Factors affecting the utilisation of silage nitrogen by ruminants

Eight ryegrass silages, which differed in their contents of dry matter, nitrogen and in the pre-ensiling treatments which were used, were fed to sheep cannulated at the rumen and abomasum. Pelleted, dried grass was fed throughout to confirm that inter-experiment compar­isons could be safely made. Dietary digestibility was measured by total faecal collection and diurnal variation in ruminal metabolism was monitored. Passage of the components of digesta at the abomasum was determined by reference to the dilution of dual phase markers (tris (l, 10 phenanthroline) ruthenium (II) chloride and the chromium complex of ethylenediamine tetraacetic acid) which were continuously infused into the rumen. The contribution of microbial nitrogen to the total flow of nitrogen at the abomasum was assessed by using nucleic acid as an endogenous microbial marker.All silages were well preserved. Fermentation was restricted by pre-treatment with formic acid (3.0 and3.4 1 t-^"), a mixture of formic acid and formalin (4:1»4.5 1 t ■*") or by wilting. Proteolysis was also reduced by these treatments, with wilting to high levels of dry matter (416 and 438 g kg having the greatest effect.There was a tendency for higher concentration of nitrogen in silage to increase the apparent digestibility of the component. Silages with high concentrations of nitrogen therefore had disproportionately high contents of digestible crude protein.In absolute terms, the peak ruminal ammonia concen­tration was higher when dried grass, rather than silage, was fed (mean = 34-9 and 239 mg ammonia N 1 ^ respectively. However the ruminal ammonia concentration before feeding and the content of nitrogen in the diet were both lower, and the ration was eaten more slowly, when silage was fed. Thus the consumption of silage induced bigger changes in ruminal ammonia levels per unit intake of nitrogen, probably reflecting the soluble nature of a large propor­tion of the nitrogenous components of the silages.The mean values for the proportion of digestible organic matter apparently digested in the rumen (0.614.) and the efficiency of synthesis of microbial nitrogen (26.2 g N kg ^ DOMR, 0.91 g N MJ ^ ME intake) were similar to those which had been reported previously for silages. However, the mean degradability of silage nitrogen between the mouth and the abomasum (0.47) was lower than many previous estimates.When silage was fed, the flow of non-ammonia nitrogen at the abomasum was negatively related to the concentra­tion of nitrogen in the dietary digestible organic matter. This reduced efficiency of transfer of nitrogen from the diet to the small intestine at high dietary concentrations has been established before for dried diets, but not for silages