Sensitivity of weed emergence and dynamics to life-traits of annual spring-emerging weeds in contrasting cropping systems, using weed beet (Beta vulgarisssp.vulgaris) as an example
Authors: COLBACH N. , CHAUVEL B. , DARMENCY H. , TRICAULT Y. 
Published: March 2011

Cropping systems contain a diverse multi-species weed flora including several species that cross-breed with and/or descend from crops, including weed beet (Beta vulgarisssp.vulgaris). The effects of cropping systems on this weed flora are complex because of their large range of variation and their numerous interactions with climate and soil conditions. In order to study and quantify the long-term effects of cropping system components (crop succession and cultural techniques) on weed population dynamics, a biophysical process-based model called GENESYS-Beet has previously been developed for weed beet. In the present paper, the model was modified to remove the crop–weed connection and employed to identify and rank the weed life-traits as a function of their effect on weed emergence timing and density as well as on weed densities at plant, adult and seed bank stages, using a global sensitivity analysis to model parameters. A similar method has already been used with the complete GENESYS-Beet model (i.e. including the crop–weed connection) based on Monte Carlo simulations with simultaneous randomization of all life-trait parameters and run in three cropping systems differing in their risk of infestation by weed beet. Simulated weed emergence timing and density, as well as surviving plant, adult and seed bank densities, were then analysed with regression models as a function of model parameters to rank life-cycle processes and related life-traits and quantify their effects. The comparison of the present, crop-independent results to those of the previous, crop-dependent study showed that the crop-relative weed beet can be considered as a typical crop-independent spring weed as long as no traits conferring a selective advantage are inherited and in rotations where crops favouring weed emergence and reproduction are frequent. In such rotations, advice for controlling the crop-relative and the crop-independent weed is more or less identical. The rarer these favourable crops, the more important pre-emergence processes become for the crop-independent weed; management advice should thus focus more on seed bank survival and seedling emergence. For the crop-relative, post-emergence processes become dominant because of the increasing necessity for a new population founding event; management advice should mostly concern the avoidance of crop bolters. In both studies, the key parameters were more or less the same, i.e. those determining the timing and success of growth, development, seed maturation and the physiological end of seed production. Timing parameters were usually more important than success parameters, showing for instance that optimal timing of weed management operations is often more important than its exact efficacy. Comparison with previous sensitivity analyses carried out for autumn-emerging weed species showed that some of the present conclusions are probably specific to spring-emerging weed species only. For autumn-emerging species, pre-emergence traits would be more important. In the rotations with frequent favourable crops and insufficient weed control, interactions between traits were small, indicating that diverse populations and species with contrasting traits could prosper, potentially leading to a diverse multi-species weed flora. Conversely, when favourable crops were rare and weed control optimal, traits had little impact individually, indicating that a small number of optimal combinations of traits would be successful, thus limiting both intra- and inter-specific variability.