The simultaneous use of ribonucleic acid,35S, 2,6-diaminopimelic acid and 2-aminoethylphosphonic acid as markers of microbial nitrogen entering the duodenum of sheep

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Abstract
1. Three sheep, each fitted with a ruminal cannula and duodenal re-entrant cannulas were given three isonitrogenous, isoenergetic diets in a Latin-Square design. Each diet contained (/kg) approximately 400 g N as white fish meal, soya-bean meal or urea and approximately 600 g dry matter (dm) was barley grain. The diets were fed continuously and supplied about 28 g N/d.2. Total duodenal digesta was collected manually for 72 h and the proportions of microbial N in that digesta were simultaneously estimated for all sheep using RNA, radioactive sulphur (35S), diaminopimelic acid (DAPA) and aminoethylphosphonic acid (AEPA) as markers.3. Three of the estimation methods showed that the variable source of dietary N had the greatest (RNAP< 0.05,35SP< 0.005, DAPAP< 0.1) effect on the proportions of microbial N in duodenal digesta, though differences between sheep accounted for some variation.4. These methods also ranked the diets in the order: urea > soya-bean meal > fish meal with respect to the proportions of digesta N that were microbial in origin; the respective mean values for these diets with the different markers were: RNA 0.98, 0.70, 0.56;35S 0.92, 0.64, 0.54; DAPA 0.80, 0.47, 0.42.5. AEPA was found to be present in substantial quantities not only in isolated rumen protozoa, but also in dietary and bacterial material; an observation that invalidated its further use as a protozoal marker.6. Calculations using values obtained from the35S procedure showed that the proportions of dietary N degraded within the rumen were 0.38, 0.43 and 0.89 for the white fish meal, soya-bean meal and barley respectively.7. The marker methods are compared and their advantages and disadvantages (real and apparent) are discussed. It is concluded that where microbial N estimates of a more general and comparative nature are required, the use of RNA as a marker is probably adequate. Where information for more exacting purposes is required, the use of35S appears to be more appropriate.

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