Influence of sward botanical composition on performance of cool temperate grassland under contrasting nitrogen fertilization regimes

Eight field experiments, which tested the effects of lime, phosphorus (P), and two rates of nitrogen (N) on a range of permanent grassland swards in Northern Ireland (NI), were re-examined to investigate the influence of sward botanical composition on aerial dry matter (DM) productivity under 'low' and 'high' levels of N input. The aim of the investigation was to verify results previously obtained using 'pure' single-species swards, which suggested that secondary grasses are more productive than perennial ryegrass at low levels of N input (i.e., <100 kg N ha(-1) a(-1)). The swards used in the investigation varied considerably in botanical composition, but were largely dominated by three main species: perennial ryegrass (Lolium perenne L.), creeping bent (Agrostis stolonifera L.), and rough stalked meadow grass (Poa trivialis L.), all of which were characteristic of fertile habitats. The results confirmed that under reduced N input management, swards dominated by these secondary grasses are capable of out-performing those in which perennial ryegrass is a major component. The superior (aerial) productivity of the secondary grasses was attributed to their ability to conserve resources and maintain high shoot-to-root ratios under 'moderate' N stress conditions.