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1373
Optimal blood sampling time points to determine bioavailability of rumen-protected Met products using the plasma free AA dose-response method
Determination of bioavailability of rumen-protected AA products using the plasma free AA dose-response method has relied on blood sampling 2, 4, 6 and 8 h after the morning feeding the last 3 d of each period in Latin square experiments with cows fed every 8 h. The objective of this study was to determine if this sampling protocol captured the diurnal variation in plasma Met concentrations that exists and adequately measures the bioavailability of Met in Smartamine M (SM; Adisseo Inc., Alpharetta, GA). Five multiparous lactating Holstein cows were used in a 5 × 5 Latin square design with 7-d periods. Treatments were: 1) control diet with no supplemental Met; 2) 12 g/d of abomasally-infused Met; 3) 24 g/d of abomasally-infused Met; 4) 15 g/d of fed Met from SM; and 5) 30 g/d of fed Met from SM. Blood samples were collected via jugular catheters every 2 h starting at 0700 h on d 5, 6, and 7 of each period. Plasma Met analysis was conducted using gas chromatography after chloroformate derivatization (EZ:faast, Phenomenex). Data were analyzed using the MIXED procedure of SAS. Plasma Met concentrations (µM) increased with infused Met or supplemental SM (P < 0.001). There was no diurnal variation in plasma Met concentrations (P = 0.18). Plasma Met concentrations were averaged across days for the 2-8, 10-16, 18-24, and 2-24 h blood sampling periods. Plasma Met concentrations were regressed on 0, 12, and 24 g of infused Met and 0, 15, and 30 g of fed Met using the REG procedure of SAS. Slopes for the 2-8, 10-16, 18-24, and 2-24 h sampling periods for infused Met were 1.356 (SE = 0.145), 1.369 (SE = 0.147), 1.329 (SE = 0.120), and 1.346 (SE = 0.123), respectively. Slopes for the 2-8, 10-16, 18-24, and 2-24 h sampling periods for fed Met were 1.148 (SE = 0.038), 1.156 (SE = 0.059), 1.197 (SE = 0.038), and 1.140 (SE = 0.030), respectively. There was no effect of sampling period on the slopes for infused (P ≥ 0.91) or fed Met (P≥ 0.26). The bioavailabilities of Met in SM averaged 84.7, 84.4, 90.0, and 84.6% for the 2-8, 10-16, 18-24, and 2-24 h sampling periods, respectively. The similarity in estimates of bioavailability for SM for the 2-8 and 2-24 h sampling periods indicates our blood sampling protocol is adequate for determining the bioavailability of RP-Met products.
Keywords: Bioavailability, methionine, sampling