slowing weed evolution with integrated weed management
TRANSCRIPT
Slowing weed evolution with integrated weed management
K. N. Harker
Agriculture and Agri-Food Canada, Lacombe Research Centre, 6000 C & E Trail, Lacombe, Alberta, CanadaT4L 1W1 (e-mail: [email protected]). Received 12 February 2013, accepted 24 April 2013.
Harker, K. N. 2013. Slowing weed evolution with integrated weed management. Can. J. Plant Sci. 93: 759�764. Formillennia, weeds have slowly evolved in response to ever-changing environments and crop production practices. Weeds arenow evolving much more quickly due to consistently repeated cropping systems and intense herbicide selection pressures.Weed resistance to herbicides now threatens cropping system sustainability in several industrialized nations. Integratedweed management (IWM) provides opportunities to reduce selection pressure for weed resistance while maintainingcurrent crop yields. Combining optimal IWM tactics that discourage weeds by minimizing disturbance (no till, direct-seeding), adopting diverse crop rotations, and attempting to preclude resource acquisition by weeds are encouraged. Newresearch knowledge on practical IWM systems is available, but despite current and looming threats of major weedresistance, most crop producers will require greater incentives than those currently available to more-fully adopt IWMsystems in the near future.
Key words: Cultural weed management, diverse crop rotation, seeding rate, no-till, weed ecology, weed resistance to herbicides
Harker, K. N. 2013. Freiner l’evolution des mauvaises herbes par la lutte integree. Can. J. Plant Sci. 93: 759�764. Depuis desmillenaires, les adventices evoluent lentement, s’adaptant a un milieu et a des pratiques agricoles qui changentconstamment. Les mauvaises herbes progressent desormais beaucoup plus vite a cause des systemes agricoles qui serepetent et de la vive selection engendree par l’emploi des herbicides. La resistance des mauvaises herbes aux desherbantsmenace dorenavant la perennite de l’agriculture dans plusieurs pays industrialises. La lutte integree permettrait d’attenuerla pression selective qui rend les adventices resistantes tout en preservant le rendement des cultures. On preconise decombiner les tactiques optimales de lutte integree qui freinent les mauvaises herbes en perturbant le moins possible le sol(non-travail, semis directs), en variant les assolements et en empechant au maximum les mauvaises herbes d’acquerir desatouts. La recherche a ajoute de nouvelles connaissances sur les methodes pratiques de lutte integree, cependant, malgre lesmenaces actuelle et immanente de resistance chez d’importantes adventices, la plupart des cultivateurs auront besoin deplus grandes mesures d’encouragement que les incitatifs courants pour adopter pleinement la lutte integree dans un procheavenir.
Mots cles: Lutte contre les mauvaises herbes dans les cultures, variete des assolements, densite des semis, non-travail du sol,ecologie des mauvaises herbes, resistance des mauvaises herbes aux herbicides
Given their capability for viable mutations, and theirsubsequent inherent genetic diversity, weeds adapt toany consistent practice used against them. For the samereason, human pathogenic bacteria develop resistance toanitbiotics that are repeatedly prescribed to controlthem. A remarkable example of weed adaptation inrice (Oryza sativa L.) production is described by Barrett(1983). Many years of continuous hand-weeding inbarnyardgrass [Echinochloa crus-galli (L.) Beauv.] se-lected for a rice-mimic barnyardgrass biotype that wasdifficult to distinguish from actual rice plants. The lattersuggests that it is ill-advised to dismiss weed resistanceto any over-used weed management tactic.
WEED RESISTANCE TO HERBICIDESIn modern agriculture, the practice that is most oftenemployed against weeds involves herbicide application.In western Canada, more is spent on herbicides for wildoat (Avena fatua L.) control ($500 million annually)than on any other weed species (Leeson et al. 2006).However, Gould (1991) warned that ‘‘Many of theshort-term triumphs of pest control have carried within
them the seeds of longer-term failure.’’ Therefore, itshould not be surprising that wild oat is the mostdominant herbicide-resistant (HR) weed in westernCanada (Beckie et al. 2013). The overuse of herbicidesglobally has led to 396 HR weed biotypes among 210HR weed species (Heap 2013). Integrated weed manage-ment (IWM) necessitates the use of weed managementtools other than just herbicides. Adopting diversifiedIWM practices has the potential to slow the evolutionof weed resistance to herbicides and other weed manage-ment techniques.
REAL INTEGRATED WEED MANAGEMENTIntegrated weed management is practiced at varyinglevels all over the world. Real integrated pest manage-ment or real IWM requires that pests or weeds aremanaged with more than just fungicides or herbicides(Ehler 2006). Some promote ‘‘integrated herbicidemanagement’’ as IWM (Harker and O’Donovan 2013),
Abbreviations: HR, herbicide-resistant; IWM, integrated weedmanagement
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but many are more careful to integrate differentmethods of weed management into their researchprograms and cropping systems.
Many researchers are developing alternative methodsof weed management. For example, relatively re-cent innovations suggest that air-propelled corn grit(Forcella 2012), band-steaming (Melander et al. 2002),cryogenic salts (Jitsuyama and Ichikawa 2011), theHarrington seed destructor (Walsh et al. 2012), roboticweeders (Blasco et al. 2002), uniform spatial planting(Weiner et al. 2001), and weed-suppressing Brassica seedmeal (Handiseni et al. 2011) all have potential as weedmanagement alternatives to herbicides. Nazarko et al.(2005) provide an in-depth review of systems and tacticsfor herbicide use reduction in Canadian field crops. It isnot, however, encouraging that even among weedresearchers, herbicidal weed control is overwhelminglythe most-commonly published method of weed manage-ment (Harker and O’Donovan 2013). Cropping systemand herbicide sustainability will require much moreresearch on alternatives to herbicides and greateradoption of real IWM.
MANAGING WEED EVOLUTIONThe rate and extent of weed evolution is dependent uponmany factors including fecundity, mating patterns, traitdominance, heritability, gene flow, mutation frequency,and selection pressure. Most of these factors areinherent to specific weed species and cannot be manipu-lated by farmers. Crop producers can have some impacton weed fecundity and gene flow, but their greatestopportunity to influence weed evolution is via theselection pressure their specific cropping systems andweed management practices exert on weed populations(Jasieniuk et al. 1996).
BIG HAMMERS VERSUS MANY LITTLE HAMMERSThe majority of Canadian cropland receives at least oneherbicide application every year (Statistics Canada2006). In terms of their impacts on weed evolution,herbicides can be considered to be ‘‘big hammers’’. Onthe basis of the mean number of herbicide applicationsrequired before weed resistance is detected, there isconsiderable variation in herbicide group risk ratings(Beckie 2006). Nevertheless, it took only 3 yr of ACCaseherbicide (‘‘High’’ risk) applications to select ACCase-resistant wild oat populations (Legere et al. 2000;Uludag et al. 2008) and ]6 yr of glyphosate (‘‘Low’’risk) applications to select glyphosate-resistant Palmeramaranth (Amaranthus palmeri S. Wats) (Steckel et al.2008). Although selection for glyphosate-resistance isconsidered to be ‘‘Low’’ risk (Beckie 2006), weed speciesshifts in rotations with continuous glyphosate-resistantcrops can be detected in only 3 yr (Harker et al. 2005).Because triallate (‘‘Moderate’’ risk) is applied only onceeach growing season and is not as efficacious or over-used as glyphosate or ACCase herbicides, 18 yr oftriallate applications in continuous wheat production
were required before wild oat resistance to triallatewas detectable (Beckie and Jana 2000). Perhaps thereason Canadian Prairie farmers never experienced wildoat resistance to barban (Carbyne) was due to itsrelatively low and variable efficacy that failed to exertsufficient selection pressure on susceptible wild oatpopulations.
Liebman and Gallandt (1997) coined the expression‘‘many little hammers’’ to suggest that several relativelylow efficacy weed management tactics can lead toadequate weed control when used in combination. Non-herbicidal weed management tactics may include: direct-seeding (no-till), using certified seed, planting relativelylarge seed, higher than normal crop seeding rates,strategic fertilizer placement, growing competitive culti-vars and crop species, narrow crop row spacing, alter-nating crop life cycles, and purposefully diversifying cropseeding dates. Almost none of these practices used alonewill provide adequate weed management, but whenapplied in varying combinations, they can not onlyadequately manage weeds, but also slow weed evolutionand adaptation to any single practice. It would take sometime for weed populations to adapt to a cropping systemthat combined crops of different life cycles with severalcultural weed management practices. From a weedevolution standpoint, the diversified selection pressureexerted against weeds with several simultaneous andconstantly changing combined techniques would bedramatically lower than the selection pressure from anysingle, repeated practice.
TACTICS THAT DISCOURAGE WEEDSThe most effective ‘‘little hammers’’ are likely to varyacross agroecozones and within different weed-cropscenarios. Booth et al. (2003b) suggest that there arethree main habitat traits that facilitate weed invasions:disturbance, low species richness, and high resourceavailability. Accordingly, successful IWM tactics woulddiscourage weeds by minimizing disturbance (no till,direct-seeding), adopting diverse crop rotations, and,where feasible, attempting to preclude light, nutrientand water resource acquisition by weeds. These tacticsmaximize niche occupation by crop species and therebyreduce niche availability to weeds.
Minimizing Soil DisturbanceTillage generally encourages weed seed germination(Mohler 2001). However, many early no-till studiesdocumented higher weed populations following theadoption of no-till practices (Cardina et al. 1991; Brandt1992; Blackshaw et al. 1994, 2001; Vencill and Banks1994). Leaving weed seeds on the soil surface where theycan readily germinate can increase initial weed popula-tions, but the soil surface in no-till fields cannot beconsidered to be a ‘‘safe site’’ (Booth 2003a) for long-term weed seed survival; longevity will decrease(Roberts and Feast 1972, 1973; Derksen et al. 1996;
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Menalled et al. 2000; Blackshaw 2005; Harker et al.2009a).
Eliminating tillage can reduce weed populations overtime (Anderson 2008). Derksen et al. (1993) showed thatspecific tillage practices also led toweed population shifts,but that weed communities were impacted more bylocation and year effects than tillage systems. At the soilsurface, environmental extremes and predation are mostprevalent and seed survival is reduced over the long-term(Sagar and Mortimer 1976; Mohler 2001). O’Donovanand McAndrew (2000) showed that even though severalweeds had higher weed seedbanks under no-till, actualweed seedling recruitment was lower in no-till than inother tillage regimes. Anderson (2005) observed that after3 yr of no-till, weed seedling emergence was eightfoldlower than in a tilled system. After 18 yr in a winterwheat�sorghum�fallow rotation, Wicks et al. (1988)observed three- to fivefold reductions in weed seedlingsin no-till versus tillage treatments. Furthermore, buryingcanola seed via tillage operations promotes secondarydormancy and increases volunteer canola persistence inthe soil seed bank (Pekrun et al. 1998; Gulden et al. 2003;Harker et al. 2006). It is also likely that allelochemicals insome crop residues would be more concentrated andinhibitory to weed seeds germinating at or near the soilsurface in no-till fields.
Diverse Crop RotationsWeeds fortunate enough to grow in simple, repeatedcropping systems have little difficulty adapting (evol-ving) and thriving in consistently repeated environments.On the Canadian prairies, after climatic influences, croprotation is the most influential practice governing weedcommunities (Blackshaw et al. 2006). Diverse rotationsprovide farmers with opportunities to employ variablecrop life cycles, planting dates, harvest dates, cropresidues, tillage and weed management practices torestrict weed evolution (Liebman and Staver 2001).Derksen et al. (2002) reported that weed densities wereminimized when diverse cropping systems were em-ployed to constantly alter the selection pressure onweed communities.
In concept, it is relatively simple to design croppingsystems to disadvantage specific, major weeds. Forexample, if the predominant weed community includessummer annual weeds such as wild oat and wildbuckwheat (Polygonum convolvulus L.), one can disruptthe life cycle of those weeds by growing winter annual orperennial crops. The life cycle of summer annual weedscan also be disrupted in summer annual crops byaltering normal seeding and harvesting dates. Whenbarley silage was cut early for 3 consecutive years, wildoat populations in the absence of herbicides werereduced to levels similar to those in a barley graincrop with full herbicide rates (Harker et al. 2003b).Alternatively, if the dominant weed is a winter annualsuch as downy brome (Bromus tectorum L.), the best
way to disadvantage that weed is by growing a summerannual crop such as canola (Blackshaw 1994).
In practice, growers are pressured by short-termeconomic realities (cash flow) that cause most of themto repeatedly grow the most profitable crops. In recentyears, perennial forage and winter cereal crops, thatwould readily discourage most major Canadian prairieweeds, are only grown by the most forward-thinkinggrowers; spring canola and wheat are so profitable!Right now, the most common rotation on the Canadianprairies is spring-sown canola�wheat. At some point,after years of minimal diversity, our best technologieswill flounder, and diversity will be enforced by nature.
Reduce Resource Acquisition by WeedsRapid response to high resource availability is aphysiological attribute of most early succession plants,especially weeds (Baker 1974; Bazzaz 1979). However, itis also true that weeds can be quickly disadvantagedwhen resources are not readily available to them.Generally, weed’s small seed size relative to crops isassociated with higher relative growth rates and greaterbiomass partitioning to ‘‘thin’’ leaves (Mohler 2001);this puts them at a disadvantage when resources are notsecured early in their life cycle. For example, weeds arerelatively susceptible to the negative influences of shade(Fenner 1978; Mohler 2001). Similarly, without readyaccess to crop nutrients, weeds suffer relatively greatergrowth declines than crops (Vengis et al. 1955). There-fore, it is important to place fertilizer strategically sothat the crop has better access to nutrients than weeds(Blackshaw et al. 2004).
Clements et al. (1929) noted that almost all of theadvantages of competing plant species could be de-scribed by two words � amount and rate: more rapidand complete germination, more rapid growth of rootsand shoots, taller and more branching stems, deeper andmore spreading roots, and larger leaves. Practices thatencourage rapid and uniform crop emergence such asusing certified seed and seeding shallow in moist soilscan improve the outcome of crop�weed competitioninteractions. Weeds emerging at times similar to thecrop cause greater yield loss than those that emerge later(O’Donovan et al. 1985; Bosnic and Swanton 1997).Residues left on the surface of no-till fields increasesoil moisture levels at crop seeding and can enhance andspeed crop germination and emergence to facilitatepre-emptive resource acquisition (Harker et al. 2009a).
STRATEGIC TACTIC COMBINATIONS(INTEGRATED WEED MANAGEMENT)
A single ‘‘big hammer’’ speeds weed evolution; a single‘‘little hammer’’ has insufficient efficacy on weeds tosignificantly influence selection pressure. Combiningmultiple tactics can delay weed evolution and beefficacious. Anderson (2000) found that, in the absenceof herbicides, combining several practices with no-till(higher seeding rate, banded nitrogen, taller cultivars,
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delayed seeding) reduced pigweed (Amaranthus spp.)biomass and seed production by at least 85% andeliminated proso millet (Panicum miliaceum L.) yieldloss. In several western Canada studies, combiningoptimal cultural practices (e.g., crop rotation, variableseeding date, higher seeding rate, competitive cultivars)with low herbicide rates provided weed managementand yield benefits similar to those in less diverse systemsemploying higher herbicide rates (O’Donovan et al.2001, 2013; Blackshaw et al. 2005; Holm et al. 2006;Harker et al. 2009b). For example, Harker et al. (2009b)showed that combined optimal weed managementtactics interacted synergistically against wild oat bio-mass (Table 1). Whereas single tactics reduced wild oatbiomass by two- to threefold, combined double or tripletactics reduced biomass six- to seven- or 19-fold,respectively. Yield and weed management benefitsfrom combined cultural practices are also evident evenwhen herbicides are applied at full rates (Harker et al.2003a).
FARM ADOPTION OF INTEGRATED WEEDMANAGEMENT PRACTICES
When will the majority of crop producers adopt morediverse weed management strategies? While it is en-couraging that many western Canadian growers haveadopted higher than normal seeding rates for yield, cropmaturity and even IWM considerations, the mostimportant IWM practice, diverse crop rotation, isusually ignored in favour of short-term profits. Farmsize is increasing and the efficiencies and simple deci-sions required to grow the same crops year after yearappear to be too enticing (Van Acker 2008). Beckie(2006) suggests that growers do not generally changepractices to avoid weed resistance; most act only whenweed resistance is an issue on their land. Rapid weedevolution and adaptation to simple cropping systemsand crucial herbicide tools will continue until weedresistance, environmental considerations or economicincentives persuade growers to adopt diverse croppingsystems and IWM.
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Table 1. Treatments and treatment combinations that improve wild oat
biomass reductions (x-fold) versus a less optimal treatment combinationz
No. factors Treatment combinationy (x)
1 2� vs. 1� seeding rate 2.9Tall (T) vs. Short (S) cultivar 1.9Rotation (R) vs. Continuous (C) barley 2.7
2 2� & T vs. 1� & S 6.32� & R vs. 1� & C 7.7T & R vs. S & C 7.3
3 2� & T & R vs. 1� & S & C 18.7
zTable adapted from data in Harker et al. (2009b).ySingle and double treatment combinations (1 or 2 factors) areaveraged over the remaining factor(s).
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