Comparison of plasma methionine response to 3 rumen-protected methionine products in lactating Holstein dairy cows

The primary aim of this work was to compare the plasma Met response to supplementation with 3 rumen-protected Met products. Secondary aims were to evaluate how time of sampling affected results and how pooling of samples may have affected results. Ten multiparous Holstein cows (280 ± 73 DIM) were used in a replicated 3 × 3 Latin square design with 7-d experimental periods. Cows were fed every 8 h, and treatments consisted of supplementation of 12 g/d (4 g/feeding) of 1 of 3 rumen-protected Met (RPM) products. The products evaluated were the newly developed RPM-K and 2 existing products, RPM-S and RPM-M, with known differences in bioavailability. During d 5 to 7 of each period, blood samples were collected at 2, 4, 6, and 8 h after the morning feeding for plasma free AA analysis. Plasma Met data were analyzed using the full data set as well as mean values from individual cows for each day or each period. Plasma Met was not different between RPM-S and RPM-K (32.7 vs. 33.0 μM, respectively; P = 0.79), and both were greater than RPM-M (30.1 μM; P ≤ 0.001). Plasma Met was affected by time of sampling (P = 0.001), due to reduced plasma Met at 4 h (30.2 μM) than at 2, 6, and 8 h (31.9–33.0 μM). Using the daily and period mean values of plasma Met, differences observed in the full model were maintained when daily means were evaluated, but period means resulted in only a tendency for a treatment effect. Bioavailability of RPM-K was similar to RPM-S and greater than RPM-M. Pooling samples by day within cow would have likely yielded similar results.