The conversion of plant nitrogen to microbial nitrogen in the rumen of the sheep.

An estimate of the extent of conversion of plant nitrogen to microbial nitrogen in the rumen of sheep fed on wheaten hay was reported in an earlier communication (Gray & Pilgrim, 1953). The basis of separation of plant from microbial material was simply the filtration of the rumen contents through muslin with subsequent limited washing of the fibrous plant residues to free them from the remaining bacteria. This procedure must have involved two main sources of error-contamination of the washed fibres with bacteria, and contamination of the filtrate and washings with very fine plant particles, but no attempt was made at that time to assess the magnitude of these errors. It was known that considerable amounts of nitrogen could be removed from the fibres by further washings, but at the same time it was realized that the physical separation of plant and microbial elements was not necessarily being improved, since further quantities of fine plant particles were also being washed into the filtrate. Later a chemical basis was adopted for assessment of the contamination of the washed fibres by bacteria, and a,€-diaminopimelic acid was selected for this purpose since it is present in the bacteria of the rumen in sheep, but not in the protozoa or in the plant substances of the fodder. Because it is absent from the protozoa it was hoped to use it also as a basis for the separation of bacterial and protozoal nitrogen. Since it was possible that the bacteria contaminating the washed fibres might have a different composition from the rest of the bacteria, it was considered desirable to check the suitability of the amino-acid as a marker of bacterial protein by comparing it in this respect with some other, quite separate substance present in bacteria but not in plants. For this purpose vitamin B,, was selected. The present experiments extend the findings previously reported and show that diaminopimelic acid can be used as a marker of bacterial nitrogen to reduce a major source of error in the former calculations. An attempt was also made to correct for the second source of error-the contamination of the microbial fractions by fine plant particles. It is not possible, of course, to know the precise error remaining, but there can be no doubt that these extensions to the procedure have improved the original estimates of the extent of conversion of plant nitrogen to microbial nitrogen in the rumen.