Diagnosis of limiting factors of organic oilseed rape yield. A survey of farmers' fields

The yield of organic winter oilseed rape (WOSR) crop is low and variable in organic farming, but the reasons responsible for this problem are not clear. Aiming to identify the factors affecting organic oilseed rape yields, we carried out a diagnostic study on a network of 19 farmers' fields in four contrasting regions of France over a two-year period. Weed density and biomass, nitrogen absorption by the crop and by weeds, and nutrient availability in the soil, as well as the occurrence of pests and diseases, were assessed at four developmental stages. A range of possible limiting factors throughout the crop cycle led us to use a regression procedure for different periods, using the yield components during spring and biomass during autumn as dependent variables. We identified factors limiting yield and the critical time points at which they acted, which has never been done. Yield variation was strongly correlated with number of grains.m$^{-2}$. Dry biomass of weeds, nitrogen uptake by weeds and density of weeds appeared to explain 39% of variation in crop biomass in early winter, while pollen beetle and rape stem weevil damage explained 4.5% and 12% of variation in the number of flowers.m$^{-2}$ and the number of branches.m$^{-2}$, respectively. Crop nitrogen deficiencies occurring before branching were strongly correlated with weed biomass in flowering. Analysis of covariance showed that sowing date, soil management and the quantity of organic manure application may have significantly affected the nitrogen nutrition index, nitrogen uptake by plants and ratio of weed biomass: a late sowing date and shallow soil tillage were related to a low crop nitrogen nutrition index and high ratio of weed biomass. From a practical point of view, this study demonstrated that it should be possible to reduce weed competition by ensuring that the amount of nitrogen in soil before sowing is around 100 kg/ha