Markers for Quantifying Microbial Protein Synthesis in the Rumen

Measurement of ruminal microbial protein is necessary to quantify ruminal escape of dietary protein and microbial yields. Microbial markers used most widely have been the internal markers, diaminopimelic acid and nucleic acids (RNA, DNA, individual purines and pyrimidines, or total purines), and the external isotopic markers (e.g., ¹⁵N and ³⁵S). Combined with digesta flow markers in ruminally and abomasally or intestinally cannulated ruminants, microbial yields can be estimated. An ideal marker system must account for both the bacterial and protozoal pools associated with both the fluid and particulate phases of digesta. No marker has proven completely satisfactory; hence, yield estimates are relative rather than absolute. Total purines represent robust microbial markers that should be adaptable by most investigators. Principal concerns about total purines relate to unequal purine:N ratios in protozoal and bacterial pools and to the need to assume that dietary purines are completely degraded in the rumen. A theoretically sounder, but more costly, method is continuous intraruminal infusion of ¹⁵N ammonium salts. However, ¹⁵N enrichments of bacterial and protozoal pools are not equal, so the basis for calculating microbial yield in faunated ruminants is uncertain. Urinary purine excretion may prove to be a noninvasive method for estimating microbial protein yields in intact dairy cows.