STRATEGIC NITROGEN-FERTILIZER USE ON PERENNIAL RYEGRASS AND WHITE CLOVER PASTURE IN NORTH-WESTERN TASMANIA

The strategic application of nitrogen (N) fertiliser on temperate past ure is a popular management tool for boosting pasture production, part icularly when low soil temperatures limit clover-derived N availabilit y. The responsiveness of a perennial ryegrass and white clover pasture to increasing rates of N fertiliser (0, 15, 30, 45 and 60 kg N/ha), a pplied as a single application, was investigated for 5 pre and 5 post mid winter N application dates. The trial was laid out in a randomised split-plot design, replicated 4 times, with N application date in the whole plots and N fertiliser rate in subplots. Herbage yields were es timated by mowing small plots, with animals excluded for the duration of a single regrowth period on each plot. The data are presented as av erages fbr the 5 pre and 5 post mid winter responses separately, as th e means of the pre and post winter partitions were significantly (P < 0.001) different at all rates of N fertiliser for both years. Signific ant (P < 0.05) yield responses were noted between increments of N fert iliser up to the 45 kg N/ha application rate. In contrast, no signific ant (P > 0.05) increases in dry matter yield were noted between the 45 and 60 kg N/ha rates in all but one response, consistent with diminis hing returns. The efficiency with which N fertiliser was utilised rang ed between 9.0 and 15.9 kg N dry matter/kg. Nitrogen efficiency was co nsistently lower at the 60 kg N/ha rate than for lower rates of N fert iliser. The application of N fertiliser increased pasture dry matter y ields by between 582 and 703 kg dry matter/ha. Based on the 1997 cost of urea, the additional forage produced cost between 6.4 and 10.1 cent s/kg dry matter. The data are reviewed in terms of the potential for s trategic N fertiliser applications during the winter, while still maxi mising utilisation of clover-derived N during the warmer months.