A Tactical Decision Support System to Optimise Nitrogen Management.

In the context of precision nitrogen (N) management, since 2002, the Walloon Region (Belgium) transposed the European Nitrate Directive 91/676/EEC under the Sustainable Nitrogen Management for Scheme Agriculture (PGDA). An ensemble of "good practices" were defined to reduce the environmental pressure linked to excessive N management while maintaining productivity of farmers.

Process-based crop growth models are dedicated tools to assess the impacts of different cropping systems inputs, i.e. agro-environmental conditions, management and weather, on the crop harvestable organs. However, N management optimization remains very complex as the expected impacts are delayed in time from the moment of N application. Quantifying the risk associated to hypothetical climatic time-series representative of the local conditions is a suitable approach.

The STICS model was used to simulate the growth of a winter wheat crop.  300 synthetic weather time-series representative of the local weather were derived from 30-years historical records using the LARS-Weather Generator. Decision rules, integrating agronomic, economic and environmental objectives, were derived to optimize farmers’ revenues while minimizing detrimental environmental impacts, when optimizing N management practice.

To improve the decision-making process, an advanced tool was designed, embedding all these components and including a retro-active loop accounting for phenological development. The tool determines first the timing of N application for Zadoks stages 23, 30 and 39. Two fix doses of 60kgN.ha^-1are always applied at stages 23 and 30. At stage 39, the tool enters in a tactical N optimization process, evaluating the best N rate using 300 climatic projections.

Results suggest that the third N application should most of the time be reduced in comparison to the current farmers' practice. Following economic constraints (selling price and N cost) and environmental pressure, 3 years out of 4, 10 to 30kgN.ha^-1 could be systematically saved, increasing the N use efficiency by 5 to 20%.