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RESEARCH PRODUCT

Combining expert knowledge and models in participatory workshops with farmers to design sustainable weed management strategies

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International audience; In order to design cropping systems reconciling cropproduction, biodiversity and reduced herbicide use, weorganized participatory workshops with farmers in theChampagne region (North-Eastern France). Methodscombined cropping-system prototyping by farmers, expertopinion and models. In a first meeting, farmers determinedtheir objectives and constraints, and chose areference system existing in one of their farms (oilseedrape/winter wheat/winter wheat/spring barley heavilyinfested by autumnal grass weeds). In a secondmeeting, two sets of prototypes were designed by twoseparate groups, using the Mission Ecophyt’eau® tool(https://ecophytopic.fr/pour-aller-plus-loin/outil-mission-ecophyteaur) as support.The reference and the prototypes were evaluated interms of (1) technical feasability from farmers’ andscientists’ expertise, (2) weed harmfulness for cropproduction and weed contribution to biodiversity withFlorSys which simulates crop and weed growth and reproductionin cropping systems at a daily time step overseveral years (Colbach et al., Weed Res 2014), and (3)economic, social and environmental sustainability withthe DEXiPM (Pelzer et al, Ecol Indicators 2012) model.Steps 2 and 3 were carried out after the meeting. Ata third meeting, these results were presented to thefarmers who continued improving the prototypes, usingthe DeciFlorSys model which includes decision treesto guide farmers during cropping-system design and afast calculator estimating weed harmfulness and biodiversityof cropping system prototyes (Colas et al., Eur JAgron 2020). Both prototypes presented increased cropdiversification, introducing legumes and other broadleavedcrops, crop mixtures and cover crops. PrototypeA was based on reduced tillage and annual crops only.Prototype B also included multi-annual lucerne andoptimised both tillage and mechanical weeding. Bothprototypes reduced weed harmfulness for crop production(yield loss, harvest contamination, field infestation)and herbicide use intensity (approx. -60%) comparedto the reference system. Prototype B also improvedweed-based trophic resources for carabids and pollinators.The best solution was a suboption of prototypeB replacing most of the herbicides by mechanicalweeding, dividing yield loss by nearly two comparedto the reference and improving biodiversity by 5-10%.The workshop participants appreciated the knowledgeon agroecosystem functioning and the complementarityof models: DeciFlorSys allowed a direct evaluationduring workshops, FlorSys produced a detailed diagnosisof the technical and meteorological causes of thecropping systems’ performance and DEXiPM assessedworking times and economic viability of the prototypes.Following the workshops, some participants investedin new implements for mechanical weeding and introducedmore spring crops into their rotations.