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.
