Nitrogen exchanges in the rumen

Nitrogenous compounds in the rumen can be classified according to chemical nature (proteins, nucleic acids etc.), origin (feed, microbial or endogenous) or physical form (undissolved, dissolved and gas).Numerous possible exchange reactions between these 'pools' of rumen nitrogen, and relative importance of these exchanges determines the total amount of amino acid-N leaving the rumen.One may attempt to describe these exchanges by considering a pool of undissolved N exchanging continuously with a pool of dissolved N. We shall deal with the size and composition of these pools, refer to supply of these substances from feed and endogenous sources and describe the exchange reactions.Quantitative results when given, are estimated averages or extremes, referring to the rumen of generalized sheep, weighing 45 to 65 kg and fed 800 to 1000 g of roughage OM daily, as most available results were obtained under these conditions.Composition, size and supply of pools Undissolved N. The N in the undissolved pool is contained within the fibre, smaller food particles, micro-organisms and sloughed rumen epithelial cells.The OM (500 g) in a total volume of about 5 1, contains 5 to 35 g of total N (TN), depending on time after feeding, nature and N content of the ration.Irrespective of time after feeding, undissolved N represents 90% of rumen T N and contains 60 to 90% of microbial-N (Weller et al. 1958).Little is known about the relative proportions of protozoal-and bacterial-N and these have been assumed as being equal (Hungate et al. 1971).About half the bacteria are closely associated with the larger fibre particles (Weller et al. 1958) which also contain adhering protozoa (Bauchop, 1979).N is continuously redistributed among particles of different sizes in this pool and bacteria, protozoa as well as other particles leave the rumen with different fractional outflow rates, all lower than the liquid phase (Faichney & Griffiths, 1978).N in this pool is also present in sloughed epithelial cells, partly digested by bacteria, adhering to the rumen wall (Cheng et al. 1979) and from the results of Wallace et al. (1979), this fraction can be estimated as being less than I g.Undissolved N is mainly present as protein-N (PN) and nucleic acid-N (NAN), the latter representing 6 to 12% of feed T N (Smith & McAllan, 1970) and 6 to 25% of microbial TN, with lower values for protozoa than for bacteria (Ellis & Pfander, 1965;Smith, 1969;Ling & Buttery, 1978).N enters the rumen undissolved N pool with feed PN and NAN (5 to 30 g), sloughed and partly digested epithelial cells (less than I g) and by fixation of gaseous N, into bacterial-N.A maximal estimate for the latter process was 750 mg/d during feeding of N, fixing bacteria (Jones & Thomas, 1974) but only negligible amounts could be