The effects of strategic nitrogen fertiliser application during the cool season on perennial ryegrass-white clover pastures in the Western Cape Province 2. Dry matter production
The response of an irrigated perennial ryegrass-white clover pasture to fertiliser nitrogen application rates of 0, 50, 100 and 150 kg N ha-1 applied in either autumn, early winter, late winter, early spring or late spring was investigated over a three-year period. Primary dry matter production (PDM) and residual dry matter production (RDM) were recorded five and ten weeks after fertiliser N application, respectively. PDM responses were determined mainly by fertiliser N application rate. On average the highest PDM was obtained following N applications in spring and lowest when N was applied in early winter. Although RDM production generally also increased with increasing fertiliser N rates, responses were much smaller than those recorded for PDM and were affected by the season of fertiliser N application. The highest RDM was obtained when N was applied in autumn and winter while application in spring had no, or even a negative effect, on RDM. Total dry matter production (PDM+RDM) increased (P<0.05) with increasing N application rates. Except for the 0 kg N ha-1 treatment in 2002, which recorded lower DM yields compared to fertilised plots, total annual dry matter production of the pasture was not affected by N fertiliser application rate. Nitrogen application can therefore be used to improve dry matter production during a short predetermined period. Although the 150 kg N ha-1 applied in early and late spring generally gave the best results in terms of PDM, the 50 kg N ha-1 application treatments resulted in a more efficient conversion of N applied to additional DM produced.