Diversifying cereal-based rotations to improve weed control. Evaluation with the AlomySys model quantifying the effect of cropping systems on a grass weed

Simplified rotations often select weed flora consisting of one or several dominant species. In rotations consisting mainly of winter cereals, one of the most frequent weeds in Atlantic European countries is blackgrass ( Alopecurus myosuroides Huds.). In order to reduce environmental impacts and avoid the selection of herbicide-resistant populations, alternative weed management strategies are necessary. The objective of the present study was to develop a methodology for using a weed dynamics model called ALOMYSYS for evaluating prospective diversified crop rotations based on expert opinion. These prospective rotations were developed for a particular region aiming at reducing herbicide use while keeping weed infestation similar to that in current cropping systems. The prospective systems were also evaluated economically by calculating costs and margins for the farmer. The simulations showed that the more diverse the rotation, the better blackgrass was controlled and the less herbicides (rates and frequencies) were necessary. Optimal herbicide spraying conditions and mouldboard ploughing were also less essential in diverse rotations. It was though essential to reason herbicide programs over the whole rotation and not simply as function of the preceding crop. The economic evaluation identified the interest of spring or winter pea either replacing or preceding oilseed rape (OSR) in OSR/wheat/barley rotations.