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, earl y May, mid-May, early June, and mid-June 1996 onto a grazed pasture. Dry ma tter yield was estimated when perennial ryegrass in the 45 kg N/ha treatmen t reached the 3-leaf stage of development. Species composition estimates we re 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 t he pastures were returned to the grazing routine of the farm. Increasing levels of N fertiliser resulted in a linear (Site 1) and curvili near (Site 2) primary DM yield response. Residual DM yield responses were l inear 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 respon se 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 (earl y June application) to 47 (mid-April application) days for the primary harv est, and from 46 (early May application) to 72 (early June application) day s for the residual harvest. At Site 2, the highest primary DM yield respons e to N was 15.8 kg DM/kg N at 45 kg N/ha (mid-April application), whereas t he highest residual N response was 8.3 kg DM/kg N (mid-June application). T he time taken for the 45 kg N/ha treatment to reach the 3-leaf stage of dev elopment 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 cu rrent 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 treatm ent effects on botanical composition of the sward at either site the follow ing spring (November). It is concluded that a 'once-off' strategic N applic ation up to 60 kg N/ha during autumn to midwinter is not likely to influenc e dairy pasture composition in western Victoria.