Nathalie Colbach

GeneSys: a model of the influence of cropping system on gene escape from herbicide tolerant rapeseed crops to rape volunteers: I. Temporal evolution of a population of rapeseed volunteers in a field

Abstract The aim of model G ene S ys is to rank cropping systems according to their risk of gene escape from genetically modified, herbicide tolerant winter oilseed rape cultivars to rapeseed volunteers. The model integrates the effects of crop succession and crop management at the level of a region. The first part of the model presented in this paper describes the temporal evolution of rapeseed volunteers in a field, using an annual life-cycle comprising stages such as seed bank, seedlings, adult plants, flowers or freshly produced seeds. The relationships between the various stages depend on the crops grown each year and the cultivation techniques (stubble breaking, soil tillage, sowing date and density, herbicides, cutting and harvesting). Parameter values were either deduced from existing models and literature, or estimated from experimental studies and field surveys. The extension of the temporal sub-model to include the genetic evolution of rapeseed volunteers and the spatial dimension is presented in a second paper [Colbach, N., Clermont-Dauphin, C., Meynard, J.M., 2001. Agric. Ecosyst. Environ.].

GeneSys: a model of the influence of cropping system on gene escape from herbicide tolerant rapeseed crops to rape volunteers: II. Genetic exchanges among volunteer and cropped populations in a small region

Abstract The aim of the model G ene S ys is to rank cropping systems according to their risk of gene escape from genetically modified, herbicide tolerant winter oilseed rape cultivars to rapeseed volunteers. The model integrates the effects of crop succession and crop management at the level of a region. The first part of the model presented by Colbach et al. [Colbach, N., Clermont-Dauphin, C., Meynard, J.M., 2001. G ene S ys : a model of the influence of cropping system on gene escape from herbicide tolerant rapeseed crops to rape volunteers. I. Temporal evolution of a population of rapeseed volunteers in a field. Agric. Ecosyst. Environ., in press], describes the temporal evolution of rapeseed volunteers in a field. In this paper, the temporal sub-model was extended to include the evolution of genotype proportions with time and the effect of genotype on herbicide efficacy, on seed and pollen production. The spatial dimension was introduced by modelling demographic and genetic evolution of rapeseed volunteers in all the plots of a region, whether they are cropped fields or uncultivated field margins and waysides. Each year, pollen and seed are exchanged between plots and the importance of these exchanges depends on plot areas and distances, and, in the case of pollen, on flowering dates.

Influence of crop management on take-all development and disease cycles on winter wheat

ABSTRACT Wheat was assessed at four crop growth stages for take-all (Gaeumannomyces graminis var. tritici) in a series of field trials that studied the effects of five wheat management practices: sowing date, plant density, nitrogen fertilizer dose and form, and removal/burial of cereal straw. An equation expressing disease level as a function of degree days was fitted to the observed disease levels. This equation was based on take-all epidemiology and depended on two parameters reflecting the importance of the primary and secondary infection cycles, respectively. Early sowing always increased disease frequency via primary infection cycle; its influence on the secondary cycle was variable. Primary infection and earliness of disease onset were increased by high density; however, at mid-season take-all was positively correlated to the root number per plant, which was itself negatively correlated to plant density. At late stages of development, neither plant density nor root number per plant had any influence on disease. A high nitrogen dose increased both take-all on seminal roots and severity of primary infection cycle but decreased take-all on nodal roots and secondary infection cycle. Ammonium (versus ammonium nitrate) fertilizer always decreased disease levels and infection cycles, whereas straw treatment (burial versus removal of straw from the previous cereal crop) had no influence.

Spatial and temporal stability of weed populations over five years

Abstract The size, location, and variation in time of weed patches within an arable field were analyzed with the ultimate goal of simplifying weed mapping. Annual and perennial weeds were sampled yearly from 1993 to 1997 at 410 permanent grid points in a 1.3-ha no-till field sown to row crops each year. Geostatistical techniques were used to examine the data as follows: (1) spatial structure within years; (2) relationships of spatial structure to literature-derived population parameters, such as seed production and seed longevity; and (3) stability of weed patches across years. Within years, densities were more variable across crop rows and patches were elongated along rows. Aggregation of seedlings into patches was strongest for annuals and, more generally, for species whose seeds were dispersed by combine harvesting. Patches were most persistent for perennials and, more generally, for species whose seeds dispersed prior to expected dates of combine harvesting. For the most abundant weed in the field, the annual, Setaria viridis, locations of patches in the current year could be used to predict patch locations in the following year, but not thereafter. Nomenclature: Amaranthus retroflexus L. AMARE, redroot pigweed; Asclepias syriaca L. ASCSY, common milkweed; Brassica kaber (DC.) L.C. Wheeler SINAR, wild mustard; Chenopodium album L. CHEAL, common lambsquarters; Cirsium arvense (L.) Scop CIRAR, Canada thistle; Elytrigia repens (L.) Nevski AGRRE, quackgrass; Setaria viridis (L.) Beauv. SETVI, green foxtail; Glycine max (L.) Merr., soybean.

Evaluation of cropping systems for management of herbicide-resistant populations of blackgrass (Alopecurus myosuroides Huds.).

Abstract Simplification of cropping systems often leads to an increase in weed populations which require an intensive use of herbicides to maintain populations at an acceptable level. Due to a heavy reliance on herbicides and a lack of cultural control measures, herbicide-resistant blackgrass (Alopecurus myosuroides Huds.) biotypes appeared recently in France. An experiment was conducted to evaluate the effects of different cropping systems on a population of herbicide-resistant blackgrass. Two crop rotations, one consisting exclusively of winter crops and another including spring crops, were assessed over a three-year period. Crop rotation was combined with different cultural practices (mouldboard plough, delayed sowing dates, reduced nitrogen fertiliser applications and effective herbicides on resistant blackgrass). Blackgrass densities decreased in all the cropping systems, but blackgrass control by herbicides was most effective when combined with non-chemical practices. The benefits of the different weed management systems are discussed in relation to their effect on blackgrass density and their cost to the farmer. In our conditions, the introduction of spring crops into the rotation gave the best results, both from an economical and weed management point of view.

Seed mortality in the soil is related to seed coat thickness

Models that quantify the effects of cropping systems on weed dynamics are useful tools for testing innovative cropping systems. In these models, seed mortality in the soil is a key parameter to account for the cumulated effect of cropping systems over time via the soil seed-bank. Since seed mortality is difficult to measure, our objective was to develop a method to estimate it from easily accessible information. Seeds of 13 weed species were buried 30 cm deep in fields and were recovered regularly for 2 years to measure their viability. Seed mass, dimensions, shape, and protein and lipid contents as well as coat thickness were measured. To estimate seed mortality of species not included in the study, we searched for relationships between mortality rates and seed traits. Seed viability mainly decreased during the second year of burial, with mortality rates ranging from 0.01 to 0.63 seeds·seeds 21 ·year 21 , depending on the species. Seed mortality decreased with increasing seed coat thickness. No correlation was found with other measured traits or with seed persistence data in the literature. These results were confirmed when the effects of phylogenetic relatedness with phylogenetically independent contrasts were included. The thickness of the seed coat, which varied between 17 and 231mm over the range of species studied, can protect the seed from external attacks in the soil and slow down seed decay. This trait can be easily measured via X-ray images and could be used to estimate the seed mortality rate for a wider range of species.

Describing and locating cropping systems on a regional scale. A review.

At regional scale issues such as diffuse pollution, water scarcity and pollen transfer are closely related to the diversity and location of cropping systems because agriculture interacts with many other activities. Although sustainable land use solutions for territorial development and natural resource management are needed, very few agro-environmental studies account for both the coherence and the spatial variability of cropping systems. The originality of this article is to review methods that describe and locate cropping systems within large areas. We mainly based our analysis on four case studies using the concept of cropping systems on a regional scale, but differing in their objectives and extents. We found that describing and locating cropping systems in space meets not only decision-making stakes but also a scientific stake that allows multi-simulations over large areas when models require cropping system information. Simulation models are indeed necessary when the study aims at estimating cropping system externalities. Then, the involved process determines the extent, and the model determines the support unit, unless socio-economic considerations prevail. In this case, as well as when no model is involved, it is often considerations related to stakeholders that determine extent and support unit choices. On a regional scale, the cropping system must be described by only a few variables whose selection depends on the study objective and the involved processes. Collecting cropping system information for all support units is often simplified by identifying determining factors of cropping systems. However, obtaining deterministic relations between easily accessible factors and cropping system variables is not always possible, and sometime accessing modalities of determining factors for all support units is also difficult. We found that describing and locating cropping systems relied very much on expertise and detailed survey data. The development of land management practice monitoring would facilitate this description work.

Modelling vertical and lateral weed seed movements during mouldboard ploughing with a skim-coulter

Abstract The vertical distribution of weed seeds in soil is crucial because seedling emergence varies with seed depth, whereas lateral soil displacement during mouldboard ploughing contributes to weed dispersal within the tilled field. In order to model vertical and lateral seed displacements during ploughing, an existing model describing soil particle movements for different ploughing characteristics (depth and width) and soil structures was adapted to integrate the effect of a skim-coulter. This model was tested in two field trials, in Northern France, using coloured plastic beads to imitate weed seeds. The trial in Dijon was set up on an eutric cambisol and comprised both compacted and uncompacted soil. The second trial was set up at Grignon, on an orthic luvisol which was left uncompacted before ploughing. The model correctly simulated the lateral displacement (LD) and the final vertical co-ordinate of the beads as a function of their initial location, soil structure before ploughing and ploughing parameters (ploughing depth and width; skim-coulter depth and width). The model was then used to calculate seed transfer matrixes describing vertical seed movements between seed bank layers and vertical seed distributions for different conditions and plough modes. The results were consistent with those of Cousens and Moss [Weed Res. 30 (1990) 61–70]. Simulations were performed to test the effect of different ploughing modes on the time changes in the vertical distribution of weed seeds and to show how the model can be used to manage weed seed concentration in the top layer by soil tillage. Furthermore, simulations showed that the vertical distribution of the seed bank could be extremely variable, depending on plough characteristics, soil structure or initial seed distribution. Although further studies are needed on the long-term seed movements under the influence of secondary tillage and climate, this model can be useful for evaluating different tillage modes on seed dispersal within the ploughed layers.

Modelling vertical and lateral seed bank movements during mouldboard ploughing

Abstract The vertical distribution of weed seeds in the soil is of fundamental importance because seedling emergence depends on seed depth. The lateral displacement of the earth during mouldboard ploughing contributes to the dispersal of the weeds inside the tilled field. In order to model vertical and lateral seed displacements during ploughing, an existing model describing soil particle movements for different ploughing characteristics (depth and width) and soil structures was tested on a multilocal field trial. The trials were carried out in 1996 and 1997 and comprised two soil textures and three soil structures; tillage was performed with a mouldboard plough at varying ploughing widths and depths. Seeds were simulated by beads that were introduced immediately before ploughing with an auger at different depths and lateral positions (relative to the future passage of the coulter) within and just below the ploughed horizon. Lateral displacement and the final vertical position of the beads were measured and compared to the simulations obtained with the model. The model correctly simulated the final vertical seed co-ordinate and lateral seed displacement as a function of soil structure, ploughing width and depth and initial seed position, if ploughing depth is lower than ploughing width. If, however, the former exceeds the latter and/or if the furrows are not properly rotated, the model does not simulate the seed movements correctly. The model was then used to calculate seed transfer matrices describing vertical seed movements between seed bank layers for different conditions and plough modes and to determine the optimal ploughing mode for a given seed bank distribution. For instance, if most seeds are located in the top layer ploughing should be as deep as possible, with a low depth to width ratio to maximise soil inversion and seed burial. If, however, the seeds are concentrated in the bottom layer, the model can be used to decide how shallowly to plough in order to avoid disturbing the deeper seeds and what ploughing width to associate to this depth in order to minimise soil inversion and leave as many seeds as possible undisturbed. Ways of improving the model are suggested, particularly the necessity to simulate the effect of a skim coulter.

How to model and simulate the effects of cropping systems on population dynamics and gene flow at the landscape level: example of oilseed rape volunteers and their role for co-existence of GM and non-GM crops

Background, aim and scopeAgricultural landscapes comprise cultivated fields and semi-natural areas. Biological components of these compartments such as weeds, insect pests and pathogenic fungi can disperse sometimes over very large distances, colonise new habitats via insect flight, spores, pollen or seeds and are responsible for losses in crop yield (e.g. weeds, pathogens) and biodiversity (e.g. invasive weeds). The spatiotemporal dynamics of these biological components interact with crop locations, successions and management as well as the location and management of semi-natural areas such as roadverges. The objective of this investigation was to establish a modelling and simulation methodology for describing, analysing and predicting spatiotemporal dynamics and genetics of biological components of agricultural landscapes. The ultimate aim of the models was to evaluate and propose innovative cropping systems adapted to particular agricultural concerns. The method was applied to oilseed rape (OSR) volunteers playing a key role for the coexistence of genetically modified (GM) and non-GM oilseed rape crops, where the adventitious presence of GM seeds in non-GM harvests (AGMP) could result in financial losses for farmers and cooperatives.Material and methodsA multi-year, spatially explicit model was built, using field patterns, climate, cropping systems and OSR varieties as input variables, focusing on processes and cultivation techniques crucial for plant densities and pollen flow. The sensitivity of the model to input variables was analysed to identify the major cropping factors. These should be modified first when searching for solutions limiting gene flow. The sensitivity to model processes and species life-traits were analysed to facilitate the future adaptation of the model to other species. The model was evaluated by comparing its simulations to independent field observations to determine its domain of validity and prediction error.ResultsThe cropping system study determined contrasted farm types, simulated the current situation and tested a large range of modifications compatible with each farm to identify solutions for reducing the AGMP. The landscape study simulated gene flow in a large number of actual and virtual field patterns, four combinations of regional OSR and GM proportions and three contrasted cropping systems. The analysis of the AGMP rate at the landscape level determined a maximum acceptable GM OSR area for the different cropping systems, depending on the regional OSR volunteer infestation. The analysis at the field level determined minimum distances between GM and non-GM crops, again for different cropping systems and volunteer infestations.DiscussionThe main challenge in building spatially explicit models of the effects of cropping systems and landscape patterns on species dynamics and gene flow is to determine the spatial extent, the time scale, the major processes and the degree of mechanistic description to include in the model, depending on the species characteristics and the model objective.ConclusionsThese models can be used to study the effects of cropping systems and landscape patterns over a large range of situations. The interactions between the two aspects make it impossible to extrapolate conclusions from individual studies to other cases. The advantage of the present method was to produce conclusions for several contrasted farm types and to establish recommendations valid for a large range of situations by testing numerous landscapes with contrasted cropping systems. Depending on the level of investigation (region or field), these recommendations concern different decision-makers, either farmers and technical advisors or cooperatives and public decision-makers.Recommendations and perspectivesThe present simulation study showed that gene flow between coexisting GM and non-GM varieties is inevitable. The management of OSR volunteers is crucial for containing gene flow, and the cropping system study identified solutions for reducing these volunteers and ferals in and outside fields. Only if these are controlled can additional measures such as isolation distances between GM and non-GM crops or limiting the proportion of the region grown with GM OSR be efficient. In addition, particular OSR varieties contribute to limit gene flow. The technical, organisational and financial feasibility of the proposed measures remains to be evaluated by a multi-disciplinary team.