Use of ‘virtual’ trials to fill gaps in experimental evidence on enhanced efficiency fertilisers

Enhanced-efficiency fertilisers (EEF) are of interest to the sugarcane industry because they have potential to increase N-use efficiency and reduce nitrogen (N) loss. However, it has been challenging to demonstrate the agronomic and/or environmental benefits of EEF experimentally. Using the APSIM-Sugar farming systems model we can run 'virtual' trials that mimic field trials but with many more treatments, N rates and locations, and over more seasons. For a selection of contrasting soil types from the Herbert mill area we simulated 273,360 virtual treatment-years to obtain complete yield N-response curves for urea and EEF under a variety of seasonal climate and management conditions. The virtual trial results explain that the mixed and often inconclusive results of field trials are due to the high seasonal variability of the yield response to both N rate and EEF. The responses to EEF can be classified into four types: A, increase in maximum yield; B, reduction in optimum N; C1, N responsive but no response to EEF; C2, no response to N rate or EEF. Daily output of variables from the model can explain these response types and provide interpretation of experimental results. The many virtual trials also allow specification of the likelihood of different response types under different conditions (e.g. soil and timing of ratooning/fertiliser application). While the virtual trials can be used to fill the large gaps in experimental evidence on EEF and enhance our understanding of the drivers, they do rely on the model to capture the system dynamics correctly. Ongoing experimental verification of the model’s representation of different processes and, in particular, N responses is critical, especially when transitioning from using the model to understand the responses to using it for more quantitative decision support. Key words Enhanced-efficiency fertilisers, controlled-release fertilisers, cropping systems modelling, nitrogen management, sugarcane, APSIM