1 A REVISED LEACHING MODEL FOR OVERSEER ® NUTRIENT BUDGETS David Wheeler 1 , Rogerio Cichota 2 , Val Snow 3 and Mark Shepherd 1 AgResearch: 1 Ruakura Research Centre, Private Bag 3123, Hamilton 3240; 2 Grasslands Research Centre, Private Bag 11008, Palmerston North 4442; 3 Lincoln Research Centre, Private Bag 4749, Christchurch 8140 Introduction OVERSEER ® Nutrient Budgets (Overseer) is a whole-farm nutrient budgeting tool, calculating budgets on an annual basis (Wheeler et al., 2006). The model is structured around ‘management blocks’, i.e. areas of the farm where management, soils and climate are similar. Previous versions of the model have separated pastoral and cropping based farms, but users have requested a number of changes to Overseer so that it is better able to represent a wider range of farms. These changes have included: integration of cropping and pastoral modules; inclusion of pasture blocks used mainly to grow supplement; and blocks for winter/summer forages. However, to accommodate these changes whilst still providing reliable estimates of N leaching under all the management options available, the N model needed to be modified. This paper outlines the approach that is being adopted. Requirements of the N leaching model Overseer is an empirically-based nutrient budgeting tool, rather than a detailed process-based model. The challenge continually is being able to model the transfer and fate of nutrients around the farm system whilst maintaining a level of user input that is practical and achievable (Shepherd & Wheeler, 2010). Amongst other outputs, Overseer calculates the long-term annual average N leaching from the management block(s) and the farm. Thus, the model has to respond to the full range of inputs that Overseer has (e.g. stocking rate, soil- type, and rainfall) and it has to be driven by parameters that the user knows, or suitable defaults need to be available. Overseer estimates the amount of N returned to the soil surface each month for each animal enterprise, as described below. The model then needs to estimate the proportion of each month’s excretal N load that is leached. The challenge is that leaching of N could occur months after the N is applied, depending on the pattern of drainage (Snow et al., 2011) and in the meantime will be subjected to a range of other processes (immobilisation, volatilisation, denitrification and plant uptake). Much of the N leaching in a grazed system will be due to losses from urine patches (Monaghan et al., 2007). However, the model also needs to be able to account for losses from different sources of N inputs added to a block such as excreta, fertiliser, effluents and N in irrigation water. Basis of the N leaching model Overseer calculates the nitrogen intake for the grazing herd as a mass balance (Figure 1). The energy requirements of the herd are first calculated using a standard metabolic model (Freer et al., 2006). This is then converted to dry matter (DM) intake based on the feed sources fed to the animals and default values for their metabolisable energy (ME) contents. Based on these DM intakes and default values for N content of the feed sources, N intake can be calculated. This is then partitioned between animal products (based on user-provided