Bioavailability of multiple rumen-protected amino acids by a stable-isotope technique in dairy cattle

Multiple EAA drive milk protein production in dairy cows, including His, Ile, Leu, Lys, Met, and Thr. Therefore, determining bioavailability of rumen-protected AA (RPAA) is essential for optimal EAA supply. We aimed to evaluate bioavailability of lipid-encapsulated prototypes of His, Ile, Lys, Met, and Thr and an isopropyl ester of 2-hydroxy-4- (methylthio) butanoic acid (HMBi). Six Holstein heifers (518 ± 28 kg BW) were allocated in a 6 × 8 Youden square design, with 8 periods and 8 treatments. Treatments were (1) a base diet (BD, 1.72 Mcal of NE_L/ kg and 15.8% of CP); (2) BD + RP-Lys1 + Met1; (3) BD + RP-His2 + RP-Lys2 + RP-Met2; (4) BD + RP-His3 + RP-Lys3 + RP-Met3; (5) BD + RP-His4 + RP-Lys4 + RP-Met4 + RP-Thr4; (6) BD + RP-Ile5 + RP-Lys5 + RP-Met5; (7) and (8) extra treatments not presented here. In each 14-d period, heifers were fed 1×/d for 10 d and 12×/d at 95% for 4 d to establish a steady state. On d 14, a saline solution with stableisotope U-[¹³C]-labeled AA was infused into the jugular vein for 720 min, and blood samples were collected upstream. Isotopic ratio (IR) of plasma ¹³C labeled AA was evaluated using an IR mass spectrometer coupled to gas chromatography by a combustion oven. Amino acid entry rates model was used to derive fractional availability values (g AA appearing in plasma/g AA consumed) for each RPAA by regression (Estes et al., 2018, J. Dairy Sci. 101:7040). Rumen-undegraded protein (RUP) was predicted by rumen kinetic degradation (0 to 96 h) using a passage rate of 7.11%/h. We could quantify EAA bioavailability for all RPAA except Thr (Table 1). The RUP values do not always represent the bioavailability of AA in RPAA.

Key Words: amino acid (AA) absorption, rumen undegradable protein (RUP), availability