Effects of grazing on the vegetation composition of Lolium perenne and Trifolium repens mixed pasture

In order to understand effects of grazing intensities on vegetation composition of Lolium perenne and Trifolium repens mixed pasture, the chemical characteristics of soil, L. perenne tillers, T. repens stolons, the number of leaves, the stem and leaf composition, the height and biomass of herbage under four grazing intensities (light, moderate, heavy and over grazing; with postgrazing height at 7.5 cm, 5.5 cm, 3.5 cm and2.0 cm, respectively) were studied at Weining, Guizhou. The results showed that the response of soil available nutrient contents to the grazing intensity was more sensitive, but their stability was lower. Tiller density and leaf biomass of L. perenne increased significantly with increasing grazing intensity, while tiller weight of L. perenne and stolon density of T. repens, height and biomass of herbages decreased significantly. Grazing intensities had no effects on the number of leaves of L. perenne and T. repens, the ratio of stem to leaf and stolon weight of T. repens. The homogeneity of growth characteristics of L. perenne and T. repens was reduced and their plasticity was enhanced by the grazing treatments. The response of L. perenne to grazing intensity was more sensitive than that did T. repens, whereas the plastical growth of the former was lower than the latter. Dactylis glomerata and native species had higher coefficients of variation under different grazing intensities. Thus, the effect of grazing animals on grassland vegetation could be revealed from three levels: individual plants, populations and communities. The low component sown grass (D. glomerata) and native grass species were the key species during the change of vegetation composition of mixed pasture.