Applications of Mass Spectrometry for the Determination of Microbial Crude Protein Synthesis in Ruminants

The importance of quantifying ruminal microbial crude protein synthesis has promoted the development and comparison of several different methods for precise determination of both the amount and rate of synthesis. One major challenge is in estimating and differentiating protein in the rumen between microbial, dietary, and endogenous fractions, and to correctly isolate the solid and liquid microbial fraction of the rumen contents. This is further complicated by the goal of using non-invasive methods as much as is feasible, such as avoiding the use of fistulated animals; the selection of an appropriate microbial marker, specifically one that behaves similarly in the solid-associated and liquid-associated microbial fractions. It is also vital to be able to accurately estimate the contribution of microbial protein to overall nitrogen used by the animal, which can be accomplished by the use of 15N labeled, as assimilated by ruminal bacteria, and by the quantification of labeled nitrogen via mass spectrometry (15N/14N). This review focuses on challenges regarding accurate quantification of microbial crude protein synthesis in the rumen, as well as providing the methodology for quantification using the 15N marker. This review is based on the collection of scientific papers from the main research groups in feed and animal nutrition in ruminants.

Keywords: Endogenous excretion, 15N, Microbial protein, Purine derivatives, Ruminants.