Population relationships of perennial ryegrass and white clover mixtures under differing grazing intensities

The objective of this study was to investigate the effects of different grazing intensities on perennial ryegrass (Lolium perenne) tiller and white clover (Trifolium repens) stolon, size/density compensation (SDC) patterns. A rotational grazing experiment using perennial ryegrass/white clover pasture and four grazing intensities was conducted from April 1999 to December 2000. Increased grazing intensity significantly decreased weight and increased density of ryegrass tillers (P < 0.05), but had the opposite effects on white clover stolons. Tiller/stolon weight and density also varied with seasonal sampling date, and there were also significant (P < 0.05) grazing intensity × time interactions. For ryegrass, the slope for the SDC relationship log(weight) versus log(density) in year 2000 was −1.74, consistent with recently published theory that grazing intensity gradients will produce SDC relationships steeper than −3/2 as a result of lower sward LAI at higher grazing intensity. The comparable slope for the reverse SDC pattern in white clover was −0.98, indicating that clover achieved near constant herbage mass across grazing intensities. Intercepts for the SDC relationship of both species moved upward in spring to a maximum in summer and downward in autumn to a minimum in winter. Other observations included significant increases (P < 0.05) in ryegrass and white clover leaf numbers per tiller or stolon, respectively, in June, with these increases typically persisting until August. White clover also developed an increased (P < 0.05) number of leaves per stolon at higher grazing intensity in June and August. Under all grazing intensities, the population biomasses of ryegrass and clover were positively correlated across micro-sites within plots (P < 0.05 or less). Operation of a reverse SDC pattern in white clover associated with constant mass across razing intensities may partly explain at a functional ecology level, the well-known stability of ryegrass–white clover species associations.