Evaluating the Plasma Free Amino Acid Dose-Response Method to Determine the Content of Metabolizable Methionine in a Rumen-Protected Methionine Supplement
The plasma free AA dose-response approach has been proposed as the standardized method for evaluating rumen-protected Lys supplements. The method has the advantage of providing animal-derived estimates of efficacy under conditions of commercial use. However, before using the approach for evaluating rumen-protected Met (RP-Met) supplements, it is necessary to confirm that a positive linear relationship exists between increasing amounts of absorbed Met and plasma Met concentrations. The primary objective of this experiment was to confirm linearity in plasma Met response with up to 24 g/d of supplemental MP-Met by abomasal infusion or feeding a RP-Met product. A secondary objective was to determine if technique precision could be improved by including the other plasma sulfur AA (cystine, cystothionine + allocystothionine, homocystine and taurine) with Met (total sulfur AA) as an indicator of Met absorption. Five rumen-cannulated lactating Holstein cows (90-155 DIM), fed a Met-deficient diet, were assigned to a 5 x 5 Latin square with 7-d experimental periods. Treatments (per 25.0 kg/d of DMI) were 0 g/d Met (negative control), 12 and 24 g/d abomasally infused Met, and 12 and 24 g/d of assumed MP-Met from a RP-Met supplement. Blood samples were taken from the tail vein every 2 h, 4 times daily, the last 3 d of each period, centrifuged, deproteinized, and composited into 1 daily sample/cow. Data for plasma AA concentrations were analyzed using the PROC MIXED and PROC REG procedures of SAS 9.2. The basal diet was confirmed to be Met-deficient by observed increases in milk protein concentration (+0.10 and 0.12 percentage units for infused and fed Met, respectively; P < 0.05) with the first level of both infused and fed Met. All plasma sulfur AA responded in a significant linear fashion to both infused and fed Met (P < 0.05). Estimates of the MP-Met content of the RP-Met supplement were the same using either plasma Met or plasma total sulfur AA. The plasma free AA dose-response method is applicable for determining the MP-Met content of RP-Met supplements.
Keywords: Rumen-Protected, Methionine, Evaluation