A comparison of methods for the estimation of microbial nitrogen in duodenal digesta of sheep

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Abstract
1. Six sheep, each fitted with a rumen cannula and re-entrant cannulas in the proximal duodenum and distal ileum, were given two diets (600 g dry matter (DM)/d) consisting of either grass silage (32·1 g nitrogen/kg DM) or dried grass (18·3 g N/kg DM). A net loss of N occurred between mouth and duodenum with the silage diet, indicating extensive ruminal degradation of dietary N, compared with a net gain on the dried-grass diet. Consequently, despite higher N intakes when silage was given, N flow at the duodenum was similar for both diets.2. The proportion of microbial N in duodenal digesta N was estimated using diaminopimelic acid (DAPA), [35S]methionine (35S),15N-enriched non-ammonia-N (15NAN) and amino acid profiles (AAP) as microbial markers. Isotopic labelling of rumen micro-organisms was achieved by intraruminal infusions of Na235SO4and (15NH4)2SO4.3. A comparison of all methods was made based on the marker concentrations in microbial fractions isolated by differential centrifuagation of strained rumen contents. With both diets, DAPA gave the highest estimates and AAP the lowest. Estimates based on35S and15NAN were intermediate and did not differ significantly (P> 0·05).4. For the15NAN,35S and AAP methods, the effect of site of sampling of the microbial fraction, i.e. from rumen contents or duodenal digesta, was examined and in all instances mean estimates based on duodenally-derived microbes were higher. However, the differences were significant for only15NAN with both diets (P< 0·001), for35S with the dried grass (P< 0·05), and for AAP with the silage (P< 0·05). Estimates based on duodenally-derived microbes were higher (P< 0·05) using15NAN than those obtained using35S with both diets.5. Depending on the method used for estimating microbial N, estimates of the efficieny ofmicrobial N synthesis in the rumen (g microbial N flow at duodenum/kg organic matter apparently digested in the rumen) ranged between 16 and 38 for the silage diet and 10 and46 for the dried grass diet. Similarly, estimates of feed N degradability in the rumen ranged between 0·62 and 0·97 for the silage and 0·00 and 0·93 for the dried grass.