The efficiency of use of dietary nitrogen (N) in cattle is influenced by the rate of degradation of dietary protein to peptides, amino acids and ammonia and the concomitant rate of assimilation of these nitrogenous products by rumen micro-organisms for their protein synthesis. The latter depends heavily on fermentable energy availability (FME). Ammonia that is not assimilated is absorbed across the rumen wall and is mostly excreted. This excretion represents inefficient use of dietary N. The stable isotope 15N provides a useful means of making quantitative estimates of N utilisation in ruminants. However, owing to technical challenges associated with implementation of tracer dilution methods, measurement of 15N content in biological materials and cost of 15N-labelled materials, there have been relatively few studies using 15N in larger animals. Our objective was to develop and evaluate tracer dilution methods using 15N that would be appropriate for studying the efficiency of use of the N in perennial ryegrass silages when used for feeding cattle.