Agronomical and physiological responses of white clover (Trifolium repens) and perennial ryegrass (Lolium perenne) to nitrogen fertiliser applied in autumn and winter

Mixed pastures of white clover and ryegrass are profitable and widely used. However, loss of clover is common with time due to N fertiliser applications in late winter. The factors that cause this loss have yet to be determined. This work investigated reasons for the competitive disadvantage suffered by white clover relative to ryegrass in autumn and winter. White clover (Trifolium repens cv. 'Grasslands Huia') and ryegrass (Lolium perenne cv. 'Grasslands Nui') were sown in March in Canterbury, New Zealand, and grown as single plants in silica sand. Nitrogen (N; 0.5 mol m(-3) N); was applied daily simulating an unfertilised soil; solution N concentration was increased to 5.0 mol m(-3) N for treatments in autumn (May) or winter (August), simulating fertiliser treatment. Net photosynthesis was measured before each harvest and over a diurnal period at 123 days after sowing. Plants were destructively harvested six times (33-185 days after sowing) and plant dry weight and N concentration (%) measured. White clover had a significantly (P < 0.05) lower dry weight than ryegrass. Extra N applied from May significantly increased (P < 0.05) dry weight of ryegrass. No significant changes in shoot N concentration were found. Except at the last harvest, net photosynthesis was significantly lower (P < 0.05) in white clover than in ryegrass; this was also apparent for diurnal net photosynthetic rates 123 days after sowing. Results indicated that white clover growth was limited by temperature, whereas ryegrass growth was limited by N supply.