Effects of fertiliser nitrogen management on nitrate leaching risk from grazed dairy pasture

Dairy farms are under pressure to increase productivity while reducing environmental impacts. Effective fertiliser management practices are critical to achieve this. We investigated the effects of N fertiliser management on pasture production and modelled N losses, either via direct leaching of fertiliser N, or indirectly through N uptake and subsequent excretion via dairy cow grazing. The Agricultural Production Systems Simulator (APSIM) was first tested with experimental data from fertiliser response experiments conducted on a well-drained soil in the Waikato region of New Zealand. The model was then used in a 20- year simulation to investigate the effect of fertiliser management on pasture response and the impacts on potential leaching losses. The risk of direct leaching from applied fertiliser was generally low, but at an annual rate of 220 kg N/ha exceeded that from urine patches in one out of 10 years. The main effect of N fertiliser on leaching risk was indirect via the urine patch by providing higher pasture yields and N concentrations. Best management practices could include identification of high risk periods based on environmental conditions (e.g. soil moisture, plant growth), avoidance of fertiliser applications in these periods and the use of duration controlled grazing (DCG) to prevent excreta deposition onto the grazing area during critical times. Keywords: Modelling, APSIM, N fertilisation rates, N fertilisation timing, direct and indirect leaching, urine patches