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Pastoral systems are increasingly under scrutiny for the nitrogen (N) deposited via urine, faeces and fertiliser that is subsequently transported through the soil profile into water ways. Variation in land management, fertiliser N application rates, soil type, climate and year-to-year variation in weather all affect the rate and timing of N losses via leaching. To mitigate N leaching, a better understanding of the environmental conditions and the identification of early indicators that lead to high risk of N leaching are needed. To identify early indicators for risk of N leaching from urine patches we used the APSIM (Agricultural Production Systems SIMulator) model to produce an extensive N leaching dataset for the Waikato region of New Zealand. In total 198000 simulation runs with different combinations of urine deposition date, N loading, soil type and irrigation were done. The risk forecasting indicators were chosen based on their practicality: being easy to be measured on farm (soil water content, temperature and pasture growth) or that could be centrally measured (such as weather forecasting). The thresholds of the early indicators that are likely to forecast a period for high risk of N leaching were determined via classification and regression tree (CART) analysis. This concept is illustrated in Figure 1 for a soil with a medium soil depth, with soil plant available water of 125-150 mm. The identified early indicators were then used within a N risk tool, based on a traffic light system, to trigger duration controlled grazing in a desktop evaluation to reduce N leaching on a paddock scale. Predicted reductions in N leaching ranged from 20 to 45%, depending on the number of indicators used and the risk alertness of the farmer. Figure 1. Classification and regression tree to predict the risk of nitrogen leaching in the Waikato region for a 750 kg N/ha deposition event. The bars indicate the percentage of simulations within the different risk categories with thresholds for N leaching of green: 0-15, yellow: 15-30, orange: 30-50, and red >50 kg/ha/yr.

Development and desktop-assessment of a concept to forecast and mitigate N leaching from dairy farms