Strategic nitrogen fertiliser use on perennial ryegrass and white clover pasture in north-western Tasmania

The strategic application of nitrogen (N) fertiliser on temperate pasture is a popular management tool for boosting pasture production, particularly when low soil temperatures limit clover-derived N availability. 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), applied 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 estimated by mowing small plots, with animals excluded for the duration of a single regrowth period on each plot. The data are presented as averages fbr the 5 pre and 5 post mid winter responses separately, as the means of the pre and post winter partitions were significantly (P < 0.001) different at all rates of N fertiliser for both years. Significant (P < 0.05) yield responses were noted between increments of N fertiliser up to the 45 kg N/ha application rate. In contrast, no significant (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 diminishing returns. The efficiency with which N fertiliser was utilised ranged between 9.0 and 15.9 kg N dry matter/kg. Nitrogen efficiency was consistently lower at the 60 kg N/ha rate than for lower rates of N fertiliser. The application of N fertiliser increased pasture dry matter yields 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 cents/kg dry matter. The data are reviewed in terms of the potential for strategic N fertiliser applications during the winter, while still maximising utilisation of clover-derived N during the warmer months.