In grass-legume swards, biologically fixed nitrogen (N) from the legume can support the N requirements of the grass, but legume N fixation is suppressed by additional fertilizer N application. This study sought to identify a fertilizer N application rate that maximizes herbage and N yields, N fixation and apparent N transfer from white clover to companion grasses under intensive grazing at a site with high soil-N status. During a 3-year period (2011-2013), swards of perennial ryegrass and of perennial ryegrass-white clover, receiving up to 240kg N ha(-1)year(-1), were compared using isotope dilution and N-difference methods. The presence of white clover increased herbage and N yields by 12-44% and 26-72%, respectively. Applications of N fertilizer reduced sward white clover content, but the effect was less at below 120kg N ha(-1). The proportion of N derived from the atmospheric N fixation was 25-70%. Nitrogen fixation ranged from 25 to 142kg N ha(-1) measured using the isotope dilution method in 2012 and from 52 to 291kg N ha(-1) using the N-difference method across all years. Fertilizer N application reduced the percentage and yield of fixed N. Transfer of N from white clover to grass was not confirmed, but there was an increased N content in grass and soil-N levels. Under intensive grazing, the maximum applied N rate that optimized herbage and N yields with minimal effect on white clover content and fixation rates was 60-120kg N ha(-1).