Relative fitnes of herbicide-resistant and susceptible biotypes of weeds

Au cours des dernières années, il y a eu une augmentation rapide du nombre de mauvaises herbes (plus de 100) signalées comme étant résistantes aux herbicides, de même qu'une augmentation du nombre de groupes d'herbicides auxquels la résistance a évolué. Cet article fait état des données qui suggèrent l'existence de différences d'aptitude entre les biotypes résistants et sensibles aux herbicides. Des estimés de l'aptitude sont nécessaires afin d'établir des modèles de population fiables. Ces estimés permettent de prévoir le potentiel de succès évolutif d'un génotype basé sur sa survie, sa compétitivité et en dernier lieu, sur son succès reproductif. Les différences d'aptitude entre les biotypes résistants et sensibles sont généralement dérivées des mesures de productivité relative ou de compétitivité. Pour les mauvaises herbes résistantes aux triazines, des études ont démontré que les plantes résistantes étaient généralement moins compétitives que les plantes sensibles, malgré la présence de certaines exceptions. Bien qu'il y ait moins de données disponibles sur l'aptitude des plantes résistantes aux groupes herbicides autres que les triazines, cet article résume l'information disponible sur les sulfonylurées, les urées substituées, les dinitroanilines, le paraquat, le diclofop et les arsenicaux organiques. Aucune différence d'aptitude constante n'a été observée pour les biotypes résistants et sensibles aux herbicides autres que les triazines. En général, les études ont démontré que l'aptitude relative des biotypes sensibles et résistants d'une espèce donnée dépend des conditions biologiques (incluant la variation à l'intérieur du génotype et de la populationet la compétition inter- et intra-biotypes), et des conditions environnementales telles la température, la qualité lumineuse et les pratiques de gestion.In recent years, there has been a rapid increase in the number of reported cases of herbicide-resistant weed species (over 100), as well as an increase in the types of herbicides to which resistance has evolved. This paper reviews evidence for differential fitness of herbicide-resistant and susceptible biotypes. Fitness estimates are required to produce reliable population models. Fitness measures describe the potential evolutionary success of a genotype based on survival, competitive ability and ultimately reproductive success. Differences in relative fitness between resistant and susceptible biotypes are usually inferred from measures of relative plant productivity or competitiveness. For triazine-resistant weed species, studies have indicated that resistant plants were generally less fit than susceptible plants, although exceptions did exist. Although less data are available on the fitness of plants resistant to non-triazine herbicides, information is summarized for sulfonylureas, substituted ureas, dinitroanilines, paraquat, diclofop, and organic arsenicals. No consistent differences in relative fitness were observed for non-triazine resistant and susceptible biotypes. In general, studies have indicated that the relative fitness of susceptible and resistant biotypes of a single species depends upon biological conditions, including genotype and population variation, intra- and inter-biotype competition, and environmental conditions such as temperature, light quality, and management practices. Future needs for relative fitness studies are discussed

Transgenic crops : new weed problems for Canada?

Over 25 000 transgenic field trials were conducted globally from 1986-1997, and many transgenic crops, including soybean (Glycine max), maize (Zea mays), tobacco (Nicotiana tabaccum), cotton (Gossypium hirsutum), canola (Brassica napus, B. rapa), tomato (Lycopersicon esculentum) and potato (Solarium tuberosum) have been commercially released. There has been a high adoption rate, with at least 28 million ha reported for 1998, with herbicide- and insect-resistant plants occupying 71 and 28% of the releases, respectively. The current status of commercial production of transgenic crops in Canada is summarized. Transgenic crops have the potential to change weed communities/populations in three principal ways, via: 1 ) escape and proliferation of the transgenic plants as 'weedy' volunteers with subsequent displacement of the crop, weed and/or natural vegetation; 2) hybridization with and transgene infiltration into related weedy and/or wild species, resulting in invigorated weeds and/or alteration of natural gene frequencies in these species; and 3) genetic changes in populations of unrelated species, as a result of changes to the environment, in particular herbicide-resistant (HR) transgenic crops and the development of HR weeds. Potential risk can be estimated a priori using knowledge of the systematics of crop/wild/weed complexes. Risk must be assessed on a case-by-case basis for each crop, each country/ecological region, and for each trait. Potential weed risks will be greater if crop volunteers are predisposed to becoming weedy, are well adapted to the Canadian climate and if sexually compatible wild species are present

Control of gene-stacked canola by alternative herbicides

Non-Peer ReviewedUnintentional herbicide resistance gene stacking in canola may alter the sensitivity of volunteers to herbicides of alternative modes of action commonly used for their control. Greenhouse experiments were conducted to investigate the dose response of three single herbicide-resistant (HR) cultivars (glyphosate, glufosinate, imidazolinone), one non-HR cultivar, and seven multiple (double or triple) HR experimental lines treated at the two- to three-leaf stage to 2,4-D (amine and ester), MCPA ester, and metribuzin; and of one non-HR and four HR cultivars (glyphosate, glufosinate, imidazolinone, bromoxynil) to 2,4-D amine applied at two growth stages (two to three, and five to six leaves). All canola cultivars or lines treated at the two- to three-leaf stage responded similarly to increasing doses of the three herbicides. At the five- to six-leaf stage, however, the bromoxynil HR cultivar was less sensitive to 2,4-D than the other cultivars. The results of this study suggest that canola with multiple herbicide resistance traits does not differ from cultivars that are non-HR or single HR in its sensitivity to herbicides commonly used to control volunteers. All volunteers, whether non-HR, single HR, or multiple HR, should be treated when plants are most sensitive to herbicides (two- to four-leaf stage) to reduce their interference against crops and their perpetuation of gene flow

A decade of herbicide-resistant crops in Canada

Non-Peer ReviewedThis review examines some agronomic, economic, and environmental impacts of herbicide-resistant (HR) canola, soybean, corn, and wheat in Canada after 10 years of growing HR cultivars. The rapid adoption of HR canola and soybean suggests a net economic benefit to farmers. HR crops often have improved weed management, greater yields or economic returns, and similar or reduced environmental impact compared with their non-HR crop counterparts. There are no marked changes in volunteer weed problems associated with these crops, except in zero-tillage systems when glyphosate is used alone to control canola volunteers. Although gene flow from glyphosate-HR canola to indigenous populations of bird’s rape in eastern Canada has been measured, enrichment of hybrid plants in such populations should only occur when and where herbicide selection pressure is applied. Weed shifts as a consequence of HR canola have been documented, but a reduction in weed species diversity has not been demonstrated. Reliance on HR crops in rotations using the same mode-of-action-herbicide and/or multiple in-crop herbicide applications over time can result in intense selection pressure for weed resistance and consequently, greater herbicide use in the future to control HR weed biotypes. History has repeatedly shown that cropping system diversity is the pillar of sustainable agriculture; stewardship of HR crops must adhere to this fundamental principle

Herbicide-resistant crops – new weed problems for farmers?

Non-Peer ReviewedWith the widespread adoption of herbicide-resistant (HR) crops, research is increasing required to evaluate their impact on agroecosystems. These crops will solve some weed problems for farmers, but will create new ones as well. Potential benefits and risks of HR crops are described. HR crops have the potential to change weed communities via escape and proliferation as ‘weedy’ volunteers, hybridization with and resistance gene infiltration into related weedy relatives, and increased selection for evolution of resistance due to frequent herbicide group use. Controlling HR crop volunteers in the crop rotation will require careful planning, particularly those plants with multiple resistance due to pollen flow. Sound agronomic practices and good record keeping is required by farmers to effectively manage these new potential weed problems

Sampling strategies to evaluate the status of offshore soft sediment assemblages

Reliable descriptions of the status of offshore seabed habitats usually require substantial investment in field data collection and sample analysis. While assessment of, for example, biogenic reef habitat can often include simple physical parameters (e.g. spatial extent), comparative measures for soft sediment habitats generally rely on the distribution and relative abundance of species, with a description of the associated sedimentary environment. To investigate the power of surveys to detect significant trends in assemblage structure, samples of meiofauna, macroinfauna and megafauna (i.e. representing ecological components from nematodes to demersal fish), were collected from four offshore mud and sand habitat sites in western UK shelf seas during July 2004 and 2005. Spatial arrays of samples within these sites, up to 23 km apart, were designed to optimise descriptions of assemblage structure and the patterns of spatial distribution at a local scale. Analyses of species abundance, biomass and taxonomic relatedness of the species complement at each site suggested that most assemblages represented relatively unimpacted regional conditions. The power of the sampling programme to detect a significant change in univariate community attributes was assessed. The variability in many of the community attributes indicated that intensive replicate sampling would be required to detect ecologically important changes. Improving the power of such benthic surveys to detect trends would therefore require substantial additional time and effort to be invested in sample collection and analysis. Resource analysis showed that the time from gear deployment to complete sample identification was gear-dependent, lowest per sample for meiofauna (10 h) and megafauna (6-12 h), and highest for macroinfauna (12-22 h). These results have implications for the development of meaningful indicators of habitat status for offshore soft sediment habitats, and the resources required for effective monitoring of change