The objective was to identify the long term consequence of two extreme scales of spatial heterogeneity of perennial ryegrass and white clover on the diet selected by sheep, together with plant to plant species competition and hence sward composition. Large patches (ie highly aggregated swards) offer minimum between plant species competition, but maximum likelihood of selection by the grazing animals, whereas a fine mixture of two plant species provides maximum between plant species competition and reduced opportunity for selection by the grazing animals.