The role of models in management and conservation of weeds

Weedsare both a harmful croppest and an important component of biodiversity. Moreover, herbicide use must be reduced to limit its impact on environment, and weedcontrol must now combine numerous management techniques with partial efficiency aiming at preventing weedoccurrence. Biological regulations of weedsby other biotic componentscould also contribute to control infestations. Thus, new cropping systemsare needed, combining numerous techniques and aiming at both maximising weed-related biodiversity and minimising weedharmfulness.[br/] Weeddynamics models are increasingly used to design innovative cropping systemsbut usually only consider weeddensities and cropyield. The objective of the present work was to illustrate with an existing model how these models can be improved to (1) integrate new knowledge and management techniques, (2) integrate biotic interactions and to assess weed-related harmfulness and biodiversity, and then (3) used to evaluate and design innovative cropping systems.[br/] The FLORSYS model is to date the only multispecific weeddynamics model that integrates the effect of all cropping systemcomponents ( cropsuccession, all management techniques) in interaction with pedoclimate. It is a mechanistic (i.e. process-based) model which synthesizes data from different experiments and teams and easily evolves to integrate new knowledge or management techniques. For instance, additional processes (e.g. the blocking of weedseed rain to soil seed banksby permanent grass canopies) were recently added to FLORSYS to adapt it to temporary grassland in arable croprotations (Doisy et al., this conference). These complex models are though more difficult to validate with field observations, a step still underway for FLORSYS.[br/] To integrate biotic interactions with other organism in weedmodels, new submodels are needed to quantify the effect (1) of cropping systemson the new organism (here the fungus [i] Gaeumannomyces graminis[/i] var. [i]tritici[/i] responsible for the take-all disease in cereals), (2) of the new organism on weeds(here the decrease in seed production of diseased weedplants) and (3) of weedson the new organism (here pathogen propagation by diseased weedplants). To assess weed-related harmfulness and biodiversity in cropping systems, the predicted weeddensities must be translated into indicators. In the present work, five harmfulness indicators ( cropyield loss, technical harvest problems, harvest pollution, field infestation and additional cropdisease incidence caused by fungi-transmitting weedspecies) and five biodiversity indicators( speciesrichness, species equitability, seed resource for birds and insects, pollen/nectar resource for pollinators) were constructed and connected to FLORSYS (Meziere et al., this conference). These models are interesting for a large range of applications. At short-term, they can be used to optimize individual management techniques in different weedflora contexts, e.g. FLORSYS was used to evaluate different cropsowing strategies (sowing densities and patterns, cropassociations etc.). More interestingly, these models can simulate existing cropping systemsover several years and with different climate scenarios to test their long-term and climatic robustness. FLORSYS was thus used to evaluate a large range of cropping systemsidentified in farm surveys and to identify cultural practices pertinent for controlling weedharmful and preserving biodiversity. However, the major interest of models is to test prospective cultural techniques and cropping systems. FLORSYS is now used to evaluate the changes in agricultural practices (e.g. simplified tillage and rotations, no-till, temporary crops, Colbach et al., this conference). Currently, our team is working on a simulation-based methodology to design prospective cropping systemswith low herbicide use, minimising weed-related harmfulness and maximising biodiversity.