% total AA (µmol basis)) to determine the slope of plasma Lys in response to treatment. Using the calculated slope for each product, relative estimated bioavailability of A2G was determined using the slope-ratio assay technique. Dry matter intake and milk yield did not differ (P > 0.10) among treatments. Plasma Lys was greater (P < 0.05) for 150 g/d AJI (93.8 ± 2.9 µmol) and 150 g/d A2G (95.0 ± 2.8 µmol) when compared to 0 g/d Lys (83.6 ± 2.9 µmol). The slope for A2G treatment was numerically greater (0.007; r² = 0.91) when compared to the slope for AJI treatment (0.005; r²= 0.99) when expressing the concentration of plasma Lys relative to that of total AA. This resulted in the calculated bioavailability of A2G being 132.1% of the bioavailability of AJI. Both first and second generation AjiPro-L products increased plasma LYS in lactating dairy cows with some comparative advantage for the second generation product.

Keywords: bioavailability, rumen-protected lysine, dairy cow