Farmstar-Rapeseed : a decision support system for rapeseed nitrogen fertilization based on satellite data

Farmstar -Rapeseed is a decision support system developed in France by Infoterra France and Cetiom, based on satellite data to estimate the optimum economical amount of nitrogen (N) fertilizer for a winter rapeseed crop in each field and in each point of the field. The fertilization rate is calculated using a simplified previsional N balance method fitted to the rapeseed crop. This method takes into account several parameters including the N amount in the whole plants at the beginning and at the end of the winter which can be very variable for this crop (about 30 to 200 kgN/ha). This N amount in the whole plants is difficult to measure for the farmers. Cetiom has developed a method to estimate it from the fresh matter weight of aerial part (FMW) which is easier to measure in the field. Nevertheless, only few farmers weigh their rapeseed plants. A model has been developed to estimate the FMW from satellite data at the field scale. For this purpose, a radiative transfer model is used to derive from the reflectance the Leaf Area Index (LAI). This information is later transformed in FMW by a specific relationship depending on the architecture of the plants, the spacing between seedling rows and the mass per area unit of the leaf limbs. All the Farmstar-Rapeseed products are based on this relationship. For each campaign, a network of fields managed by Cetiom is used to weigh the FMW in order to monitor the quality of the relationship. Farmstar-Rapeseed has been marketed during the last 4 years. In 2006, Farmstar-Rapeseed made it possible to advise producers on 80 000 ha of rapeseed crop in France and to improve the way they manage the N fertilization at the field scale. This represents a much higher penetration level than any other decision support system for rapeseed N fertilization developed in the past. Another interest of this method is that being based on satellite data, thus in coordinates system, it allows to modulate the N fertilizer amount inside the field, depending on the crop heterogeneity.