The objective of this study was to determine and compare relative bioavailability (BA) of commercial RP-Met products using the plasma free AA dose-response (PDR) and selenomethionine (Se-Met) methods.A total of 36 lactating cows were used in the experiment and started with a 14-d background phase during which all cows were fed a basal diet (metabolizable Met at 83% of the requirement) containing a Se-yeast product at 0.15% of dietary DM.After the background phase, cows entered the experimental phase in a randomized complete block design for 14 d.Treatment diets were the background diet supplemented with 1 of 4 commercial RP-Met products (RP-Met A, B, C, and D).The inclusion of RP-Met products in the diet was based on providing 20 g/d of Met from each RP-Met product.Blood and milk samples were collected in each phase to determine plasma Met concentration and milk N and Se concentrations.The BA of RP-Met A, B, and C were determined relative to RP-Met D with an assumed BA of 80%.The changes in the proportion of plasma Met in total AA and milk Se/N were key variables to determine the BA of RP-Met A, B, and C relative to RP-Met D for individual cows using the PDR and Se-Met methods, respectively.The DMI and production of milk and milk components did not differ among the RP-Met products.Plasma concentration of Met was greater and milk Se/N was lower for cows during the experimental phase compared with the background phase, indicating additional supply of metabolizable Met from RP-Met products.During the experimental phase, plasma Met as a proportion of total AA was affected (1.35, 1.19, 1.54, and 1.48% for RP-Met A, B, C, and D, respectively) by the RP-Met products.As a result, the BA of the RP-Met A, B, and C relative to RP-Met D using the PDR method differed and were 55.6, 30.9, and 80.5%, respectively.However, milk Se/N did not differ among the RP-Met products, and the BA of RP-Met A, B, and C relative to RP-Met D using the Se-Met method were not different (60.8, 48.6, and 77.2%, respectively).From the comparison analysis between the PDR and Se-Met methods, BA of RP-Met were different but did not differ between the methods.No interaction between RP-Met and Method was observed.The results indicate that the ranking of RP-Met for BA was agreed between PDR and Se-Met (RP-Met B < A < C), but a clear difference in detection sensitivity was observed where PDR was more sensitive to detect the difference in BA among the RP-Met products.The difference in detection sensitivity occurred because the error associated with the Se-Met method was twice as large as that associated with the PDR method.In conclusion, both PDR and Se-Met methods were appropriate to determine the ranking of the RP-Met products.However, the absolute values of relative BA obtained from the PDR method were likely more reliable due to smaller errors associated with the method in the current experimental condition.
Kim et al. (Fri,) studied this question.