Manipulation of nitrogenous supply to increase efficiency of net microbial cell synthesis in the rumen

Abstract

Protozoa appeared to have an important role in fibre digestion in the rumen only when the concentration of ruminal fluid ammonia was below 40 mgN/l. Increased efficiency of net microbial cell synthesis in the rumen by increasing the availability of ruminal fluid ammonia appeared to be associated with a reduction in protozoal pool in ruminal fluid while the specific growth rate (indicated by purine:total-N ratio) of bacteria remained unchanged. This indicates that increasing the efficiency of net microbial cell synthesis is due to reduced turnover of microbial cells within the rumen. Since dry matter digested in the rumen is either fermented to VFA or used in the synthesis of microbial cells, an increasing microbial growth efficiency will have large effects on the ratio of microbial protein to VFA available for digestion and absorption respectively. For a forage that is low in crude protein, therefore, the ratios of protein:energy and acetogenic:glucogenic substrates available for digestion and absorption from the digestive tract of ruminants may be optimised by adjusting the concentration of ruminal fluid ammonia to or above 200 mgN/l.