Herbage and nitrogen yields, fixation and transfer by white clover to companion grasses in grazed swards under different rates of nitrogen fertilization

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 240 kg 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 120 kg N ha −1 . The proportion of N derived from the atmospheric N fixation was 25–70%. Nitrogen fixation ranged from 25 to 142 kg N ha −1 measured using the isotope dilution method in 2012 and from 52 to 291 kg 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 60–120 kg N ha −1 .