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a Agric. and Agri-Food Can., Box 1000A RR 3, Brandon, MB, Canada R7A 5Y3
b USDA, Cent. Plains Res. Cent., Box 400, Akron, CO 80720
c Agric. and Agri-Food Can., Box 3000, Lethbridge, AB, Canada T1J 4B1
d Dep. of Land Resour. and Environ. Sci., Univ. of Montana, Bozeman, MT 59717-0312
* Corresponding author (derksen{at}em.agr.ca)
Received for publication January 2, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONHISTORIC PERSPECTIVEWEED COMMUNITY CHANGESNOVEL AND FUTURE CROPPING...SPECIFIC WEED MANAGEMENT ISSUES...INTEGRATED WEED MANAGEMENT...CONCLUSIONSREFERENCES |
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Abbreviations: IWM, integrated weed management • NGP, northern Great Plains
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONHISTORIC PERSPECTIVEWEED COMMUNITY CHANGESNOVEL AND FUTURE CROPPING...SPECIFIC WEED MANAGEMENT ISSUES...INTEGRATED WEED MANAGEMENT...CONCLUSIONSREFERENCES |
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Weeds are the major pests in the NGP, with herbicides comprising approximately 85% of the pesticides used. Crop diseases and insects can be serious problems on a cyclical basis, but many problems have been dealt with by crop breeding and rotations. Weeds, being present every year, can cause significant economic losses (Swanton et al., 1993).
Herbicide expenditures typically comprise 20 to 30% of input costs for producers in the NGP. Therefore, efforts to reduce producer reliance on herbicides while maintaining yields can have a large positive impact on net returns. For example, reducing herbicide usage by 10% without affecting crop yield would increase net returns significantly across the NGP. Production systems are being developed for the region that give crops a competitive advantage over weeds, minimize densities of weeds as crops establish, and keep weed communities out of equilibrium to reduce the long-term buildup of problem weed species (Derksen, 1997).
The objective of this paper is to provide information on evolving approaches to weed management in the context of new cropping systems in the NGP by understanding weed community dynamics and developing a rationale to use varied selection pressure as a key weed management tool.
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HISTORIC PERSPECTIVE |
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TOPABSTRACTINTRODUCTIONHISTORIC PERSPECTIVEWEED COMMUNITY CHANGESNOVEL AND FUTURE CROPPING...SPECIFIC WEED MANAGEMENT ISSUES...INTEGRATED WEED MANAGEMENT...CONCLUSIONSREFERENCES |
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Weeds in Spring Wheat–Fallow Region
Spring wheat–fallow or spring wheat–spring wheat–fallow were common rotations throughout the spring wheat region of the NGP from the late 1800s until recently. Based on field surveys from the 1890s, 1940s, and 1960s (Alex, 1982), common weeds across the Canadian prairies were wild oat (Avena fatua L.), wild buckwheat (Polygonum convolvulus L.), common lambsquarters (Chenopodium album L.), field pennycress (Thlaspi arvense L.), and wild mustard [Brassica kaber (DC.) L.C. Wheeler], with regional problems being quackgrass [Elytrigia repens (L.) Nevski], Canada thistle [Cirsium arvensis (L.) Scop.], foxtail barley (Hordeum jubatum L.), green foxtail [Setaria viridis (L.) Beauv.], greenflower pepperweed (Lepidium densiflorum Schrad.), European sticktight (Lappula echinata Gilib.), blue lettuce [Lactuca pulchella (Pursh) DC.], Russian thistle (Salsola iberica Sennen & Pau), cowcockle (Vaccaria pyramidata Medicus), and kochia [Kochia scoparia (L.) Schrad.]. Weed problems and the state of weed management before the adoption of conservation tillage in western Canada have been summarized by Ashford and Hunter (1986), Holm and Kirkland (1986), Hunter et al. (1990), and Todd and Derksen (1986). Recent surveys indicate that green foxtail and wild oat are still the dominant grass species and wild buckwheat and field pennycress are the dominant broadleaf weeds (Thomas, 1978; Thomas and Wise, 1982, 1983, 1987, 1988; Thomas et al., 1996b, 1998). Perennial weeds, such as perennial sowthistle (Sonchus arvensis L.) and dandelion (Taraxacum officinale Weber in Wiggers), and volunteer crops are increasing in abundance. Herbicides are available to control most weed species but not in all crops.
Weeds in Winter Wheat–Fallow Region
Winter wheat–fallow has been the prevalent rotation in the southern and western region of the NGP since the 1920s and 1930s (Black, 1983). The weeds most difficult to control in winter wheat have been the winter annual grasses, downy brome (Bromus tectorum L.), jointed goatgrass (Aegilops cylindrica Host), and volunteer fall rye (Secale cereale L.), because these weeds have similar life cycles to winter wheat (Wicks and Smika, 1990). Densities of these weeds have been increasing in recent decades because producers grow semidwarf cultivars, broadcast N fertilizer, control broadleaf weeds with in-crop herbicides, and control weeds during fallow with the sweep plow (Blackshaw, 1994b; Thill et al., 1984; Wicks, 1984). Because few herbicides have been available to control these grasses in winter wheat, producers have explored alternative rotations that include summer annual crops to aid weed management (Anderson, 1997). The recent registration of sulfosulfuron {1-[2-ethylsulfonylimidazo(1,2a)pyridin-3-ylsulfonyl]-3-[4,6-dimethyoxypyrimidin-2-yl]urea} for control of downy brome in winter wheat coupled with new cropping systems will provide producers with management tools.
Predominant broadleaf weed species in the weed community of winter wheat have been Russian thistle, kochia, common lambsquarters, blue mustard [Chorispora tenella (Pallas) DC.], and pinnate tansymustard [Descurainia pinnata (Walt.) Britt.]. With available herbicides, producers have been able to control these broadleaf weeds and avoid extensive yield loss; however, herbicide resistance is making cost effective weed control more difficult, especially with kochia.
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WEED COMMUNITY CHANGES |
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TOPABSTRACTINTRODUCTIONHISTORIC PERSPECTIVEWEED COMMUNITY CHANGESNOVEL AND FUTURE CROPPING...SPECIFIC WEED MANAGEMENT ISSUES...INTEGRATED WEED MANAGEMENT...CONCLUSIONSREFERENCES |
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Coupling information from experiments and field surveys can provide a broader basis for understanding weed species response to agronomic and environmental factors. For example, the recent increase in Canada thistle populations found in field surveys on the Canadian prairies (Dale et al., 1992) has been attributed to decreased tillage frequency (Moyer et al., 1994; Thomas et al., 1997), but growing seasons were wetter than during previous surveys. In an experiment comparing tillage systems during the survey period, Canada thistle was associated with zero, minimum, or conventional tillage depending on the management practices employed (Derksen et al., 1994). Therefore, other management factors and/or environmental conditions were associated with increased populations of Canada thistle and not necessarily reduced tillage systems. For example, Canada thistle abundance was shown to be influenced more by seeding date than by the intensity of tillage or herbicides used (Dale et al., 1992). Clearly, there is a need for more information on the biology and ecology of weeds in the NGP. Specifically, information on species response to environment and crop production strategies is needed to develop new approaches to weed management.
A means of understanding weed responses to changes in tillage system has been to group weeds based on their life cycle (Froud-Williams, 1988; Buhler, 1995; Moyer et al., 1994). However, as the green foxtail and Canada thistle examples illustrate, species generally don't respond to one factor alone, and simple categorization based on one experiment may be misleading. Research on weed community dynamics should involve a two-step process. Firstly, on an experiment-by-experiment basis, all weed species present should be used to determine response groups to agronomic variables (i.e., those weeds associated with a given practice). Secondly, based on multiyear data from numerous experiments or surveys, species characteristics and management practices that led to the response should be determined. Reversing this process by grouping weeds before analysis can be misleading because exceptions within the groups are hidden and useful understandings of weed community dynamics may be lost. For example, because wild oat and green foxtail respond differently to tillage systems over time (Derksen et al., 1998), grouping them would provide erroneous results.
Weed Changes in the Traditional Spring Wheat Region
The adoption of conservation tillage techniques and the subsequent crop diversification of the traditional spring wheat region of the NGP has brought about changes in weed communities, some of which were predicted and others not (Derksen et al., 1993, 1996b; Moyer et al., 1994; Thomas et al., 1997). Weed species present before adopting conservation tillage have not disappeared, with many species remaining numerous (Thomas et al., 1997). Some weed species traditionally found in conventional tillage have shown a preference for conservation tillage (Moyer et al., 1994; Derksen et al., 1993). Few new species have arisen, but some introduced and native weed species that were rare in conventional tillage have increased in abundance in conservation tillage systems, especially in zero-tillage.
Most annual grass weeds have not shown a consistent association with conventional or reduced tillage systems. Some annual broadleaf species have been associated with conservation tillage systems in some studies (Moyer et al., 1994), but others have been ubiquitous among systems or more common in conventional tillage (Derksen et al., 1994). Annual broadleaf species that have a greater association with conservation tillage often exhibit a winter annual habit in the protective layer of stubble created by zero-tillage. New problem winter annuals include narrowleaf hawksbeard (Crepis tectorum L.), nightflowering catchfly (Silene noctiflora L.), redstem filaree [Erodium cicutarium (L.) L'Her. ex Ait.], salsify species (Tragopogon dubius Scop. and Tra. pratensis L.), catchweed bedstraw (Galium aparine L.), greenflower pepperweed (Le. densiflorum Schrad.), horseweed [Conyza canadensis (L.) Cronq.], wood whitlowgrass (Draba nemorosa L.), American dragonhead (Dracocephalum parviflorum Nutt.), and pygmyflower [or northern rockjasmine (Androsaece septentrionalis L.)] (Derksen et al., 1996a, Moyer et al., 1994). Many traditionally problematic winter annuals, such as flixweed [De. sophia (L.) Webb. ex Prantl], narrowleaf hawksbeard, and shepherd's-purse [Capsella bursa-pastoris (L.) Medicus], have increased in zero-tillage, but field pennycress has shown a greater association with conventional tillage (Derksen et al., 1994; Moyer et al., 1994), perhaps due to its dual summer and winter annual habit (Hume, 1990).
The perennial species Canada thistle, sowthistle, and quackgrass have been associated with conventional and reduced tillage systems and, under certain management practices, have increased in zero-tillage (Derksen et al., 1994; Moyer et al., 1994) but can be managed using integrated weed management (IWM) techniques (Leoppky and Derksen, 1994). Conversely, foxtail barley, dandelion, Melilotus spp., smooth brome (Bro. inermis Leyss.), perennial sowthistle, native rose species (Rosa spp.), and yellow toadflax (Linaria vulgaris Mill.) have generally increased in abundance in zero-tillage and have been difficult to manage. Occasional tillage may be required if some of these species, such as foxtail barley, become dominant problems (Donald, 1990). However, new herbicides and IWM strategies are under development for these species (Blackshaw et al., 1998, 1999).
Volunteer crops have been associated with reduced tillage (Derksen et al., 1993; Moyer et al., 1994; Froud-Williams, 1988), but recent research has provided new insights into their dynamics. Firstly, volunteer crop plants have not responded to tillage systems as a group. Secondly, while some volunteer crops were associated with reduced tillage at crop and weed maturity, most were associated with conventional tillage as seedlings (Derksen et al., 1998). Because yield losses due to weed competition in the NGP are generally considered to occur when crops are seedlings, crop yield losses due to interference from volunteer crops may be lower in zero- than conventional tillage systems.
Weed Changes in Traditional Winter Wheat Regions
Producers have been changing their rotations to include alternative crops with winter wheat because of conservation tillage. Rotations of two spring crops in 3 yr have been successful, with corn (Zea mays L.), proso millet (Panicum miliaceum L.), sorghum [Sorghum bicolor (L.) Moench], sunflower (Helianthus annuus L.), barley (Ho. vulgare L.), or safflower (Carthamus tinctorius L.) grown after winter wheat (Black, 1983; Peterson et al., 1993). Longer rotations, such as winter wheat–corn–proso millet–fallow, also have been successful (Anderson et al., 1999).
Including summer annual grass crops in the rotation has led to a proliferation of summer annual grasses, such as sandbur [Cenchrus longispinus (Hack.) Fern.], green foxtail, and witchgrass (Pa. capillare L.) (Wicks and Smika, 1990). Broadleaf weed biotypes resistant to triazine and sulfonylurea herbicides have infested both winter wheat and summer annual crops (Lyon et al., 1996), and blue mustard densities are increasing in no-till winter wheat–fallow systems.
Perennial weeds such as field bindweed (Convolvulus arvensis L.) and Canada thistle are common in the American NGP. Density of tumblegrass [Schedonnardus paniculatus (Nutt.) Trel.], a perennial grass, has increased in no-till winter wheat–fallow systems, mainly due to its tolerance to herbicides used for weed control during fallow (Wicks and Smika, 1990).
Some winter wheat producers till after harvest with the sweep plow to shallowly incorporate weed seed in soil. Their goal is to stimulate germination of downy brome and jointed goatgrass to reduce the seedbank density of these species (Anderson, 1998b). However, this strategy increased seedling densities of summer annual weeds by 35 to 50% in corn and sunflower the following year compared with a no-till system (Anderson, 1999a).
Producers in traditional winter wheat areas believe that weeds will be more difficult to control when fallow is used less frequently. This belief is based on previous experiences with monoculture wheat production where serious weed problems developed. For example, continuous winter wheat has led to high densities of downy brome (Moyer et al., 1994) while continuous spring wheat resulted in severe infestations of foxtail species (Hume, 1982) or wild oat (Donald and Nalewaja, 1990). Weed densities increased because the weeds and crops had similar life cycles (Froud-Williams, 1988). A solution would be to vary crop life cycles to avoid creating a niche that favors any one weed.
Weeds and Fallow
Fallow generally reduces weed densities in subsequent crops (Derksen et al., 1994), but it is not a neutral selection force and can cause shifts in weed communities. For example, field pennycress and common lambsquarters have been strongly associated with mechanical–fallow rotations in southeastern Saskatchewan (Hume, 1982). Whereas herbicides can replace tillage for managing weeds in fallow years, strictly relying on them can select for difficult-to-control species, such as foxtail barley, cutleaf nightshade (Solanum triflorum Nutt.), and dandelion (Derksen et al., 1994; Lafond et al., 1994), and this approach may not be agronomically or economically sustainable. A combination of chemical and mechanical fallow has been a good option in areas where fallow is required (Blackshaw and Lindwall, 1995).
General Considerations Regarding Weed Changes, Tillage Systems, and Crop Rotations
Diversifying rotations to include both spring and winter crops has helped producers control weeds (Moyer et al., 1994; Blackshaw et al., 1994). Adding a perennial crop to rotations can reduce weed seed production (Kegode et al., 1999). Crop diversification provides more control opportunities and disrupts life cycles of weeds that are crop mimics (Anderson, 1997; Patriquin, 1988). Weed communities become more diverse in diverse cropping systems, thus minimizing the predominance of any one weed (Froud-Williams, 1988; Anderson, 1998a; Derksen et al., 1995). Additionally, more diverse crop rotations allow growers to vary the timing and modes of action of herbicides, thus delaying the evolution of herbicide-resistant biotypes (Jordan and Donaldson, 1996).
Crop choice and the sequence in which crops are grown have had a greater impact on weed community composition than tillage system (Derksen et al., 1996b; Thomas et al., 1996a). Grower experience has shown that merely changing tillage practices without increasing crop diversity within rotations has generally led to increased weed problems, especially if monoculture is practiced.
Weather patterns can influence weed responses to tillage systems and crop rotations. For example, over a 12-yr period including two dry periods in Saskatchewan, weed densities generally were lower in zero-tillage compared with conventional tillage in dry years. However, when wet fall and spring conditions followed several years of drought, zero-tillage plots were weedier (Watson et al., 1999). Weed recruitment from a dry layer of crop residue during wet years was thought to be the cause. Long-term research is required to fully understand weed community dynamics, and producers need to be made aware of conditions that favor weed growth in their respective systems.
Weed community composition and density change, depending on the intensity and frequency with which a selection pressure is applied. Therefore, varying selection pressure to keep weed communities from equilibrium can minimize weed densities and reduce adverse changes in the weed community. A diverse cropping system inherently includes varying seeding date, crop life cycle, herbicide modes of action, herbicide timing (preseeding, in-crop, preharvest, or postharvest), crop residues layers, and soil disturbance and provides an economical means of managing weeds by reducing weed densities and reliance on herbicides.
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NOVEL AND FUTURE CROPPING SYSTEMS AND WEEDS IN THE NORTHERN GREAT PLAINS |
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TOPABSTRACTINTRODUCTIONHISTORIC PERSPECTIVEWEED COMMUNITY CHANGESNOVEL AND FUTURE CROPPING...SPECIFIC WEED MANAGEMENT ISSUES...INTEGRATED WEED MANAGEMENT...CONCLUSIONSREFERENCES |
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Diversifying crops within crop types to provide a 4-yr cropping interval within a cycle of grass and broadleaf crops may further improve weed control by providing more planting date and herbicide options. For example, the concept of rotating cool-season and warm-season broadleaf and grass crops to intensify rotations in South Dakota (Beck, 1999) varies seeding date and herbicide selection pressures by default. The success of this approach may have as much to do with this intrinsic variation in selection pressure as with the rotation of crops with different photosynthetic pathways; however, further research is required to fully develop the concept.
Stacked Rotations
Recently, agronomic and economic success has been achieved using the novel concept of stacked rotations (Beck, 1999). Determining underlying reasons for the success of a stacked rotation of soybean [Glycine max. (L.) Merr.]–soybean–corn–corn–winter wheat–winter wheat compared with a typical 3-yr rotation of these crops would determine its applicability to other areas of the NGP. The primary reasons for success may be the inherently different selection pressure of corn, soybean, and winter wheat in terms of seeding dates, herbicide options, and competitive ability coupled with the 4-yr replanting break. If this is true, then merely stacking two or three similar crops may not be successful in the long term. The principle of varied selection pressure is likely more important than stacking rotations per se, but the concept of 2 yr of similar selection pressure followed by a 4-yr break deserves further exploration.
Reduced-Input Rotations
It has been commonly assumed that reducing inputs, especially herbicides, will lead to increased weed problems and reduced crop yields. In a recent study in Saskatchewan, weed seedling and seedbank densities were similar or lower in low-input rotations compared with high-input rotations (Fig. 3)
. This was accomplished by increasing variation in seeding dates of the spring crops. The lowest weed seedbank densities occurred where delayed seeding was used in a rotation of spring wheat and annual legumes to reduce herbicide and fertilizer usage. Conversely, the highest weed seedling densities occurred in a fully fertilized conventional tillage rotation using an aggressive postemergent herbicide strategy, but the variation in seeding date was not as intensive (Fig. 4)
. Therefore, reduced-input or, perhaps more correctly, appropriate-input systems have the potential to enhance the sustainability of cropping systems in the NGP.
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Organic Production
Organic farming has been practiced for some time in the NGP and has recently expanded to the point where more land is devoted to organic production in Saskatchewan than anywhere else in the world (Neil Strayer, personal communication, 1999). Organic farming may be at the state that zero-tillage was 20 yr ago in that some farmers have been viable in the system for up to 20 yr, indicating potential for this approach, but there is a need for research to facilitate sustained and broad-scale adoption of the system. Organic production that relies heavily on tillage for weed control increases soil erodability and is at odds with sustainable production principles; however, long-term organic farms have been found to use less tillage than many neighboring conventional tillage farms the same way that many long-term zero tillers used less herbicides than their neighbors (Clancy et al., 1993).
The key element to success of both organic and reduced tillage systems has been diverse crop rotations where weed management was a long-term strategy (Clancy et al., 1993). Organic production systems, however, may provide a unique environment for weed germination, establishment, and management. Further understanding weed population dynamics in organic fields would provide insight into managing weeds in conventional cropping systems, especially in low-input rotations. Furthermore, consumer interest in organic production is increasing, and there is very little research support for this expanding production system. Martin Entz (personal communication, 1999) at the University of Manitoba recently completed a 12-yr study that compared various input management systems, including organic. A long-term study comparing organic, reduced tillage, and conventional systems has completed one rotational cycle at Scott, SK (Brandt et al., 1999), and the fourth year of an organic rotation study at Lethbridge, AB, has been completed.
Cover Crops, Perennial Forages, and Crop Residue Management
Crop residue management can provide a means of improving weed control. Winter wheat residue reduced weed seedling emergence by 45% (Crutchfield et al., 1986) and biomass by 60% (Wicks et al., 1994a) in corn. Residues suppress weed emergence by reducing light penetration and soil temperature fluctuations (Teasdale and Mohler, 1993). Weed suppression has been related to residue quantity, with 3000 kg ha-1 being the minimum required for suppression (Crutchfield et al., 1986; Vander Vorst et al., 1983). Weed suppressant effects of cover crops were increased twofold when combined with no-till compared with a reduced till system (Anderson, 1999a). Tilling with the sweep plow minimizes the positive effect of residues on weed emergence by burying weed seeds and residues in soil.
Cover crops grown in rotation have the potential to aid weed management in all production systems. Excellent weed suppression has been shown with underseeded biennial sweetclover [Melilotus officianalis (L.) Lam.] (Moyer et al., 1997). Similarly, the weed suppressant effect of perennial alfalfa (Medicago sativa L.) has been shown to last for 3 yr for wild oat and other, but not all, weed species (Entz et al., 1995; Ominski et al., 1999). Enhancing the natural biological, physical, and chemical suppressant abilities of crop residues and forage crops in reduced tillage and reduced pesticide systems would benefit producers and the environment in the NGP.
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SPECIFIC WEED MANAGEMENT ISSUES IN THE NORTHERN GREAT PLAINS |
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TOPABSTRACTINTRODUCTIONHISTORIC PERSPECTIVEWEED COMMUNITY CHANGESNOVEL AND FUTURE CROPPING...SPECIFIC WEED MANAGEMENT ISSUES...INTEGRATED WEED MANAGEMENT...CONCLUSIONSREFERENCES |
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The severity of the resistance problems identified in Table 1 depends on the geographical distribution across the NGP and on what alternative herbicides are available to control the resistant weed. Currently, the most severe problem is wild oat resistance to Group 1 herbicides. Wild oat is a dominant weed in western Canada in terms of distribution and ability to cause crop yield reductions. Group 1 herbicides are widely used because they are very efficacious. Group 2 resistance may become as or more severe than Group 1 resistance because of the large number of broadleaf and grass weeds that are targeted by Group 2 (imidazolinone) herbicides. With increased pulse production in the NGP, new Group 2 herbicides for wild oat, and the development of wheat and canola resistant to Group 2 herbicides, wild oat resistance to Group 2 herbicides is expected to increase. The use of canola varieties resistant to Group 9 (glyphosate) and 10 {glufosinate [2-amino-4-(hydroxymethylphosphinyl)butanoic acid]} herbicides has provided new mechanisms of action to combat resistant patches of weeds in the short term, but if applied frequently enough, they may cause selection for resistant or cross-resistant biotypes.
Herbicide-Tolerant Crops
Herbicide-tolerant canola represents a relatively new means of controlling problem species in canola production, such as wild mustard, field pennycress, catchweed bedstraw, redstem filaree, and shepherd's-purse, and they have been readily adopted in western Canada. Currently, glyphosate-, glufosinate-, and imidazolinone-tolerant cultivars are available commercially. The first two types are transgenic in origin. Herbicide-tolerant canola accounted for 75% of the 6 million ha of canola grown in 1999 in western Canada (Warwick et al., 1999). Glyphosate-tolerant canola alone accounted for more than half of the herbicide-tolerant canola production. To date, shifts in weed community composition due to the adoption of this technology have not been documented, but the potential exists to reduce densities of previously uncontrolled Brassica spp. weeds, reduce patches of herbicide-resistant weed species, and select for species that are not controlled well by low rates of glyphosate, such as those in the smartweed family (Polygonaceae family); field horsetail (Equisetum arvense L.); round-leaved mallow (Malva pusilla L.); and overwintering species such as horseweed, narrowleaf hawksbeard, scentless chamomile (Matricaria perforata Merat), and dandelion (Derksen et al., 1999a).
Apart from concerns regarding the safety of transgenic crops, the usage of herbicide-tolerant canola has raised an number of agronomic concerns. Although generally thought to be unlikely, gene flow has been documented between crops and related weed species (Warwick et al., 1999). Should this occur with herbicide tolerance, the complexity of weed management would increase. The problem could become even greater if the field occurrence of multiple herbicide-resistant volunteer canola becomes widespread.
With the possible release of glyphosate-, glufosinate-, and imidazolinone-resistant wheat, selection pressure for resistance of common weeds, such as wild oat, to these herbicides will intensify. This will be especially true for glyphosate in zero-tillage where reliance on glyphosate is already high and for the commonly used imidazolinone herbicides with which resistance problems have developed quickly. Increases in the cost and complexity of volunteer herbicide-tolerant crop management may jeopardize the sustainability of conservation tillage systems in the NGP. Utilizing IWM strategies to minimize the problems associated with herbicide-tolerant crops and maximize their benefits will be important in the near future.
Soil Residual Herbicides
Changes in NGP cropping patterns toward a diversified reduced tillage system has implications for herbicide use patterns. Cropping systems in the NGP differ markedly from the U.S. Midwest. Cereals and crops such as canola and tame mustard (B. hirta Moench) are more competitive with weeds than row-crop corn and soybean, and thus require less herbicide inputs. Additionally, economics dictate that less herbicide inputs will be used in lower-yielding and lower-value crops. Choice of herbicides also differs between these regions. There is less dependence on soil-applied herbicides, such as atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine), alachlor [2-chloro-2',6'-diethyl-N-(methoxymethyl)acetanilide], metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)N-(2-methoxy-1-methylethyl)acetamide], butylate (S-ethyl di-isobutylthiocarbamate), and trifluralin (
,,-Trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine). This is due to the herbicides being less efficacious in these typically dry soils, differences in crops grown, less of a need to control multiple weed flushes throughout the growing season in narrow-row crops, and an increase in availability of postemergence herbicide options. Ethalfluralin [N-ethyl-N-(2-methyl2-propenyl)-2,6-dinitro-4-(trifluromethyl)benzenamine], trifluralin, and triallate, the more common soil-applied herbicides used in the NGP, have been found to be efficacious in reduced tillage as a nonincorporated surface application (Kirkland and Johnson, 1999). The low water solubilities of these herbicides and low precipitation typical of the NGP greatly reduces the potential for surface runoff and deep leaching of soil-applied herbicides.
The majority of herbicides used in the NGP are applied postemergence at low use rates and have short persistence in the environment. Typical examples are tralkoxydim {2-[1-(ethoxyimino)propyl]-3-hydroxy-5-(2, 4,6-trimethylphenyl)-2-cyclohexen-1-one}, fenoxaprop {2-[4-[(6-chloro-2-benzoxazolyl)oxy]phenoxy] propanoic acid}, sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one}, thifensulfuron {3-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]amino]sulfonyl]-2-thiophenecarboxylic acid}, tribenuron {2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)methylamino]carbonyl]amino]sulfonyl]benzoic acid}, bromoxynil (3,5-dibromo-4-hydroxybenzonitrile), 2,4-D (2,4-dichlorophenoxyacetic acid), and dicamba (3,6-dichloro-2-methoxybenzoic acid). The potential for herbicide loss as surface runoff or by leaching to ground water is less in the NGP than in the U.S. Corn Belt. In some cases, herbicide damage to subsequent sensitive crops has been shown to be less, and in other cases greater, in reduced tillage (Holm, 1994). Further research is required to compare efficacy and runoff from typical herbicides used in the evolving diversified reduced tillage systems of the NGP.
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INTEGRATED WEED MANAGEMENT STRATEGIES AND CROPPING SYSTEMS |
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TOPABSTRACTINTRODUCTIONHISTORIC PERSPECTIVEWEED COMMUNITY CHANGESNOVEL AND FUTURE CROPPING...SPECIFIC WEED MANAGEMENT ISSUES...INTEGRATED WEED MANAGEMENT...CONCLUSIONSREFERENCES |
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In winter wheat, yield loss due to downy brome was reduced twofold by growing taller cultivars (Challaiah et al., 1986; Blackshaw, 1994a), whereas yield loss in safflower due to green foxtail was reduced threefold by increasing safflower planting density (Blackshaw, 1993). Spring wheat's tolerance of wild oat and green foxtail was increased when N fertilizer was banded near the seed compared with broadcast applications, resulting in 12% more grain (Kirkland and Beckie, 1998). In no-till systems, effect of N placement on barley tolerance to weeds was even greater (O'Donovan et al., 1997). However, adequate separation between seed and fertilizer was essential, especially at wide-row spacings; otherwise, suppression of crop growth could lead to increased weed problems (Derksen et al., 1999b).
Cultural practices that favor crop competitiveness also reduce seed production of weeds infesting the crop, subsequently reducing weed densities in future crops (Wicks et al., 1994b). Seed production of jointed goatgrass was reduced 10% by a taller winter wheat cultivar or increased seeding rate (Anderson, 1997), whereas increasing safflower density reduced seed production of green foxtail by 87% (Blackshaw, 1993).
Producers can enhance the cultural effect on crop growth by combining several practices into a cultural system, leading to a synergistic improvement in control. For example, corn suppression of foxtail species was improved 10 to 15% when a cultural practice such as N banding, narrow rows, or increased crop density was used alone (Anderson, 1999b). Combining two of the practices reduced weed biomass by 25 to 30%, whereas a cultural system comprised of all three practices reduced weed biomass 70%. This approach also synergistically increased corn yield. When a single cultural practice was used, grain yield loss due to weeds was 29% compared with a 33% loss in the conventional system (Fig. 5) . When two cultural practices were combined, yield loss was 23%; in contrast, yield loss was only 8% with the cultural system where all three cultural practices were applied (a threefold decrease in yield loss compared with the system using two cultural practices).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONHISTORIC PERSPECTIVEWEED COMMUNITY CHANGESNOVEL AND FUTURE CROPPING...SPECIFIC WEED MANAGEMENT ISSUES...INTEGRATED WEED MANAGEMENT...CONCLUSIONSREFERENCES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONHISTORIC PERSPECTIVEWEED COMMUNITY CHANGESNOVEL AND FUTURE CROPPING...SPECIFIC WEED MANAGEMENT ISSUES...INTEGRATED WEED MANAGEMENT...CONCLUSIONSREFERENCES |
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