Anne Légère

Hybridization between transgenic Brassica napus L. and its wild relatives: Brassica rapa L., Raphanus raphanistrum L., Sinapis arvensis L., and Erucastrum gallicum (Willd.) O.E. Schulz

Abstract. The frequency of gene flow from Brassica napus L. (canola) to four wild relatives, Brassica rapa L., Raphanus raphanistrum L., Sinapis arvensis L. and Erucastrum gallicum (Willd.) O.E. Schulz, was assessed in greenhouse and/or field experiments, and actual rates measured in commercial fields in Canada. Various marker systems were used to detect hybrid individuals: herbicide resistance traits (HR), green fluorescent protein marker (GFP), species-specific amplified fragment length polymorphisms (AFLPs) and ploidy level. Hybridization between B. rapa and B. napus occurred in two field experiments (frequency approximately 7%) and in wild populations in commercial fields (approximately 13.6%). The higher frequency in commercial fields was most likely due to greater distance between B. rapa plants. All F1 hybrids were morphologically similar to B. rapa, had B. napus- and B. rapa-specific AFLP markers and were triploid (AAC, 2n = 29 chromosomes). They had reduced pollen viability (about 55%) and segregated for both self-incompatible and self-compatible individuals (the latter being a B. napus trait). In contrast, gene flow between R. raphanistrum and B. napus was very rare. A single R. raphanistrum × B. napus F1 hybrid was detected in 32,821 seedlings from the HR B. napus field experiment. The hybrid was morphologically similar to R. raphanistrum except for the presence of valves, a B. napus trait, in the distorted seed pods. It had a genomic structure consistent with the fusion of an unreduced gamete of R. raphanistrum and a reduced gamete of B. napus (RrRrAC, 2n = 37), both B. napus- and R. raphanistrum-specific AFLP markers, and had <1% pollen viability. No hybrids were detected in the greenhouse experiments (1,534 seedlings), the GFP field experiment (4,059 seedlings) or in commercial fields in Québec and Alberta (22,114 seedlings). No S. arvensis or E. gallicum × B. napus hybrids were detected (42,828 and 21,841 seedlings, respectively) from commercial fields in Saskatchewan. These findings suggest that the probability of gene flow from transgenic B. napus to R. raphanistrum, S. arvensis or E. gallicum is very low (<2–5 × 10–5). However, transgenes can disperse in the environment via wild B. rapa in eastern Canada and possibly via commercial B. rapa volunteers in western Canada.

A decade of herbicide-resistant crops in Canada

This review examines some agronomic, economic, and environmental impacts of herbicide-resistant (HR) canola, soybean, corn, and wheat in Canada after 10 yr 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 wild populations of bird’s rape (Brassica rapa L.) 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. However, reli...

The Frequency and Persistence of Volunteer Canola (Brassica napus) in Quebec Cropping Systems

The presence of volunteer canola is becoming a significant agro-ecological concern, given the large-scale use of herbicide-tolerant varieties in some areas. Our goal was to estimate the frequency and persistence of volunteer canola in Québec cropping systems by surveying fields that included a single canola crop since 1995. A survey was conducted in 131 fields in the main canola-growing areas of Québec: in the Saguenay-Lac Saint-Jean region and the Québec City–La Pocatière area. Volunteer canola plants were counted in 0.25-m2 quadrats every 10 m along a W pattern, and every 15 m along the margins of 88 fields. Volunteer canola plants were found in 90% of the fields surveyed and in a wide range of crops, including cereal, corn, and soybean. Average densities of 4.9 and 3.9 plants/m2 were found 1 yr after canola production in fields and field margins, respectively. Volunteer canola densities decreased significantly over time. However, volunteer plants were still present at low densities 4 and 5 yr after production. Dense stands of volunteer canola were found before postemergence herbicide application in no-till fields (9.8 ± 4.1 plants/m2), suggesting that, contrary to what was suggested in the literature, seeds could become dormant in no-till as well as in tilled systems. A small proportion of the volunteer canola plants observed in no-till fields near Québec City and Ottawa included plants that had overwintered, either originating from fall-germinated seedlings, harvested adult plants that had grown new leaves before the onset of winter, or spring regrowth from the base of unharvested adult plants from experimental plots. The presence and persistence of low densities of volunteer canola may not have been a cause of concern until now. However, producers should be made more aware of the potential short-and long-term problems associated with potential gene flow between different herbicide-tolerant canola (HT canola) varieties and also between HT canola and related weed species. Nomenclature: Canola, Brassica napus L.; corn, Zea mays L.; soybean, Glycine max (L.) Merr. Additional index words: Herbicide-tolerant canola, oilseed rape, weed survey. Abbreviations: HT, herbicide-tolerant (canola).

Symposium A multistudy approach to understanding weed population shifts in medium- to long-term tillage systems

Abstract Production systems based on reduced-tillage practices account for over 60% of the cropped land on the Canadian Prairies. Concerns have been expressed regarding potential shifts in weed communities as a result of changing tillage practices. Study objectives were to (1) determine the feasibility of combining and analyzing weed abundance data from 10 medium- to long-term studies on the Canadian Prairies that compared conventional-, reduced-, and zero-tillage systems, (2) identify species that are associated with specific tillage systems, and (3) place species into plant response groups according to the similarity of their tillage system response. Conventional-tillage systems were defined as including both a fall and spring sweep-plow operation before seeding spring crops, whereas reduced tillage consisted of only one sweep-plow operation shortly before seeding. Crops within zero-tillage systems were planted directly into the previous crops stubble. The association between weed species and tillage systems was investigated using indicator species analysis. Species were assigned to tillage response groups on the basis of the results of the analysis and the expertise of the project scientists. Perennial species such as Canada thistle and perennial sowthistle were associated with reduced- and zero-tillage systems, but annual species were associated with a range of tillage systems. Field pennycress was placed in the conventional-tillage response group, Russian thistle in the zero-tillage group, and wild buckwheat and common lambsquarters were equally abundant in all tillage systems. The goal of classifying weed species based on common functional traits in relation to responses to tillage systems was not realized, in part, because the required information on species biology and ecology was either unavailable or not applicable to local conditions. Nomenclature: Canada thistle, Cirsium arvense (L.) Scop. CIRAR; common lambsquarters, Chenopodium album L. CHEAL; field pennycress, Thlaspi arvense L. THLAR; perennial sowthistle, Sonchus arvensis L. SONAR; Russian thistle, Salsola iberica Sennen & Pau SASKR; wild buckwheat, Polygonum convolvulus L. POLCO.

Survey of Management Practices Affecting the Occurrence of Wild Oat (Avena fatua) Resistance to Acetyl-CoA Carboxylase Inhibitors'

Abstract: A survey conducted across agricultural ecoregions of Saskatchewan in 1996 revealed that wild oat (Avena fatua) populations resistant to acetyl-CoA carboxylase (ACCase) inhibitors were present in approximately 10% of Saskatchewan fields (2.4 million ha). In the Aspen Parkland and Boreal Transition ecoregions, this increased to 17%. The objective of this study was to determine if agronomic practices promoted or delayed resistance and to assess producer awareness of herbicide resistance. Weed resistance and management questionnaire data from the 1996 resistance survey and management questionnaire data from the 1995 Saskatchewan weed survey were submitted to multiway frequency analysis. The frequency of occurrence of herbicide-resistant wild oat was related directly to ACCase inhibitor use. Resistance to cyclohexanedione (CHD) herbicides was not related to CHD use but to frequency of ACCase inhibitor use (i.e., CHD + aryloxyphenoxypropanoate [AOPP]), suggesting that the pressure imposed by AOPPs contributed to the selection of CHD resistance in wild oat. ACCase inhibitor use was more extensive in the Aspen Parkland and Boreal Transition ecoregions than in the Mixed and Moist Mixed Grassland ecoregions. Crop rotations were not conducive to rotation of herbicides with different sites of action. Frequency of ACCase inhibitor use increased with frequency of annual crops, in spite of the inclusion of cereal and dicot crops in the rotation. Producers utilizing conservation tillage practices in the Grassland ecoregions used proportionally more ACCase inhibitors than those using conventional tillage practices. This increase in ACCase use in conservation tillage systems did not result in an increased incidence of wild oat populations resistant to ACCase inhibitors. Producers reporting troublesome wild oat populations tended to have proportionally more ACCase-resistant wild oat. Producers who reported practicing weed sanitation were less likely to have resistant wild oat than those who were less careful. Increased awareness and implementation of management practices that will reduce the dependency on ACCase herbicides are required to better enable producers to prevent, delay, or manage herbicide-resistant wild oat populations. Nomenclature: Wild oat, Avena fatua L. #3 AVEFA. Additional index words: Weed survey, multiway frequency analysis, aryloxyphenoxypropanoate, cyclohexanedione, management of herbicide resistance. Abbreviations: ACCase, acetyl-CoA carboxylase; AOPP, aryloxyphenoxypropanoate; CHD, cyclohexanedione.

Contrasting Responses of Weed Communities and Crops to 12 Years of tillage and Fertilization Treatments

Minimizing inputs such as fertilizers, herbicides, or tillage may be sought by producers to satisfy economic as well as environmental goals. One of the challenges in reducing inputs, whether synthetic fertilizers or herbicides, or substituting a synthetic nutrient with an organic source, is to identify practices that will provide optimum growing conditions for the crop while maintaining an adequate level of weed control. Our objective was to measure the cumulative effects of 12 yr of nitrogen (N) and phosphorus (P) fertilization treatments applied to two tillage systems [conventional tillage (CT) vs. no tillage (NT)] in a corn–soybean rotation on weed communities and crop yields. Residual (postherbicide treatment) weed species assembly was determined by multivariate analysis and was influenced mainly by tillage, with weeds more strongly associated with NT than with CT. Diversity of weed communities as measured by richness, evenness (E), and a diversity index (H′), and total weed biomass were greater for NT than for CT. Nutrient treatments had little or no effect on these parameters. Corn yields were reduced by 70% in the absence of N and by 25% in NT compared to CT treatments. Soybean yields were reduced in NT with increasing P rates compared to other treatments, but reductions never exceeded 10%. Overall, corn and soybean had different responses to treatments, with corn yields being far more affected by fertilization and tillage than soybean yields. Conversely, the absence of tillage had a much greater effect than the absence of nutrient input on weed community assembly and biomass, suggesting the importance of a weed management program specifically tailored for NT systems. Nomenclature: Corn, Zea mays L., soybean, Glycine max (L.) Merr

Symposium Tillage and weed management effects on weeds in barley-red clover cropping systems

Abstract The main study objective was to measure the effects of tillage (moldboard plow, chisel plow, and no-till) and weed management (intensive, moderate, and minimum) on weeds and crops in a spring barley monoculture compared with a spring barley–red clover rotation. The study was initiated in 1987 and conducted at two sites. Residual effects of treatments were measured in a wheat test crop at the loam site in 1994 and at the clay site in 1995–1996. Weed seed bank densities ranged from less than 300 to nearly 30,000 seeds m−2 and plant densities from 30 to 6,000 plants m−2. Seven species were recorded on average per plot aboveground and 10 species per plot in the seed bank. Species number in the seed bank varied little with treatments compared with species numbers aboveground. Crop rotation and tillage had little effect on weed species diversity but affected relative species dominance. The presence and abundance of species was also influenced by their degree of tolerance to the herbicides used in each system. Annual dicots largely dominated in minimum weed management treatments. Their relative importance in each rotation varied with their level of susceptibility to the different postemergence herbicides. Perennials were not exclusively found in reduced tillage systems. The relationship between perennials and tillage was dependent on the response of perennating structures to the type and frequency of soil disturbance. For example, quackgrass dominated in chisel and moldboard plow systems where rhizomes would be frequently fragmented. Field horsetail, also a rhizomatous species, dominated in the monoculture/direct-seeded no-till treatment under minimum weed management. Its absence from the rotation was explained by the regular removal of aboveground biomass during the forage production year. Overall, weed response was regulated by agronomic factors but was largely determined by specific biological attributes and environmental conditions. Nomenclature: Field horsetail, Equisetum arvense L. EQUAR; quackgrass, Elytrigia repens (L.) Nevski AGRRE; barley, Hordeum vulgare L. HORVX; red clover, Trifolium pratense L. TRFPR; wheat, Triticum aestivum L.

The Selective Memory of Weed Seedbanks after 18 Years of Conservation Tillage

A conservation tillage study provided the opportunity to test whether tillage effects on the germinable weed seedbank would be consistent across different crop rotations and to investigate the potential residual effects of herbicide treatments terminated 12 yr earlier. Our objective was to measure the effects of tillage (moldboard plow [MP] vs. chisel plow [CP] vs. no-till [NT]), crop rotation (2-yr barley–red clover followed by 4-yr barley–canola–wheat–soybean rotation, compared to a cereal monoculture), and of a prior weed management factor (three intensity levels of herbicide use) on the density, diversity, and community structure of weed seedbanks. Species richness, evenness (Shannons E), and diversity (Shannons H′) of spring seedbanks varied little across treatments and over time. Total seedbank density generally increased as tillage was reduced, with some variations due to weed management in 1993 and crop rotation in 2006. Crop rotations generally had smaller seedbanks with fewer species than the monoculture. In 1993, seedbanks with minimum weed management were twice as dense as those with intensive or moderate weed management (approximately 6,000 vs. 3,000 seed m−2). By 2006, seed density averaged 6,838 seed m−2 across intensive and moderate weed management regardless of tillage, but was nearly twice as large in NT (12,188 seed m−2) compared to MP (4,770 seed m−2) and CP (7,117 seed m−2) with minimum weed management (LSD0.005  =  4488). Species with abundant seedbanks responded differently to treatments. Barnyardgrass and green foxtail had larger seedbanks in the monoculture than in the rotation. Common lambsquarters and pigweed species had large seedbanks in tilled treatments in the rotation, whereas yellow foxtail and field pennycress contributed to the large seedbanks observed in NT treatments. The latter two species were also associated with residual effects of weed management treatments (terminated 12 yr earlier) in NT. The differential seedbank response of weed species, attributed in part to contrasting weed emergence patterns and agronomic practice effects on seed rain, explained some of the weak treatment effects observed for total seedbank density and diversity. The large weed seedbanks observed in NT plots after 18 yr confirms the importance of seed rain and seedbank management for the sustainability of NT systems. Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv.; common lambsquarters, Chenopodium album L.; green foxtail, Setaria viridis (L.) Beauv.; field pennycress, Thlaspi arvense L.; pigweed species, Amaranthus sp.; yellow foxtail, Setaria pumila (Poir.) Roem & Schult.

Fitness of double vs. single herbicide-resistant canola

Abstract Since 1995, canola cultivars with herbicide resistance (HR) have been readily adopted by Canadian producers. Gene flow between these cultivars with different HR traits has led to the occurrence of double herbicide–resistant (2HR) volunteers. To evaluate the fitness of canola volunteers with double HR, we compared three 2HR combinations to each of their parent single-HR plants (1HR: glufosinate-R, imidazolinone-R, glyphosate-R) commercial canola lines in separate greenhouse experiments. The replacement series design included five ratios of 2HR vs. 1HR plants at a single density of 129 plants m−2 and three stress treatments: herbicide application with either glufosinate, imazethapyr, or glyphosate; competition with a wheat crop; and a control without herbicide or wheat competition. Fitness indicators included aboveground biomass at 5 and 12 to 16 wk, seed production, and reproductive allocation. The 2HR plants showed delayed reproductive growth but were generally as competitive as 1HR commercial lines. Plant biomass of 2HR canola was comparable to or greater than 1HR canola, whereas seed biomass of 2HR canola was less than that of 1HR canola in half of the cases, likely because of delayed reproductive growth and early harvesting. Glufosinate–glyphosate 2HR was the fittest combination. Herbicide application had little effect on 2HR biomass at harvest, except for imazethapyr, which reduced the biomass and seed production of 2HR plants with imidazolinone-glyphosate resistance by 30%. The latter effect could have been from the unsuspected presence of 2HR plants with only one of the two acetolactate synthase mutations conferring resistance to imidazolinones. Wheat competition reduced fitness values of both 2HR and 1HR canola similarly, but seed production was still 64% that of the controls. Overall, there was little indication of reduced fitness in 2HR canola compared with commercial 1HR varieties. Nomenclature: Canola, Brassica napus L.; wheat, Triticum aestivum L. ‘Voyageur’, ‘AC Pollet’.

Synchrony of flowering between canola and wild radish (Raphanus raphanistrum)

Abstract Many conditions need to be satisfied for gene flow to occur between a transgenic crop and its weedy relatives. Flowering overlap is one essential requirement for hybrid formation. Hybridization can occur between canola and its wild relative, wild radish. We studied the effects of wild radish plant density and date of emergence, canola (glyphosate resistant) planting dates, presence of other weeds, and presence of a wheat crop on the synchrony of flowering between wild radish and canola (as a crop and volunteer). Four field experiments were conducted from 2000 to 2002 in St-David de Lévis, Québec. Flowering periods of wild radish emerging after glyphosate application overlapped with early-, intermediate-, and late-seeded canola 14, 26, and 55%, respectively, of the total flowering time. Flowering periods of early-emerging wild radish and canola volunteers in uncropped treatments overlapped from mid-June until the end of July, ranging from 26 to 81% of the total flowering time. Flowering periods of wild radish and canola volunteers emerging synchronously on May 30 or June 5 as weeds in wheat overlapped 88 and 42%, respectively, of their total flowering time. For later emergence dates, few flowers or seeds were produced by both species because of wheat competition. Wild radish density in canola and wild radish and canola volunteer densities in wheat did not affect the mean flowering dates of wild radish or canola. Increasing wild radish density in uncropped plots (pure or weedy stands) hastened wild radish flowering. Our results show that if hybridization is to happen, it will be most likely with uncontrolled early-emerging weeds in crops or on roadsides, field margins, and uncultivated areas, stressing the need to control the early flush of weeds, weedy relatives, and crop volunteers in noncrop areas. Nomenclature: Glyphosate; wild radish, Raphanus raphanistrum L. RAPRA; canola, Brassica napus L. ‘Hyola 357 RR’; wheat, Triticum aestivum L.