Targeting selenometabolites in the saliva of dairy cows: size-exclusion chromatography – inductively coupled plasma mass spectrometry-based characterization

Selenium supply is an important input for milking cows, and the monitoring of selenium uptake has been carried out through total selenium determination in the blood serum samples or the concentration of selenoprotein P. While the optimum concentrations of these two parameters have already been established, the specific selenium requirement of cows' microbiomes has not been addressed. The oral cavity of cows contains basically the first, front-end microbiome of the animal. However, no information is available on its selenium-dependent/ independent strains, its distribution, and ratio. In any case, saliva would be the only readily accessible source of selenium for the microbiome of the oral cavity. Therefore, our study focused on the total selenium and selenium speciation analyses of saliva of Holstein cows, together with the determination of total selenium in whole blood and serum samples. A two-step feeding experiment was conducted with supplementing the selenite-containing daily ration with the hydroxy-analogue of selenomethionine (R,S-2-hydroxy-4-methylselenobutanoic acid, HMSeBA) at the highest authorized rate in the EU. Size-exclusion chromatography - inductively coupled plasma mass spectrometry was used for the speciation of saliva samples that revealed the presence of low molecular weight (LMW) selenometabolites in the 1.78-15.3 µg Se kg^-1 range. The supplementation of HMSeBA did not significantly increase the blood selenium concentrations, and no correlation was found between the LMW selenium concentration and blood selenium concentrations. While the identification of LMW selenometabolites with electrospray ionization high-resolution mass spectrometry could not be achieved, a dedicated sample preparation protocol has been suggested for future experiments.