Effect of grazing intensity on the annual productivity and composition of mixed Lolium perenne and Trifolium repens pasture

The pasture cover, composition and changes in annual productivity for a mixed Lolium perenne and Trifolium repens pasture were studied under four different grazing intensities (lax, moderate, heavy and overgrazing: postgrazing pasture covers were 1 200～1 300 kg/hm2, 900～1 000 kg/hm2, 600 kg/hm2 and 300 kg/hm2 respectively). The results showed that pasture cover decreased with increase in grazing intensity, but pasture utilization rate increased. The growth of stems and leaves of L.perenne showed an obvious response to grazing intensity, but the growth of stems and leaves of T.repens showed very little response. The stem to leaf ratio of L.perenne decreased with increase in grazing intensity, whereas different seasons played the key role affecting the stem to leaf ratio of T.repens. Under moderate grazing, pasture cover of L.perenne increased as a linear function with the pasture cover increase of T.repens. Under lax grazing, heavy grazing and overgrazing, the pasture cover of L.perenne increased as a power function with the pasture cover increase of T.repens. Total pasture yield and forage yield of L.perenne showed the following pattern: moderate grazingheavy grazingovergrazinglax grazing, and for T.repens was: moderate grazingover grazingheavy grazinglax grazing. From lax grazing to overgrazing, the proportion of L.perenne remained similar (about 74%), the proportion of T.repens increased, and the proportion of weeds and dead matter decreased. Therefore, grazing can increase the proportion of T.repens, restrain weed growth, reduce dead matter loss, and enhance the positive relationship between L.perenne and T.repens resulting in increased growth of both species. Moderate grazing is beneficial in maintaining stable proportions of L.perenne and T.repens in the mixed pasture sward, and in maintaining high pasture productivity. Reasonable pasture utilization is crucial to maintaining high production and high quality pasture.