Dynamics of perennial ryegrass and white clover in sown swards in NW Greece

Seasonal dynamics of white clover and perennial ryegrass were examined in s own perennial ryegrass/white clover swards subject to a 2 x 2 factorial tre atment combination of defoliation (rotational grazing by sheep and cutting) and nitrogen fertilizer application (0 and 40 kg N ha(-1) year(-1)) in NW Greece. Sward surface height and percentage cover were measured before and after five defoliation periods in 1996 within permanent microplots (30 x 30 cm, divided into nine cells) in which white clover was either initially pr esent or absent. Both white clover and perennial ryegrass achieved maximum height and cover in April-May. Defoliation treatment and whether white clov er was present initially significantly affected height and cover of both sp ecies. Total plant cover was similar prior to all defoliation periods excep t in July, a time of drought. Cover of perennial ryegrass was greater where white clover was initially absent, but total plant cover was greater in mi croplots containing white clover and the extent of the differences varied d uring the year. In contrast, N fertilizer application had little effect on species cover, other than small reductions in white clover cover. When whit e clover was present in April, it was found in virtually every microplot ce ll until July, but if it was absent in April there was little colonization of the microplot.