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.

Field studies were conducted in 1989 and 1990 at St-Augustin, Quebec, Canada, to determine the economic threshold density of quackgrass in potato. Potato yield losses due to quackgrass interference increased with quackgrass population density. Potato yield ranged from 33 to 73% in 1989, and from 19 to 44% in 1990. The relationship between potato yield losses and quackgrass densities was described by a rectangular hyperbolic function. Dry weight of quackgrass proved to be the best predictor of potato yield loss compared to shoot number. The interference thresholds for 10% potato yield loss amounted to 25 shoots m−2 or 20 g total dry biomass m−2. The economic threshold varied between 0.04 and 2 shoots m−2 or 0.0165 and 1.5 g total dry biomass m−2, depending on the variables considered.

Field studies were conducted from 1993 through 1995 to determine potential reductions in herbicide use and associated cost savings by utilizing a weed-sensing sprayer, named Detectspray, to control weeds throughout the fallow season and to control perennial weeds after crop harvest. The Detectspray system gave comparable weed control to conventional broadcast spraying on 80% of the application dates and reduced glyphosate/dicamba use over the fallow season by 19 to 60%. This reduced herbicide use resulted in cost savings of $6 to $32 ha−1. A fallow treatment that combined two herbicide applications with the Detectsprayer plus one to two wide-blade tillage operations was less costly than conventional tillage at two of 11 sites and at all sites retained more surface crop residues to reduce the risk of erosion. Postharvest glyphosate use on quackgrass with the Detectsprayer was reduced 50 to 78% compared to broadcast applications and resulted in cost savings of $16 to $25 ha−1. Clopyralid use on Canada thistle with the Detectsprayer was reduced 71 to 80%, with cost savings of $44 to $50 ha−1. The Detectspray system is a useful tool to effectively manage weeds in conservation fallow and reduced tillage cropping systems.

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.

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.

A study at Fairbanks, AK, was started in 1984 to determine soil seed longevity of 17 weed species. Seeds were buried in mesh bags 2 and 15 cm deep and were recovered 0.7, 1.7, 2.7, 3.7, 4.7, 6.7, 9.7, and 19.7 yr later. Viability was determined by germination and tetrazolium tests. Seed viability data were fit to an exponential model, separately for each depth, and the likelihood-ratio test was used to determine whether seed-viability decline was affected by burial depth. Depth of burial had a significant effect on viability decline of prostrate knotweed, marsh yellowcress, bluejoint reedgrass, and wild oat. By 19.7 years after burial (YAB), all seeds of common hempnettle, quackgrass, wild oat, foxtail barley, and bluejoint reedgrass were dead. Seeds of 12 other species were still viable: corn spurry (0.1%), prostrate knotweed (0.3% at 2 cm, 0.8% at 15 cm), flixweed (0.5%), pineapple-weed (0.6%), shepherd's-purse (1.3%), wild buckwheat (1.5%), common chickweed (1.6%), rough cinquefoil (1.8%), common lambsquarters (3.0%), Pennsylvania smartweed (3.3%), marsh yellowcress (8.5% at 2 cm, 0.3% at 15 cm), and American dragonhead (62.2%). Seed dormancy at 19.7 YAB was very low for all species (< 4%) except for American dragonhead, common lambsquarters, Pennsylvania smartweed, and shepherd's-purse, which had seed dormancies of 100, 27, 25, and 38%, respectively. Seed longevity was not increased by cold, subarctic temperatures.

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.

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.

The genetic variation of quackgrass as a species and the array of environments in which it is found indicate that selection in these different environments could lead to differentiation among quackgrass populations. Yet, a highly diverse environment might not promote the genetic divergence of quackgrass if it poses contradictory selection pressures. To assess the extent of divergence among quackgrass populations, this study compared the morphology of populations of quackgrass for 1 yr in Rosemount, MN, in a “common garden” study. The quackgrass was initially collected from three different farming systems in southeast Minnesota: corn–soybean (CS), oats–hay–corn (OHC), and permanent pasture (PP). The systems represent pasture or arable land and differ in cropping rotations and levels of disturbance. Although no differences among farming systems were detected in multivariate or univariate comparisons, a significant farming system effect was detected between CS and PP systems when the most diversified system, OHC, was excluded from the analysis. Consistent with this result, a principal components analysis suggested that plants from two of the three farming systems exemplified contrasting modes of perennial plant growth. Relative to each other, the CS plants showed more features of the “guerrilla” growth mode (longer intra-ramet distances, sparse, large patches), whereas PP plants showed more “phalanx” mode features (short intra-ramet distances, dense, smaller patches). Plants from the most diversified system, OHC, did not fit into either growth form, and for this farming system, the variation among populations was the highest. The results suggest that the CS and PP systems selected for distinct growth forms, whereas the diversified OHC system did not. This is consistent with the hypothesis that diversification of a farming system and weed management decreases the risk of evolution of a weed population highly adapted to control measures used in that farming system.