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RESEARCH PRODUCT

Ex ante evaluation of cropping systems for weed-mediated pests and environmental benefits with simulation–based indicators

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SPEEAGESTADAGROSUP; International audience; Integrated crop protection tolerates residual weed floras if they are not harmful for crop production. These weeds can host harmful crop pests, among which parasitic plants such as broomrape (Phelipanche ramosa). They can also contribute to reduce soil erosion as well as nitrate and pesticide leaching. To evaluate these weed impacts of management practices ex ante, we developed indicators for the weed dynamics model FLORSYS (Colbach et al., 2014) and then used the model to predict weed-mediated broomrape risk and environmental benefits in cropping systems from five French regions. Materials and Methods FLORSYS is a virtual field on which cropping systems can be tested. It predicts indicators of weed impact on biodiversity and production (Mézière et al., 2014). Here, several new indicators were developed (Table 1). For instance, for the potential weed contribution to reduce soil erosion, the potential rain interception by weeds is calculated each dayby summing the relative light interception (as a proxy for rain interception) for each plant p of each weed species. The daily interception Icropd by crop plants is calculated on the same principle. For a given cropping season, the indicator value Ierosion is the sum of days from crop harvest h to harvest h+1 with Iweedd > 0.1 and Icropd < 0.2. Then, 246 arable cropping systems from five French regions (Aquitaine Burgundy, Lorraine, Paris Basin, Poitou-Charentes) were simulated over 27 years and repeated 10 times with randomly chosen regional weather series.Results and Discussion Antagonisms and synergies between weed-impact indicated were analysed with Pearson correlation coefficients (Table 1). For instance, weed-mediated broomrape was positively correlated to weed-based food offer for bees and carabids, and, to a lesser degree, to vegetal biodiversity, field infestation and weed-mediated environmental benefits. Then, regression trees were used to quantify the effect of cultural practices on weed-impact indicators (Fig. 1).Conclusions There tended to be an antagonism between weed-mediated environmental benefits and biodiversity on one hand, weed-mediated pests and harmfulness on the other hand. Additional analyses and simulations will be necessary to design innovative cropping systems that reconcile high weed benefits with low harmfulness.