PSV-11 Determining Relative Bioavailability of Commercially Available Rumen Protected - Methionine Products in Lactating Dairy Cows
Authors: Clark Kirsten , Porter Noah , Rebelo Lucas , Guyader Jessie , Lee Chanhee 
Published: October 2023

Bioavailability (BA) of rumen protected Methionine (RP-Met) can be estimated by supplementing Se-yeast, in which selenomethionine from Se-yeast acts as a tracer for metabolizable methionine (mMet). The advantage of the method is that experimental and sampling procedures are easy and non-invasive. However, the method provides relative BA of RP-Met rather than absolute BA because the absorption rate of the tracer (selenomethionine) is unknown. This method was used in an experiment to estimate relative BA of commercially available RP-Met products in lactating cows. Thirty cows were used in a randomized block design (10 blocks, 3 cows each), consisting of three periods: 10-day basal period, 14-day backgrounding period, and 10-day experimental period. During the basal period, all cows received a basal diet (49:51 forage:concentrate, 16.9% crude protein, 30.7% neutral detergent fiber, 27.1% starch). During the backgrounding period, all cows received the basal diet with Se-yeast supplementation. During the experimental period, cows received the backgrounding diet supplemented with RP-Met. Cows in each block were randomly assigned to one of the three RP-Met products: MetaSmart (Adisseo), Smartamine M (Adisseo), and Mepron (Evonik). Milk Se and N concentrations were measured for the last 4 milkings in each period to determine specific activity (SA, milk Se:N) for individual cows. The backgrounding and experimental specific activities were corrected for SA from the basal period (i.e., natural SA before feeding Se-yeast), and the corrected SA was used to determine the increases in dMet flow. Relative BA between RP-Met products was calculated by the rate of increases in dMet. Production data was analyzed using the PROC MIXED of SAS (random effect: block; fixed effect: RP-Met treatments) where the backgrounding period was used as a covariate. All BA data were analyzed using the same model; however, the covariate was excluded. While dry matter intake (DMI) and milk yield were not affected, feed efficiency increased with supplementation of Mepron compared with MetaSmart and Smartamine M (1.56 vs. 1.50 kg/d; P = 0.049). This should be evaluated with caution due to the short duration of the experiment. Milk Se concentration was 0.021 mg/L during the basal period. Feeding Se-yeast increased milk Se concentration up to 0.063 mg/L during the backgrounding period. Supplementing with RP-Met diluted Se concentration in milk which decreased to 0.056 mg/L on average. While not statistically different, BA of Mepron and Metasmart was 74% and 65%, respectively, when BA of Smartamine M was considered 80%. However, due to the greater Met content, the concentration of mMet was the greatest for Mepron (629 g mMet/kg) followed by Smartamine M (600 g mMet/kg) and MetaSmart (286 g mMet/kg). To improve the Se-yeast method, further analyses are in progress to measure absorption of Se from Se-yeast to calculate absolute bioavailability rather than relative bioavailability.