Delphine Mézière

Ecological Intensification Through Pesticide Reduction: Weed Control, Weed Biodiversity and Sustainability in Arable Farming

Amongst the biodiversity components of agriculture, weeds are an interesting model for exploring management options relying on the principle of ecological intensification in arable farming. Weeds can cause severe crop yield losses, contribute to farmland functional biodiversity and are strongly associated with the generic issue of pesticide use. In this paper, we address the impacts of herbicide reduction following a causal framework starting with herbicide reduction and triggering changes in (i) the management options required to control weeds, (ii) the weed communities and functions they provide and (iii) the overall performance and sustainability of the implemented land management options. The three components of this framework were analysed in a multidisciplinary project that was conducted on 55 experimental and farmer’s fields that included conventional, integrated and organic cropping systems. Our results indicate that the reduction of herbicide use is not antagonistic with crop production, provided that alternative practices are put into place. Herbicide reduction and associated land management modified the composition of in-field weed communities and thus the functions of weeds related to biodiversity and production. Through a long-term simulation of weed communities based on alternative (?) cropping systems, some specific management pathways were identified that delivered high biodiversity gains and limited the negative impacts of weeds on crop production. Finally, the multi-criteria assessment of the environmental, economic and societal sustainability of the 55 systems suggests that integrated weed management systems fared better than their conventional and organic counterparts. These outcomes suggest that sustainable management could possibly be achieved through changes in weed management, along a pathway starting with herbicide reduction.

Using a sensitivity analysis of a weed dynamics model to develop sustainable cropping systems. II. Long-term effect of past crops and management techniques on weed infestation

Both scientists and farmers are confronted by a similar question: which current and past cropping system components will influence the present weed flora, and how? This information is necessary to optimize both cropping systems for weed control, and quality and cost in surveys and monitoring schemes. The present study addressed these questions with a sensitivity analysis to input variables of a cropping system model, AlomySys , that predicts weed dynamics in interaction with pedo-climatic conditions. The study ranked cropping system components according to their impact on weed infestation in winter wheat, showing for instance that though crop succession was crucial, current and past tillage strategies influenced grass weed densities even more. Crops were not only ranked as a function of the resulting weed risk but the latter was also linked to crop species traits, i.e. crop type, usual sowing period and emergence speed. A previous winter v. spring crop thus increased weed density by 72% in the following winter wheat; a late-sown v. early sown winter crop by 26%, a slow v. fast-emerging winter crop by 17%, and a lower competitive ability by 9%. Similarly, the characteristics of each crop management technique (tillage, catch crop, secondary crop, mowing, mechanical weeding, herbicides, nitrogen fertilizer, manure and harvest) were quantified. For instance, the timing of the first tillage operation was crucial prior to the analysed winter wheat crop while the choice of the tool used even 5 years previously still influenced weed infestation in the current year; a catch crop prior to previous spring sown crops reduced the current infestation regardless of catch crop sowing dates and densities, but the reductive effect could be lost if the field was tilled several times to destroy the catch crop. The advice synthesized here and in a companion paper (Colbach & Meziere 2012 ). will be valuable to design innovative, integrated cropping systems, indicating (1) which cropping system components to modify to produce the largest effect, (2) for how long past practices must be considered when choosing current options and (3) the optimal options for the different management techniques. Points (1) and (2) are also valuable to identify data to record in surveys, though still resulting in a total of 232 variables. In a second step, these detailed variables were therefore simplified and aggregated to determine a smaller set of 22 synthetic variables easily recorded in surveys, such as the proportion of winter and spring crops during the last 10 years (instead of the actual crop sequence), the proportion of crops sown in summer, early autumn, late autumn, early spring and late spring during the last 5 years (instead of exact sowing dates), the ploughing frequency (instead of ploughing dates and characteristics), the mean number of herbicide sprayings per year (instead of dates), etc. This reduced survey list will reduce the cost of surveys as well as increase the number and quality of surveys as more farmers will be ready to participate and there will be fewer uncertainties in the answers.