Reassessing the manipulation of protein synthesis by rumen microbes.

Protein synthesis by rumen microbes plays an important role in ruminant nutrition. Microbial proteins that are not recycled flow out of the rumen and account for more than 50% of the total proteins entering the intestines. Determination of microbial nitrogen in digesta is based on the isolation of a representative sample of rumen microbes and on the use of specific markers (purine bases, RNA, 15N incorporation). This review discusses the reliability of microbial markers and examines current methods for assessing differences in turnover rate and microbial composition among liquid and particulate compartments. Four groups of factors acting on microbial synthesis, which are also adaptable to farm conditions, are considered: I) modification of protozoal population; II) modification of liquid and solid phase kinetics in the rumen; III) balance in the components of the diet; and IV) probiotics. The effects of these factors are discussed, considering their feasibility and their usefulness for the host.