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

Abstract

AbstractIn grass–legume swards, biologically fixed nitrogen (N) from the legume can support theNrequirements of the grass, but legumeNfixation is suppressed by additional fertilizerNapplication. This study sought to identify a fertilizerNapplication rate that maximizes herbage andNyields,Nfixation and apparentNtransfer 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 kgNha−1 year−1, were compared using isotope dilution andN‐difference methods. The presence of white clover increased herbage andNyields by 12–44% and 26–72%, respectively. Applications ofNfertilizer reduced sward white clover content, but the effect was less at below 120 kgNha−1. The proportion of N derived from the atmosphericNfixation was 25–70%. Nitrogen fixation ranged from 25 to 142 kgNha−1measured using the isotope dilution method in 2012 and from 52 to 291 kgNha−1using theN‐difference method across all years. FertilizerNapplication reduced the percentage and yield of fixed N. Transfer of N from white clover to grass was not confirmed, but there was an increasedNcontent in grass and soil‐Nlevels. Under intensive grazing, the maximum applied N rate that optimized herbage andNyields with minimal effect on white clover content and fixation rates was 60–120 kgNha−1.