The development of problematic weed populations is a concern in western Canadian fields where canola and pea are grown in a 4-yr sequence with spring cereal grains. Weed densities were examined at a site near Melfort, Saskatchewan, Canada, from 1994 to 1997 in seven zero-till managed crop rotations. Four rotations that included canola, pea, or flax in at least 3 of 4 yr (HBF: high broadleaf–crop frequency) were compared with three rotations that included broadleaf crops grown in 2 of 4 yr (LBF: low broadleaf–crop frequency). Spring wheat and barley were the cereal crops in rotation. Residual (postherbicide application) weed density for each weed species in a given year was summed across all phases for each rotation to reflect the overall weed infestation. Four annual broadleaf weed species were most abundant in 1996 and a second group of three species, having a variety of reproductive strategies, became progressively less abundant as the study progressed. The difference between the HBF and LBF rotations for the density of these species varied and was most prominent in years when environmental conditions were conducive for their growth. More frequent applications of ethafluralin, with its residual weed control, best explained why wild oat and catchweed bedstraw generally were less abundant in the HBF rotations. Of particular interest was the 8 plants m−-2 greater density of dandelion and perennial sowthistle in the HBF vs. LBF rotations in the last year of the study. It is thought that the limited herbicide options for the control of these species could present a future problem if they continued to develop in the HBF rotations. Differences in herbicide use between the HBF and LBF rotations were considered the primary factor controlling the rotation effects on weed density.

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.

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.

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.

In both reduced-tillage and tilled fields, dandelion can be a problem, but the effect of dandelion on canola yield is unknown. This study was conducted to investigate the effect of dandelion infestation on spring canola yield and the influence of tillage on interference and in-field distribution of dandelion. Dandelion distribution was not associated with tillage regimen. Dandelion distribution showed some relationship to past cropping history, for example, the presence of alfalfa in the rotation. The strength of correlation between measures of dandelion infestation level and canola yield loss was associated with tillage regimen. For tilled fields, there was no correlation between reduction in canola yield and any measurements of dandelion infestation level. For reduced-tillage fields, the most reliable measures of dandelion interference level were dandelion ground cover prespray, total dandelion rosette diameter prespray, relative dandelion ground cover prespray, and total dandelion root diameter at crop harvest.

Canada thistle and dandelion are troublesome weeds found throughout the northern United States. Carbohydrate concentrations in roots of these plants change in response to freezing temperatures as plants prepare to overwinter. Herbicides applied in the fall provide more effective control of these weeds than does treatment applied in early spring. Experiments were conducted near Scottsbluff, NE, from 1999 to 2001 to examine changes in glucose, fructose, sucrose, and fructans in roots of Canada thistle and dandelion in response to the fall-applied herbicide. Dicamba applied 10 d after the first fall frost reduced the quantities of low degree-of-polymerization (DP) fructans and provided better control of Canada thistle and dandelion than did dicamba applied 11 d before the first frost. Dicamba and dicamba plus SAN 836H fall-applied were more effective in controlling Canada thistle and dandelion than was 2,4-D fall-applied. As the rate of dicamba and dicamba plus SAN 836H increased, the quantities of low-DP fructans in plant roots declined, and plant control increased. Activity of fructan 1-exohydrolase in roots of Canada thistle was increased by dicamba fall-applied and was closely associated with decline in the quantities of low-DP fructans.

Roots of dandelion were exhumed from the soil monthly from 1997 to 1999 near Scottsbluff, NE, to assess the seasonal changes in total sugars, glucose, fructose, sucrose, and fructans. In the spring, the initiation of plant growth was accompanied by increased fructose and, to a lesser degree, glucose as a percentage of total sugars. During June, July, and August, the percentage of total sugars with a middle to high degree of polymerization (DP) fructans increased. The DP in fructans changed with reduced rainfall and was associated with increased fructose and decreased high-DP fructans as a percentage of total sugars. Decreasing or freezing soil temperatures in the fall were associated with increased fructose and decreased high-DP fructans as a percentage of total sugars. When soil froze in December, the percentage of total sugars as sucrose and low-DP fructans increased and mid- to high-DP fructans decreased. The seasonal fluctuations of glucose, fructose, sucrose, and various fructan polymers may be one mechanism that allows dandelion to adapt to environmental stresses and gain a competitive advantage over other plants in the community. An understanding of these seasonal fluctuations in sugars may also allow better timing of biological, mechanical, and chemical control practices for improved plant management.

The objective of this study was to examine the effects of crop rotation (spring barley monoculture vs. spring barley–red clover 2-yr rotation), tillage (moldboard plow, chisel plow, no-till), and weed management (intensive, moderate, minimum) on plant–seedbank relationships for 19 weed species. Plant and seedbank density data were collected over 4 yr and analyzed by analysis of variance and correlation analysis to confirm treatment effects on plant–seedbank relationships. The relative frequency (difference between aboveground and seedbank frequency) of many species was more influenced by rotation, whereas species density appeared regulated more by weed management than by other factors. Frequency data confirmed that very few species were ubiquitous over time or treatment, aboveground or in the seedbank. The perennial species, field horsetail, quackgrass, white clover, and perennial sowthistle were more frequent aboveground than in the seedbank. This was also observed for annuals such as common hempnettle, sun spurge, catchweed bedstraw, and annual grasses. Treatment effects on abundance were inconsistent aboveground and in the seedbank across time for 12 of 19 species. The seven species that showed more consistent treatment response for abundance were frequent species present in 50% of the plots both aboveground and in the seedbank. For most species, plant density was correlated with either the previous or current year seedbank, but correlations were rarely of the same magnitude and significance over the years. Common chickweed was the only species for which treatment effects on the plant–seedbank relationship were confirmed for all 4 yr. Correlations between midseason plant populations and subsequent seedbanks confirmed the role of residual populations in replenishing the seedbanks, including those of perennials like quackgrass and dandelion. Overall, plant–seedbank relationships were tenuous for many weed species and varied over time with cropping practices and environment.

Biological agents, including fungal pathogens, are being considered to control weeds such as dandelion. Compatibility of a fungal pathogen, Phoma herbarum, with nonformulated 2,4-D was tested in vitro. Moderate to high concentrations (828 to 2,451 ppm) of 2,4-D reduced fungal colony diameter by 50% or less, suggesting that practical concentrations may be chosen for tank mixes. Under growth room and field conditions, integration of 2,4-D and P. herbarum produced enhanced control of dandelion. In general, synergistic responses to sublethal doses of 2,4-D followed 4, 8, or 16 d later by P. herbarum inoculation were found. Synergistic interactions, as determined by the Colby method, ranged as high as 62% greater than expected for an additive interaction. In general, dandelion response was reduced under field conditions, suggesting that environmental conditions or other factors play a role in efficacy. Integration of chemical and biological agents shows potential for enhanced weed control.

Laboratory and greenhouse experiments were conducted to determine the herbicidal effect of plant-derived oils and to identify the active ingredient in an oil with herbicide activity. Twenty-five different oils were applied to detached leaves of dandelion in the laboratory. Essential oils (1%, v/v) from red thyme, summer savory, cinnamon, and clove were the most phytotoxic and caused electrolyte leakage resulting in cell death. Each of these essential oils in aqueous concentrations from 5 to 10% (v/v) plus two adjuvants (nonionic surfactant and paraffinic oil blend at 0.2% [v/v]) were applied to shoots of common lambsquarters, common ragweed, and johnsongrass in the greenhouse; shoot death occurred within 1 h to 1 d after application. Essential oil of cinnamon had high herbicidal activity, and eugenol (2-methoxy-4-[2-propenyl]phenol) was determined to be this oil's major component (84%, v/v). Dandelion leaf disk and whole-plant assays verified that eugenol was the active ingredient in the essential oil of cinnamon. Essential oils are extracted from plants and thus may be useful as “natural product herbicides” for organic farming systems.

Ultraviolet-B (UVB, 280 to 320 nm) radiation levels reaching the earth's surface are increasing because of depletion of the stratospheric ozone layer. Adverse biological effects of this harmful radiation are a serious concern. The effects of UVB radiation on spotted chickweed, dandelion, downy brome, green foxtail, redstem filaree, and common catsear seed germination, shoot and radicle elongation, and seedling growth and development in a UVB-free environment after exposure to UVB were investigated in a greenhouse study. UVB radiation (4, 7, or 11 kJ m−2 d−1) did not influence seed germination of any species except downy brome. Postgermination UVB exposure (8 h d−1, for 10 d), however, inhibited radicle elongation in all species, except in spotted catsear. Shoot elongation was also inhibited in some species but to a lesser extent than radicle elongation. A significant inhibition by UVB treatment (11 kJ m−2 d−1) of root and shoot biomass, leaf area, and leaf number was observed in some species after transfer of UVB-treated seedlings to a UVB-free environment for 3 wk. The inhibition of postgermination shoot and radicle elongation by UVB radiation, the continuation of growth inhibition after transfer of the treated seedlings to a UVB-free environment, and the differential responses of weedy species could have significant implications for their ability to compete with each other and with associated crops in a UVB-enriched environment that is likely as the stratospheric ozone layer gets depleted.