How does the vertical and horizontal structure of a perennial ryegrass andwhite clover sward influence grazing?

Mixtures of perennial ryegrass (Lolium perenne L.) and white clover (Trifol ium repens L..) sown in alternate rows or in a thoroughly mixed matrix were grazed by sheep, either continuously or during short grazing tests, and we re used to investigate the influence of the vertical and horizontal compone nts of the sward structure on defoliation hy sheep. In an experiment under continuous grazing, the defoliation intensity was gr eater for white clover compared with perennial ryegrass leaves (0.80 and 0. 58 respectively). In spring, perennial ryegrass leaves were more defoliated than white clover leaves, whereas the reverse was observed in summer. The ratio of the proportion of white clover to perennial ryegrass leaves grazed was negatively correlated with the difference between the surface height o f the perennial ryegrass and white clover rows in spring. In both spring an d summer, white clover leaves of the same extended leaf length had a higher proportion of them grazed than perennial ryegrass leaves. In another experiment, during short grazing tests with perennial ryegrass-w hite clover swards that were grazed at the same sward surface height and at the same white clover content as in the previous experiment, there were no significant differences in the proportion of white clover and perennial ry egrass leaves grazed between strips of the two species and thoroughly mixed structures. The proportion of white clover leaves grazed was higher than t hat of perennial ryegrass leaves. These results show that the differential defoliation by sheep of perennial ryegrass and white clover leaves varies according to their vertical distrib ution in the mixed canopy, but is little affected by their horizontal distr ibution. Even small differences in sward surface height between mixed peren nial ryegrass and white clover can affect diet selection by sheep to a rath er large extent.