Influence of applied nitrogen on weed invasion of Lolium perenne pastures in a subtropical environment

Lolium perenne L. (perennial ryegrass) exhibits poor persistence in subtropical environments and this is often characterised by weed invasion. The effect of applied nitrogen (N) (120-720 kg/ha.year), on weed invasion of perennial ryegrass was examined during the establishment year. Weed tiller densities ranged from 50 tillers/m2 (at 360 kg N/ha.year) during November 1993, to 2400 tillers/m2 (at 120 kg N/ha.year) during February 1994. Increasing levels of applied N up to 360 kg/ha.year reduced both weed tiller densities and relative frequencies in perennial ryegrass pastures. Seasonally, high weed tiller densities (up to 2400 tillers/m2 at 120 kg N/ha.year) and high frequencies (46-82% for 120 and 600 kg N/ha.year, respectively) were observed over midsummer (December) to autumn (May). Applying N at levels above 360 kg/ha.year (or 30 kg/ha.month) is useful in minimising weed invasion of perennial ryegrass during its establishment under subtropical conditions.