Recent proposals (ARC, 1980) have clearly identified the need to supply adequate ruminally degradarle nitrogen (RDN) if rumen microbial protein yields are to be optimized and they provide a basis on which the required quantities of RDN can be calculated. However, apart from the suggestion that urea is used with only 80% of the efficiency of degraded protein N as a source of RDN in the synthesis of microbial protein, the present scheme fails to qualify the RDN composition (i.e. protein N (PN) vs. NPN) most likely to promote optimal microbial synthesis.

Cottrill, Beever and Osbourn (1982) examined the isonitrogenous supplementation of maize silage fed to growing calves with four different NPN:PN ratios in the supplement and showed a 29% increase in total duodenal amino acid (DAA) flow at the first increment of PN (fish meal). Of this increase, almost 87% could be accounted for by an increased duodenal microbial protein (DMP) flow. Higher levels of PN inclusion did not promote any further increases in DAA, despite increases in undegraded dietary protein (UDP) flow, due to a significant reduction in DMP flow. From these data, a further examination of total RDN supply and its composition in relation to MP synthesis was undertaken and the results are presented in this paper.