Effect of rate and time of nitrogen application from autumn to midwinter on perennial ryegrass-white clover dairy pastures in western Victoria. 1. Growth and composition

Field experiments were carried out at 2 sites to study the effects of time and rate of nitrogen (N) application from autumn to midwinter on perennial ryegrass-white clover pasture dry matter (DM) yield and species composition. Nitrogen (0, 15, 25, 30, 45, and 60 kg/ha) was applied in mid-April, early May, mid-May, early June, and mid-June 1996 onto a grazed pasture. Dry matter yield was estimated when perennial ryegrass in the 45 kg N/ha treatment reached the 3-leaf stage of development. Species composition estimates were made on all plots in autumn, late winter, and late spring (November). A second DM yield estimate was taken one regrowth cycle after the primary DM yield estimate to determine residual N impact on pasture growth, and then the pastures were returned to the grazing routine of the farm. Increasing levels of N fertiliser resulted in a linear (Site 1) and curvilinear (Site 2) primary DM yield response. Residual DM yield responses were linear at both sites. The highest primary DM yield response at Site 1 was 9.1 kg DM/kg N (mid-April application), whereas the highest residual N response was 9.4 kg DM/kg N (mid-May application). The time taken for the 45 kg N/ha treatment to reach the 3-leaf stage of development ranged from 38 (early June application) to 47 (mid-April application) days for the primary harvest, and from 46 (early May application) to 72 (early June application) days for the residual harvest. At Site 2, the highest primary DM yield response to N was 15.8 kg DM/kg N at 45 kg N/ha (mid-April application), whereas the highest residual N response was 8.3 kg DM/kg N (mid-June application). The time taken for the 45 kg N/ha treatment to reach the 3-leaf stage of development ranged from 24 (early June application) to 36 (mid-April and early May applications) days for the primary harvest, and from 30 (mid-April) to 57 (early June application) days for the residual harvest. Based on the current cost of urea, the study concluded that N fertiliser use during autumn to midwinter is economically viable for dairy farmers. Botanical composition was unaffected by N application. There were no treatment effects on botanical composition of the sward at either site the following spring (November). It is concluded that a 'once-off' strategic N application up to 60 kg N/ha during autumn to midwinter is not likely to influence dairy pasture composition in western Victoria.