EFFECTS OF CUTTING HEIGHT ON THE PRODUCTIVITY AND COMPOSITION OF PERENNIAL RYEGRASS-WHITE CLOVER SWARDS

All combinations of five cutting heights (2, 4, 6, 8 and 10 cm), two rates of P application (0 and 100 kg P/ha per year) and two irrigation treatments (0 and maximum soil water deficit 35 mm) were compared on field plots sown with a mixture of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) in Wales. The cutting and P treatments were applied for 4 1/2 years (1986-90) and the irrigation treatments for 2 years (1987 and 1988). The plots were cut at 4-week intervals from April to October each year. Applied P and irrigation had only a small effect on the productivity and composition of the swards. The proportion of white clover in the sward was inversely related to the height of cutting, so that clover was almost eliminated by 3 years of 10 cm cutting. This reduction in clover content led to a 50 % reduction in herbage yield. Close cutting increased the tiller density of ryegrass in May and June of the first harvest year, but this effect was subsequently reversed as clover competed more strongly with ryegrass under close cutting. The proportion of unsown species, predominantly Agrostis tenuis, was much higher in the third and fourth than in the first and second harvest years and was positively related to the height of cutting. Close cutting depleted soil K and soil water.