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.
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