Since the earliest microscopic description of 15-20 species of 'animalcules' in the rumen (Gruby and Delafond, 1843), a few landmark discoveries have changed scientists' understanding of the rumen, its microbial ecosystem and the relations between rumen fermentation and ruminant nutrition. The demonstration that cellulose was digested by microbial activity in the rumen (von Tappeiner, 1884) was a seminal discovery, as was the importance of the microbial fermentation products, the volatile fatty acids, as nutrients for the host animal (Barcroft et ai, 1944). By applying habitat-simulating principles to growth medium formulations Hungate (1947) was able to grow the strictly anaerobic rumen bacteria in vitro, an accomplishment that had hitherto been impossible and which enabled rumen bacteriology to begin in earnest. Orpin's (1975) discovery that strictly anaerobic fungi were significant commensal organisms also changed our perception of the rumen in a very significant way.

In terms of rumen protein and amino acid metabolism, there have been no comparable strides forward, rather a steady increase in understanding. The most elegant demonstration of the biosynthetic power of rumen microbes, in terms of amino acid biosynthesis, was made by

Virtanen (1966), who maintained lactating cow« on a protein-free diet for many months, the microbial amino acids being synthesized from urea-N. This result was to some degree predictable, however, based on the many in vitro studies carried out by Bryant and his colleagues (Bryant, 1973). The review by Chalmers and Synge (1954) was among the first to assemble all known information about nitrogen metabolism in the rumen; it is worth returning occasionally to this excellent review as a historical perspective to appreciate how our impressions of rumen N metabolism have changed.

Amino acid metabolism in the rumen is studied principally because of its nutritional implications. The ruminant relies for its amino acids on the mixture of microbial and surviving feed protein which result from rumen fermentation (Fig. 15.1). Only a fraction of the protein consumed, and to a limited degree some peptides arising from proteolysis, escape rumen fermentation. Once released, amino acids do not survive in the free form for long. They are either incorporated into microbial protein or deaminated to ammonia; free amino acid concentrations are low. In order to maximize the efficiency of amino acid production from the rumen, we would wish to minimize degradation of feed protein to the point where all the protein which is degraded is incorporated by the t-mai! cidd resss : [email protected]

© CAB International 2003. Amino Acids in Animal Nutrition, 2nd edition (ed. J.P.F. D'Mello)

Non-protein nitrogen

Absorption and excretion


Amino acids


Microbial amino acids

Undegraded dietary amino acids

Host animal

Host animal


  1. 15.1. Protein metabolism in the rumen.
  2. This might not achieve the goals of animal production, however, if providing more amino acids were to increase the rate of fermentation of the energy source and thereby increase feed intake. This chapter aims to review amino acid metabolism by rumen microbes, both in pure and in mixed culture, in a nutritional context.

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