Generalizations concerning the effects of management practices on weed community dynamics often lack robustness, most likely because of the concomitant effects of agronomic and environmental factors. However, such generalizations, when valid, provide useful grounds for predictions and are thus desirable. This study attempted to evaluate the relative importance of crop rotation, tillage, and weed management as factors affecting weed communities and tested the hypothesis of an association between management practices and weeds from certain life cycle groups. Principal component analysis (PCA) of weed density data from a 4-yr field study conducted on a Kamouraska clay and a Saint-André gravelly sandy loam at La Pocatière QC, Canada, identified groups of weed species, while an analysis of variance (ANOVA) of PCA scores associated these groups with management factors. A multivariate analysis of variance (MANOVA) of regression coefficients describing time courses of density for each species confirmed treatment effects. Species segregated roughly according to life cycles. Interactions among weed management intensity, tillage, and crop rotation mostly explained species dominance in the various cropping systems. A first group of species, mostly annual dicots, largely dominated in minimum weed management treatments; their relative importance in each rotation varied with their level of susceptibility to postemergence herbicides. A second group included annuals and perennials, whose commonality seemed to be their tolerance to herbicides; these species also had a particular affinity for chisel and no-till treatments. A third group was formed by perennial species, each with a different response to tillage. The tenuous correspondence between commonly used classification schemes and management factors suggests that other aspects of weed biology (e.g., seed size, dispersal, production, germination requirements, and seedbank longevity) should be considered when trying to explain and predict the presence and dominance of certain weed species with regard to management practices.

Weed seedling emergence is partially dependent on biotic and abiotic conditions directly surrounding the seed. When environmental conditions are appropriate, seed germination and emergence occurs. We studied the impact of seeding depth (surface, 1 to 2, 3 to 4, and 6 to 7 cm) and fluctuating soil moisture regimes (field capacity [FC]–1/3FC–FC; FC–1/6FC–FC) on percent weed emergence in a greenhouse. At FC, wild mustard and field pennycress had the greatest percent emergence when seeds were placed on or near the soil surface, whereas percent emergence of barnyardgrass and round-leaved mallow was unaffected by seeding depth. All the perennials tested had the greatest percent emergence at FC when seeds were placed near or on the soil surface, except for common milkweed which only emerged below the soil surface. When soil moisture levels fluctuated, surface seeds of barnyardgrass, catchweed bedstraw, green foxtail, wheat, and wild oat had less emergence than seeds below the soil surface; field pennycress had increased emergence when the seeds were placed on the surface; and round-leaved mallow and wild mustard emergence was unaffected by seeding depth. The emergence of curly dock, dandelion, and perennial sowthistle was unaffected by seeding depth, whereas foxtail barley and quackgrass emergence was reduced when seeds were placed on the surface and soil moisture fluctuated.

Winter annual weeds are becoming prolific in agricultural fields in the midwestern United States. The objectives of this research were to understand the roles of soil temperature (daily average and fluctuation) and moisture on the emergence of nine winter annual weed species and dandelion and to develop predictive models for weed emergence based on the accumulation of modified thermal/hydrothermal time (mHTT). Experiments were established at Lincoln, NE; Mead, NE; and at two sites (irrigated and rainfed) near Clay Center, NE, in 2010 and 2011. In July of each year, 1,000 seeds of each species were planted in 15 by 20 by 6-cm mesh baskets installed between soybean rows. Soil temperature and water content were recorded at the 2-cm depth. Emerged seedlings were counted and removed from the baskets on a weekly basis until no additional emergence was observed in the fall, resumed in late winter, and continued until emergence ceased in late spring. Weather data were used to accumulate mHTT beginning on August 1. A Weibull function was selected to fit cumulative emergence (%) on cumulative mHTT (seven base temperature [Tbase ] by six base water potential [Ψbase ] by three base temperature fluctuation [Fbase ] candidate threshold values = 126 models); it was also fit to days after August 1 (DAA1), for a total of 127 candidate models per species. The search for optimal base thresholds was based on the theoretic-model comparison approach (Akaike information criterion [AIC]). All three components (Tbase , Ψbase , and Fbase ) were only important for Virginia pepperweed. For downy brome and purslane speedwell, including Tbase and Ψbase resulted in the best fit, whereas for dandelion including Tbase and Fbase resulted in the best fit. A model including only Tbase resulted in the best fit for most species included in this study (Carolina foxtail, shepherd's-purse, pinnate tansymustard, henbit, and field pansy). For field pennycress, the model based on DAA1 resulted in the best fit. Threshold values were species specific. Soil temperature was the major environmental factor influencing winter annual weed emergence. Even though soil moisture and often temperature fluctuation are essential for seed germination, Ψbase and Fbase were not as critical in the predictive models as initially expected. Most seedlings (> 90%) of downy brome, pinnate tansymustard, Carolina foxtail, henbit, and field pansy emerged during the fall. Virginia pepperweed, purslane speedwell, dandelion, shepherd's-purse, and field pennycress seedlings emerged during both fall and spring. The results of this research provide robust information on the prediction of the time of winter annual weed emergence, which can help growers make better management decisions.

Organic vegetable producers have limited options for managing weeds. They cite weed management as their number one research priority. Studies were conducted in 2004 and 2005 at the Black Sea Agricultural Research Institute, Samsun, Turkey, to determine the weed suppressive effects of summer cover crops in organic kale production. Treatments consisted of grain sorghum, sudangrass, hairy vetch, grain amaranth, pea, and fallow. Weed density and total weed dry biomass were assessed before and at 14, 28, and 56 d after incorporation (DAI) of the cover crops. Kale was transplanted 14 DAI and hand weeded once after last weed evaluation (56 DAI). All cover crops produced at least 1 ton/ha (t/ha) biomass; grain sorghum produced more dry matter than all other cover crops in both years. After incorporation of the cover crops, hairy vetch and sorghum treatments showed fewer species, lower weed density, and total weed dry biomass compared with other treatments. Cover crops suppressed emergence of common purslane, common lambsquarters, redroot pigweed, European heliotrope, field pennycress, annual sowthistle, black nightshade, shepherd's-purse, wild mustard, sun spurge, Persian speedwell, annual mercury, and jimsonweed up to 56 DAI. Total kale yield in hairy vetch treatments was more than double that of the no cover crop, and was significantly higher than yield from the other cover crop treatments. These results indicate that hairy vetch, grain sorghum, and sudangrass have ability to suppress early-season weeds in organic kale production.

Winter annual weeds protect the soil from erosion and retain nutrients during the winter; however, they can also act as a host for crop pests and pathogens and impede planting. Increased knowledge of the reproductive biology and the seed fate of winter annuals would be useful to improve management and crop productivity. The objectives of this research were to determine the recruitment biology of shepherd's-purse, henbit, common chickweed, and field pennycress, including seed production, dispersal, dormancy, and seedling emergence, based on growing degree days (GDD). Henbit was the least prolific of the four weeds studied, producing 800 to 40,000 seeds m−2 at naturally occurring densities; shepherd's-purse was the most prolific, producing 11,000 to 400,000 seeds m−2 with 40 to 230 plants m−2. Fifty percent seed rain occurred for henbit, common chickweed, shepherd's-purse, and field pennycress at 620, 790, 880, and 1300 GDDBase ,0C, respectively. Overall, seeds were dormant for all species at the time of dispersal. In 2 of 3 yr, dormancy of later-dispersed common chickweed decreased after 6 mo of storage at natural, fluctuating temperatures in the absence of water. The emergence patterns of the four species followed the Gompertz equation and were indicative of facultative winter annuals. The emergence patterns by rate were similar between henbit and common chickweed and between shepherd's-purse and field pennycress. Seed production, dispersal, dormancy, and seedling emergence were influenced by moisture; therefore, including a precipitation or soil moisture component into a GDD model (such as the use of hydrothermal time) would improve the accuracy of predicting winter annual reproduction, seed fate, and emergence.

Weed resistance monitoring has been routinely conducted in the Northern Great Plains of Canada (Prairies) since the mid-1990s. Most recently, random surveys were conducted in Alberta in 2001, Manitoba in 2002, and Saskatchewan in 2003 totaling nearly 800 fields. In addition, nearly 1,300 weed seed samples were submitted by growers across the Prairies between 1996 and 2006 for resistance testing. Collected or submitted samples were screened for group 1 [acetyl-CoA carboxylase (ACCase) inhibitor] and/or group 2 [acetolactate synthase (ALS) inhibitor] resistance. Twenty percent of 565 sampled fields had an herbicide-resistant (HR) wild oat biotype. Most populations exhibited broad cross-resistance across various classes of group 1 or group 2 herbicides. In Manitoba, 22% of 59 fields had group 1–HR green foxtail. Group 2–HR biotypes of kochia were documented in Saskatchewan, common chickweed and spiny sowthistle in Alberta, and green foxtail and redroot pigweed in Manitoba. Across the Prairies, HR weeds are estimated to occur in fields covering an area of nearly 5 million ha. Of 1,067 wild oat seed samples submitted by growers and industry for testing between 1996 and 2006, 725 were group 1 HR, 34 group 2 HR, and 55 groups 1 and 2 HR. Of 80 submitted green foxtail samples, 26 were confirmed group 1 HR; most populations originated from southern Manitoba where the weed is most abundant. Similar to the field surveys, various group 2–HR biotypes were confirmed among submitted samples: kochia, wild mustard, field pennycress, Galium spp., common chickweed, and common hempnettle. Information from grower questionnaires indicates patterns of herbicide usage are related to location, changing with cropping system. Two herbicide modes of action most prone to select resistance, groups 1 and 2, continue to be widely and repeatedly used. There is little evidence that growers are aware of the level of resistance within their fields, but a majority have adopted herbicide rotations to proactively or reactively manage HR weeds.

We used laboratory and field feeding trials to investigate adult carabid beetle preferences for three brassicaceous weed species (rapeseed, wild mustard, and field pennycress) that are pests in canola. All carabid species preferred seeds of rapeseed most and those of field pennycress least and showed intermediate preference for wild mustard seeds. Beetles highly preferred imbibed seeds of all three weed species. Activity–density of carabids and mean weed seed removal were highly correlated in field plots of canola, with activity–density accounting for 67% of the observed variation in seed removal. Our study indicates that seed consumption among carabids is influenced by several factors, including weed species, physiological state of seeds, and carabid activity–density. Carabid seed predation is significant in canola agroecosystems; therefore, understanding these influences has implications for ecological weed management.

The critical period of weed control is the portion of the life cycle of a crop during which it must be kept weed-free to prevent yield loss due to weed interference. The advent of herbicide-resistant canola (Brassica napus L.) varieties in western Canada has meant that there are now more options for postemergence weed control in canola, and this has prompted increased interest in identifying the optimum timing for weed control in this crop. A critical period experiment was conducted at three locations in southern Manitoba in 1998 and 1999, and it consisted of two sets of treatments. In the first set of treatments, the crop was kept weed-free for increasing lengths of time to determine when emerging weeds would no longer reduce crop yield. In the second set of treatments, weeds were permitted to grow in the crop for increasing lengths of time to determine when weeds emerging with the crop began irrevocably to reduce crop yield. Results of the experiments indicated that canola must be kept weed-free in most cases until the four-leaf stage of the crop (17–38 days after crop emergence [DAE]) and, in one early-seeded experiment, until the six-leaf stage of the crop (41 DAE), in order to prevent >10% yield loss. After the four- to six-leaf stage of the canola crop, few weeds emerged, and late-emerging weeds accumulated little shoot biomass. Weeds needed to be removed by the four-leaf stage of the crop (17–38 DAE) to prevent >10% yield loss due to weed interference. In all but the early-seeded experiment, the critical weed-free period and the critical time of weed removal overlapped, such that a single weed removal at the four-leaf stage of the crop would have been sufficient to prevent >10% yield loss. This information will be useful for providing weed control recommendations to canola producers.

Defatted seedmeals from 15 glucosinolate-containing plant species were analyzed for herbicidal activity by determining inhibition of seedling emergence when added to a sandy loam soil containing wheat and sicklepod seeds at concentrations of 0.1, 0.5, and 1% (w/w). In general, the seedmeals were more phytotoxic to wheat than sicklepod. For wheat, all of the seedmeals significantly inhibited seedling emergence at the 1.0% concentration. At the 0.1% concentration three of the seedmeals (Indian mustard, money plant, and field pennycress) completely inhibited wheat emergence. For sicklepod emergence, eight of the seedmeals were completely inhibitory at the 1% level (Indian mustard, field pennycress, garden rocket, Siberian wallflower, English wallflower, garden cress, sweet alyssum, and evening stock) and four were completely inhibitory at the 0.5% level (brown mustard, garden rocket, English wallflower, and sweet alyssum). Intact glucosinolates and their corresponding hydrolysis products varied among the seedmeals with the highest activity. Major hydrolysis products produced by the seedmeals with the most phytotoxicity, respectively, included 2-propenyl (allyl) isothiocyanate (AITC) by brown mustard seedmeal, allyl thiocyanate and AITC by field pennycress seedmeal, erucin (4-methylthiobutyl isothiocyanate) by arugula seedmeal, 3-butenyl isothiocyanate and lesquerellin (6-methylthiohexyl isothiocyanate) by sweet alyssum seedmeal, and isopropyl isothiocyanate by money plant seedmeal. From our data it appears that both the type and concentration of glucosinolates and their hydrolysis products present in the seedmeals affect seed-emergence inhibition.

Improved fallow systems are needed to reduce the negative effects of increased soil erosion and reduced soil quality. Field experiments were conducted to determine weed suppression attained with yellow sweetclover grown as a green manure fallow replacement crop. Yellow sweetclover was undersown in field pea, flax, or Indian mustard and then killed in June of the following fallow year. Living yellow sweetclover competed strongly with weeds during the first fall and spring of fallow. Weed biomass accounted for <1 to 12% of the total plant biomass when yellow sweetclover was terminated in June. Yellow sweetclover residues remaining after termination of growth continued to provide excellent weed suppression. Weed densities in April before planting the succeeding wheat crop were 75 to 97% lower in yellow sweetclover than in untreated fallow treatments. Yellow sweetclover controlled the perennial weeds dandelion and perennial sowthistle, as well as the annuals kochia, flixweed, Russian thistle, and downy brome. Weed suppression was similar whether yellow sweetclover was harvested as hay or its residues were incorporated or left on the soil surface, suggesting that a portion of the weed suppression effect may be due to allelopathic compounds being released from decomposing yellow sweetclover. Results will be used to develop more sustainable agronomic practices in regions where fallow is still widely employed.

Herbicide rotations and mixtures are widely recommended to manage herbicide resistance. However, little research has quantified how these practices actually affect the selection of herbicide resistance in weeds. A 4-yr experiment was conducted in western Canada from 2004 to 2007 to examine the impact of herbicide rotation and mixture in selecting for acetolactate synthase (ALS) inhibitor resistance in the annual broadleaf weed, field pennycress, co-occurring in wheat. Treatments consisted of the ALS-inhibitor herbicide, ethametsulfuron, applied in a mixture with bromoxynil/MCPA formulated herbicide (photosystem-II inhibitor/synthetic auxin), or in rotation with the non-ALS inhibitor at an ALS-inhibitor application frequency of 0, 25, 50, 75, and 100% (i.e., zero to four applications, respectively) over the 4-yr period. The field pennycress seed bank at the start of the experiment contained 5% ethametsulfuron-resistant seed. Although weed control was only marginally reduced, resistance frequency of progeny of survivors increased markedly after one ALS-inhibitor application. At the end of the experiment, the level of resistance in the seed bank was buffered by susceptible seed, increasing from 29% of recruited seedlings after one application to 85% after four applications of the ALS inhibitor. The level of resistance in the seed bank for the mixture treatment after 4 yr remained similar to that of the nontreated (weedy) control or 0% ALS-inhibitor rotation frequency treatment. The results of this study demonstrate how rapidly ALS-inhibitor resistance can evolve as a consequence of repeated application of herbicides with this site of action, and supports epidemiological information from farmer questionnaire surveys and modeling simulations that mixtures are more effective than rotations in mitigating resistance evolution through herbicide selection.

The influence of four tillage systems, varying from intensive to zero tillage, on weed populations and the vertical distribution of weed seeds in the soil was determined at Alliance, Hairy Hill, and Wainwright in northeastern Alberta. The soil was sampled at two depths (0 to 5 and 5 to 10 cm) in fall. Weed seedling emergence in the greenhouse over the winter was assumed to represent the type and amount of weed seeds present in the soil seedbank. Emerged weed seedlings were also identified and counted in the field in spring. In the zero-tillage system, most of the weed seeds were close to the soil surface (0 to 5 cm) at Alliance and Wainwright but were deeper (5 to 10 cm) at Hairy Hill. The winter annuals, field pennycress, shepherd's-purse, and flixweed, and the summer annuals, wild buckwheat and common lambsquarters, increased in the soil seedbank as tillage was reduced, but the higher populations in the soil seedbank did not always result in higher spring seedling populations under zero tillage. In contrast to the seedbank, spring seedling populations of common lambsquarters at Alliance and field pennycress and ball mustard at Hairy Hill were lowest in the zero-tillage system, suggesting that the requirement for herbicides for controlling these weeds in the crop may be least under zero tillage. Both soil seedbank and spring seedling populations of shepherd's-purse at Wainwright and Alliance and of flixweed at Alliance were highest in the zero-tillage system. At Alliance, wild buckwheat seedling emergence in the spring tended to be highest in the minimum-tillage system (one tillage operation prior to seeding). Both soil seedbank and spring seedling populations of green foxtail decreased as tillage was reduced, indicating that green foxtail should become less of a problem under reduced tillage.

Propane flaming could be an alternative tool for PRE control or suppression of early-emerging weeds in organic and conventional crops. The objective of this study was to test the tolerance of selected early-season weeds to broadcast flaming in no-till systems. Four winter annuals (tansy mustard, henbit, cutleaf evening primrose, and field pennycress), one summer annual (common lambsquarters), and one perennial (dandelion) species were included in the study. Except for dandelion, the response to propane flaming was evaluated at two growth stages. Flaming treatments were applied using an all-terrain-vehicle-mounted flamer moving 4.8 km h−1, and propane pressure was adjusted to deliver doses of 0 (nonflamed control), 22, 34, 48, 67, and 90 kg ha−1. The response of each species to propane doses was described by log-logistic models based on visual ratings of weed control and dry matter reduction. Response to broadcast flaming varied among species and growth stages. Common lambsquarters, tansy mustard, and henbit were more susceptible to flaming than cutleaf evening primrose, field pennycress, and dandelion. On the basis of visual ratings, propane doses between 54 and 62 kg ha−1 effectively controlled (90% control) common lambsquarters at the early growth stage (five-leaf), tansy mustard at both growth stages (nine-leaf and flowering), and henbit (flowering). However, a higher propane dose (> 80 kg ha−1) was necessary to obtain 90% control of common lambsquarters in later growth stage (11-leaf) and early growth stage of henbit (nine-leaf). Cutleaf evening primrose, field pennycress, and dandelion exhibited higher levels of tolerance to broadcast flaming. A 90% control of these species was not achieved even with the highest propane dose (90 kg ha−1) utilized in the study. Results of this study indicate that a single application of broadcast flaming can be an effective tool for controlling tansy mustard, henbit, and common lambsquarters and temporary suppression of cutleaf evening primrose, field pennycress, and dandelion.

Weeds often limit productivity of organic cropping systems. Radish is a fast-growing, potentially allelopathic cover crop that has the potential to improve weed management in organic systems. To evaluate the effect of radish on density, cover, and biomass of weeds in organically managed corn, 2-yr field experiments were conducted over 4 site years. Four cover-crop planting treatments (fall-only, spring-only, fall + spring, and no cover) were tested in factorial with three cultivation treatments (standard [three to four passes], false seedbed [standard with a false seedbed], and reduced [two passes]). All plots were tilled before planting. Shoot biomass averaged 3,057 kg ha−1 for fall-seeded radish and 385 kg ha−1 for spring-seeded radish. Radish cover crops generally did not improve management of weeds during the corn growing season. However, in the absence of a false seedbed, fall-seeded radish reduced field pennycress density from 9 to < 1 plant m−2 and horseweed density from 6 to 2 plants m−2 in spring in site years where these weeds were present. Fall-seeded radish also reduced cover of summer annual weeds during the fall cover-crop growing season from 4 to 0% in 1 site year, preventing these weeds from setting seed. Radish cover crops did not affect corn grain yield.

Crop rotations, particularly those that include legumes, often result in improved soil quality and crop yields. A study was conducted to confirm the presence and persistence of the residual effects of crop rotation and weed management on a test crop and weeds in three tillage systems (moldboard plow [MP]; chisel plow [CP]; no tillage [NT]). Rotation (spring barley monoculture and spring barley–red clover rotation) and weed management (intensive, moderate, minimum) treatments, initiated in 1987, were terminated, and a test crop of spring wheat was grown in 1995 and 1996. Tillage treatments were maintained throughout. Multivariate analysis showed that weed communities were more affected by treatment termination in the rotation NT treatment with minimum weed management than in all other treatments. The former treatment was dominated by perennial broadleaf weeds but sustained adequate wheat yields (3.3 Mg ha−1) compared with the monoculture (1.0 Mg ha−1) one year after termination. Weed communities in CP and MP plots were less affected by treatment termination. Yet, changes in herbicide use at termination caused the virtual elimination of quackgrass from tilled plots and allowed field pennycress to become ubiquitous across treatments. Residual effects from crop rotation were more important than those from weed management in increasing wheat yields in tilled systems. Differences in wheat yield in NT systems 2 yr after treatment termination were attributed to residual effects from previous weed management rather than from crop rotation. Beneficial effects of crop rotation and weed management may persist for 2 yr but will vary according to tillage system.

Variation in spring emergence periodicity (both before and after crop seeding) of summer annual weeds is a potentially exploitable attribute that may be applied to weed management in canola. Tillage intensity, which is decreasing in the Great Plains of North America, may influence emergence periodicity of summer annual weeds. Emergence periodicity of common lambsquarters, field pennycress, green foxtail, redroot pigweed, wild buckwheat, wild mustard, and wild oat were monitored during the spring of 2000 in 17 producers' canola fields across southern Manitoba, Canada. The fields represented a region of approximately 2 million ha and included a broad range of soil types, agronomic practices, environmental conditions, and seedbank distributions. Fields were grouped into one of two broad tillage classifications (conventional or conservation). For most species, except redroot pigweed and wild mustard, conservation tillage promoted earlier emergence than conventional tillage in terms of both thermal and chronological time. The differences were significant even though there was only a limited range of tillage intensity for the two tillage classes within this region. Onset of canola crop emergence preceded that of all but one weed species in the conservation-tillage fields and five weed species in the conventional-tillage fields. This suggests that canola seeded in conservation- vs. conventional-tillage systems may have a competitive advantage by way of an earlier relative time of crop emergence. The influence of tillage system on weed emergence periodicity is likely due to the influence of tillage on the vertical origin of weed seedling recruitment because measurements of soil temperature and soil moisture did not help to fully explain the differences in emergence periodicity between tillage systems. The results from this study will facilitate weed control timing decisions in canola and provide validation data for weed emergence models.

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 crop's 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.