Impact of Agronomic Practices on Weed Communities ...

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Author(s): Douglas A. Derksen, A. Gordon Thomas, Guy P. Lafond, Heather A. Loeppky and. Clarence J. ... For more information about JSTOR, please contact [email protected]. ...... The authors acknowledge the technical support from the staff.
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Impact of Agronomic Practices on Weed Communities: Fallow within Tillage Systems Author(s): Douglas A. Derksen, A. Gordon Thomas, Guy P. Lafond, Heather A. Loeppky and Clarence J. Swanton Reviewed work(s): Source: Weed Science, Vol. 42, No. 2 (Apr. - Jun., 1994), pp. 184-194 Published by: Weed Science Society of America and Allen Press Stable URL: http://www.jstor.org/stable/4045392 . Accessed: 21/12/2012 14:25 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp

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Weed Science, 1994. Volume42:184-194

Impactof AgronomicPracticeson WeedCommunities:FallowWithinTilage Systems' DOUGLAS A. DERKSEN,A. GORDONTHOMAS,GUY P. LAFOND, HEATHERA. LOEPPKY,and CLARENCEJ. SWANTON2

Abstract.Continuous-cropping conservationtillage systems may providea viablealternativeto the practiceof summer fallow;however,concernshavebeenraisedregardingpotentially negativechangesin weed communitiesin continuous cropping.Field experimentswere establishedin Saskatchewan at three locations to determine the nature of weed communitydifferencesbetweena crop sequencewith and withoutfallowin zero-,minimum-,and conventional-tillage systemsfrom1986to 1990.Weedcommunitiesin continuouscroppingtreatmentstended to have greatertotal densities andweremoresimilarin compositionthancrop-fallowtreatments. Inclusionor exclusionof fallow within the rotation had a greaterimpacton weedcommunitycompositionthan did tillagesystemat Itunaand Waldron,but the reversewas true at Tadmoredue to poor crop growthin all tillagesystems.Differencesin weedcommunitycompositionweregenerally characterizedby fluctuational changes in species associations.Volunteersof summer-annualcrops, such as canola, flax, and barley, were associatedwith continuous cropping,but otherspeciesincludingperennialweeds,such as Canadathistle,perennialsowthistle,andquackgrass,were notstronglyassociatedwiththepresenceorabsenceof fallow. The practiceof fallowingland to manageweedsmay not be necessary.Nomenclature:Canadathistle, Cirsiumarvense (L.)Scop.#3 CIRAR;perennialsowthistle,Sonchusarvensis L. # SONAR; quackgrass,Elytrigia repens (L.) Nevski # AGRRE;Barley,HordeumvulgareL. # HORVX;canola, Brassica napus L. # BRSNS; flax, Linum usitatissimumL. # LINUS. Additional index words: Zero tillage, minimum tillage, sum-

merfallow,canonicaldiscriminantanalysis. INTRODUCTION

The objectives of mechanicalor chemical fallowing of crop land are to conserve soil moisture, release soil nutrients,and controlweeds (3, 4, 12, 24). Fallowinglandmechanicallycan be a majorcontributorto soil erosionand degradationdue to excessive tillage, and must be reducedto promotelong-termagricultural sustainability(7, 11, 17). Researchersas early as 1925

'Received for publicationJanuary8, 1993, and in revised form October 16, 1993. 2Res. Sci., Agric. Canada,Box 760, IndianHead, SK, SOG2K0; Res. Sci., Agric. Canada,Box 440, Regina, SK, S4P 3A2; Res. Sci., Agric. Canada,Box 760, IndianHead, SK, SOG2K0; Biol., Agric. Canada,Box 1240, Melfort, SK SOE lAO; Assoc. Prof., Univ. Guelph, Guelph, ON, NIG 2W1. Researchconductedin partialcompletionof Ph.D of first author. 3WSSA-approvedcomputercode from Composite List of Weeds, Revised 1989. AvailablefromWSSA, 1508 WestUniversityAve., Champaign,IL 618213133.

attemptedto develop cropping sequences without fallow that were suitable to the short growing season and semiaridconditions in Saskatchewan(6); however, in 1985, fallow accounted for 30% of Saskatchewancropland(22). Thereasonsfor fallowinglandmay no longerbe valid.Fallow is an inefficient methodof increasingsoil moisturestorage(22) but may be a necessary practice under the arid and semiarid conditions of the brown and dark-brownsoil zones of the prairies, especially in droughtyears (4, 8). Historically,fallow has been used to supply nutrientsto crop plants, but the benefit of releasingnutrientsfrom the soil decreasesas soil organicmatter levels decrease (6). Currently,fallowing may not provide adequatenutrientsfor crop productionin Saskatchewan(22). Weedspecies varyin theirresponseto fallow.In some studies, crop-fallowsequenceshave been found to decreaseweed numbers comparedto continuouscropping,particularlyweeds with shortdormancyperiodsandcertainperennialweeds (5, 10, 13), but the density of some species, such as common lambsquarters andfield pennycress,increaseswith fallow (13). The cumulative effect of variedcrop competitiveabilities,dates of seeding, and weed managementpractices in crop rotationsis to reduce the dominanceof specific weeds (1, 12);therefore,cropsequencing, even withoutfallowing,generallydecreasesweed problems(18, 23). Crop sequencing coupled with conservationtillage may conservesoil moisture,increasesoil organicmatter,andsuppress weeds, therebyprovidingan alternativeto fallow. East-centralSaskatchewanis in the moist black chernozemic soil zone wherefallow may not be necessarybutis still practiced. TIheelucidationof weed communitydynamicsin conservation tillage, practicedwithout fallow, may aid in the furtheranceof conservationtillage practicesandthe abatementof soil degradation. The objective of this study was to determinethe influence of fallow on weed communities in a zero-, minimum-, and conventional-tillagecrop sequence.

ANDMETHODS MATERIALS

Field researchwas conductedin Saskatchewanat Itunaand Waldronfrom 1986 to 1990 and at Tadmorefrom 1986 to 1988 on orthic black chernozemsoils (Udic Boroll). The soil types wereanOxbowloam,Yorktonlight loam, anda Meotafine sandy loam, respectively.The field history of all three sites included crop-fallowsequences;therefore,in orderto ensurethatthe data reflected treatment responses and not experimental start-up anomalies,only datafrom 1988 through1990 were used for the analysis of weed communities. Experimental design. The experiments were designed in a randomizedsplit-split-plotdesign with fourreplicationsat Ituna andWaldron,andsix replicationsatTadmore.Tillagesystemwas the main plot, the inclusion or exclusion of fallow within a crop

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WEED SCIENCE Table1. Cropsequence from 1986 to 1990 for continuous-croppingand crop-fallowtreatments.a Crop-fallowb Phase 3

Phase 2

Phase 1

Continuous-cropping Year

Phase 1

Phase 2

Phase 3

a

b

a

b

a

b

1986 1987 1988 1989 1990

Canola Wheat 1 Wheat 2 Flax Barley

Wheat 1 Wheat 2 Canola Barley Wheat 2

Wheat 2 Canola Wheat 1 Wheat 2 Flax

Canola Fallow Wheat 1 Fallow Wheat2

Fallow Canola Fallow Barley Fallow

Wheat 1 Fallow Wheat2 Fallow Flax

Fallow Wheat 1 Fallow Wheat2 Fallow

Wheat2 Fallow Canola Fallow Barley

Fallow Wheat2 Fallow Flax Fallow

a'Westar'Canola,Wheat 1-'Hy320' Canadaprairiespringwheat, Wheat2-'Katepwa' hardred springwheat, 'Norlin'flax, 'Harrington'barley,Fallow year. bEntrypoint of fallow within crop sequence.

sequence was the subplot, and the crops grown in sequence comprised the sub-subplots. All phases of the rotations were presenteach year at each site; therefore,the datawere balanced at the level of tillage system and crop sequence (Table 1). Soil disturbanceoccurredonly at seedlingin zerotillage.Plots werecultivatedin minimumtillage in the springpriorto seeding, and cultivationoccurredboth in the fall and springin the conventional-tillagesystem. A field cultivatorwas used for tillage operations.In place of fall cultivation,herbicideswere used to controlwinter annualweeds in zero- and minimum-tillagesystems (Table2). Continuous-croppingand crop-fallow subplots consisted of the crops grown in sequence with and without a fallow year betweeneach crop (Table1). The crop sequencewas selected on the basis of rotatingcerealandbroadleafcropscommonlygrown in the area.The crop plots were 6.7 m by 12.5 m in size. Duringfallow years, weeds were controlledby herbicidesin zero-tillage (Table 2) early-season herbicide use followed by cultivationin minimum-tillage,and by mechanicalmeansin the conventional-tillageplots. Weed managementin fallow years dependedon weed density, and the frequencyof control measures were adjustedaccordinglyto ensure thatweeds did not set seed. Zero-tillageplots received threeto four herbicideapplications. Minimum-tillageplots received one to three herbicide treatmentsfollowed by two to four cultivations per year. In conventional-tillageplots, fall tillageplus threeto six subsequent tillage operationswere conductedin fallow years. Crop agronomy. 'Westar'canola was seeded at 7.7 to 8.7 kg ha-', 'Katepwa'hard red spring wheat at 132 to 139 kg ha-', 'Hy320'prairiespringwheat(TriticumaestivumL.) at 150 to 164 kg ha-', 'Harrington'barleyat 82 kg ha-', and 'Norlin'flax at 56 kg ha-'. Seedingof all cropsat each site was generallycompleted within 2 d of the initiation of seeding. In orderto reduce confounding effects due to the use of differentseeding equipment, seeding was done in all tillage systems with a modifiedcommercial Edwards4hoe drill with 20-cm row spacings equipped to place fertilizerwith the seed andin a midrowbandbetweenevery othercrop row using a double offset disk. Acceptable seed and 4EdwardsRodweeder,Inc., Lethbridge,AB. 5TeejetModel SS80015. SprayingSystems Co., Wheaton,IL 60187.

fertilizerplacementoccurredin all tillage systems. Soil samples were takenin the fall of each year to determinesoil fertilityand fertilizerwas addedto adjustnutrientsto recommendedlevels at seeding (22). Based on similarambientfertilitylevels, the three tillage systems were given the same level of added nutrients within a site, for a given year. Recommendedherbicides(19) were appliedin all yearsto all croppedplots (Table2), and the same nonresidualPOST herbicides were appliedwithin crops in all three tillage systems at a given site, with severalexceptions.Clopyralid,which has some soil residualproperties,was appliedpostemergencein canolafor Canadathistlecontrolin 1987 and 1988 atItunain all threetillage systems as a spot spray.In 1988, the conventional-and minimum-tillagecanola plots at Tadmorereceived a preplant-incorporated application of trifluralin, while zero-tillage plots received sethoxydim.To avoid the potentiallyconfoundingeffect of a predeterminedherbicideprogram,herbicide selection variedby site and year dependingon weed species composition. Herbicides were applied with a tractor-mountedsprayer that delivered 110 1ha-' of spraysolutionthroughfan-typenozzles5. Weed assessment and analysis. Weedswere countedby species 6 to 8 wk afterwithin-cropherbicideapplicationin mid-Julyof each year. Thereby,the residual weed communitiesremaining afterthe effects of weed managementandcropcompetitionwere assessed. Weeds within twenty 0.5- by 0.5-m quadratswere assessed per croppedplot for a total of 1440 quadratsat each samplingtime at Itunaand Waldron,and 2160 quadratsat Tadmore. Different quadratswere assessed at each sampling date. Due to the different timing of tillage versus chemical weed managementin fallow years,the effect of fallow was determined indirectlyby comparingweed communitieswithin the cropped plots in the sequenceswith and withoutfallow. Weed density data were used to compare total community densities by treatment.In order to overcome the problemof a nonuniformweed distribution,relative abundance,a synthetic importancevalue that includes density and frequency components, was calculatedfor use in the analysisof relative community composition.Relativeabundancevalues were calculatedby plot for each weed species as follows: (relativedensity+ relative frequency)/2.Relative density was calculatedas the numberof individualsfor a given species within the 20 quadratsfor each

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DERKSENET AL.: IMPACTOF AGRONOMICPRACTICESON WEED COMMUNITIES Table2. List of herbicidetreatmentsappliedwithin crops, priorto seeding, or to fallow areasat Ituna,Tadmore,andWaldronfrom 1988 to 1990.a,b

Herbicide

Rate

Waldron

Tadmore

Ituna 1988

1989

1990

x

x

-

1988

1988

1989

1990

kghaSpringwheat: Diclofop Fenoxaprop+ bromoxynil+ MCPA Fenoxaprop+ MCA + trifensulfuron Bromoxynil+ MCPA Dicamba+ 2,4-D MCPA+ mecoprop+ dicamba Barley: Diclofop Dicamba+ 2,4-D Canola: Sethoxydim Clopyralid Trifluralin Flax: Sethoxydim+ bromoxynil+ MCPA+ Atplus 300 plus 83% mineraloil Fall, preseeding,or fallow: Glyphosate Glyphosate+ 2,4-D Glyphosate+ 2,4-D + bromoxynil Glyphosate+ 2,4-D + dicamba Glyphosate+ dicamba Glyphosate+ bromoxynil Paraquat 2,4-D Bromoxynil MCPA+ mecoprop+ dicamba Dicamba+ 2,4-D Spot spray: Clopyralid(canola)

0.700 0.350 + 0.280 + 0.280 0.310 + 0.420 + 0.015 0.280 + 0.280 0.110 + 0.420 0.275 + 0.063 + 0.063

x

x x

-

x

-

-

x

-

x

x

x x x

0.700 0.110 + 0.420 0.184 0.150 1.100

x

x x

x x

x x

x

x

fl fl, ps fa fl fl ps

fa fl, ps

ps fl, ps

fa, fl

fl

fa fl

fa, ps, fl

fl

fa fa, fl

fa, fl

-

x x

x x x

0.184 + 0.280 + 0.280 + 17% 0.900 0.330 + 0.150 0.330 + 0.150 + 0.280 0.330 + 0.150 + 0.070 0.330 + 0.150 0.330 + 0.280 0.425 0.420 0.280 0.275 + 0.063 + 0.063 0.110 + 0.420

fl ps fa, fl

0.150

x

x

x

ps, fl fl

ps, fl ps, fl

-

fl ps, fl

ps fl

fl fl fa, fl, ps

ps fa, ps fa fa fa

fa

fa

fl

acid; bromoxynil, acid; fenoxaprop,(?)-2-[4-[(6-chloro-2-benzoxazolyl)oxy]phenoxy]propanoic aDiclofop, (?)-2-[4-(2,4-dichlorophenoxy)phenoxy]propanoic 3,5-dibromo-4-hydroxybenzonitrile;MCPA, (4-chloro-2-methylphenoxy)acetic acid; thifensulfuron, 3-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]caracid; dicamba, 3,6-dichloro-2-methoxybenzoicacid; 2,4-D, (2,4-dichlorophenoxy)aceticacid; mecoprop, (?)-2-(4bonyl]amino]sulfonyl]-2-thiophenecarboxylic clopyralid, chloro-2-methylphenoxy)propanoic acid; sethoxydim, 2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one; glyphosate, N-(phosphonomethyl)glycine;paraquat, 3,6-dichloro-2-pyridinecarboxylicacid; trifluralin,2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine; ion. 1,1'-dimethyl-4,4'-bipyridinium bAnx denotesthe herbicidewas applied,fl denotes herbicideappliedin fallow to zero- andminimum-tillageplots, ps denotesherbicideappliedin springpriorto seeding, fa denotes herbicideappliedto fall plots.

plot divided by the total numberof individualswithin the plot. Relative frequencywas calculatedas the proportionof quadrats in which the species was present per plot divided by the total frequencyof all species. Canonical discriminant analysis (CDA)6 is an ordination technique that can be used to determineif weed communities differ among experimentaltreatmentsbased on their relative species compositionusing all the weed species presentas vari6Abbreviations:CDA, canonical discriminantanalysis; M2, Mahalanobis squareddistances between cluster centroids in CDA; ZTCC, zero-tillage continuous-cropping;ZTCF,zero-tillagecrop-fallow;MTCC,minimum-tillageconcrop-fallow; CTCC, MTCF, minimum-tillage tinuous-cropping; conventional-tillagecontinuous-cropping;CTCF,conventional-tillagecrop-fallow. 7SAS Institute,Inc., Box 8000, Cary,NC 27511-8000. 8Personalcommunication,L. P. Lefkovitch. Agric. Canada,StatisticalRes. Eng., and StatisticalRes. Ctr.,Ottawa,ON.

ables as describedby Derksenet al. (9). CDA was performedon arcsine square root-transformedrelative abundancevalues in orderto increasehomogeneityof variance.Analyses were done using SAS7. CDA and other clustering techniques have primarilybeen used on data from unstructureddata sets (14, 20). CDA cannot determinethe presence of interactionsamong experimentaleffects. In orderto avoid the problemof not addressinginteractions betweentillage systemsandcropsequences,treatmentcombinations were formed between these experimentaleffects for subsequent analysis8. Treatmentcombinations were: zero-tillage continuous-cropping(ZTCC)6,zero-tillagecrop-fallow(ZTCF), minimum-tillagecontinuous-cropping(MTCC),minimum-tillage crop-fallow(MTCF),conventional-tillagecontinuous-cropping (CTCC), and conventional-tillage crop-fallow (CTCF) treatments.

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WEED SCIENCE

In CDA, experimentalplots are placed on ordinationaxes of compositionalvariation(14). The numberof canonicalfunctions formedby CDA is one less thanthe numberof treatments.In this case five canonicalfunctionsand subsequentaxes were formed. Since the first two axes explained a large proportionof the variation,two-dimensionalbiplots were createdfrom canonical axes one and two in orderto visually representthe data and to determine the association of weed species with experimental treatments.Biplots were formed by overlaying the scatterdiagramof treatmentclustersor clustercentroidsfromCDA andthe species vector diagrambased on the total canonicalcoefficients of each species from the canonicalfunctions(14). These coefficients represent the degree to which a species increases or decreasesalong an axis; therefore,the directionandlengthof the vectorsindicatesthe degreeof associationbetweeneach species and treatment. Clustersformed if plots of the same treatmentwere in close proximityto each other (i.e., of similarweed communitycomposition). Significant differencesamong tillage system clusters were determinedby comparingthe Mahalanobissquared(M2) distances between cluster centroids, as calculatedby SAS, to Chi2 tables (14). Since the weed communities were naturally occurringand variable, P values of < 0.10 were presentedfor consideration.This techniquehas been previouslydescribedfor use in weed science (9).

125 Ituna 100 755 ..

fil

50

25t

1988

L"0

I[i^

1989

1990

CN

E E 125v YeI~~~~~~adron

n

C 100

.

4,75"

.

00 Silm

0

~~~1988

Q

ti100 19a89

l jilf1b 1990

RESULTSAND DISCUSSION

Weed flora. Naturalweed communitiesat Ituna,Tadmore,and Waldronwere comprisedof volunteercrops,and grassand dicot weed species of variedperennation(Table3), whichwere typical of Saskatchewanweed flora in cultivatedfields (21). Weed densities. No apparenttrendwas observedbetween total weed densitiesandtillage treatments.For example,althoughthe zero-tillagetreatmentsat Itunahad greatertotal weed densities in 1989 and 1990 than minimum- or conventional-tillagesystems (Figure 1), this was not a consistent trendfor these treatments at all locations. Total weed densities were, however, generallygreaterin the continuous-croppingtreatmentsof zero-, minimum-, and conventional-tillagethan fallow treatmentsat Waldronand Ituna,but this did not occur consistentlywith any one specific treatmentor in all situations. For example, the greatestweed density occurredat ZTCC at Itunain 1990 and in CTCC at Waldronin 1989. Hume (13) also found a more dense weed communityin continuouswheat plots thanin fallow rotations. Althoughgreaterweed densities usually signify greatercrop yield loss, it is difficult to directly infer yield losses from this study since the date of weed emergenceand weed biomass were not recordedand assessed against crop yield. The potentialfor greater weed densities in continuous cropping exists because weed controlis never 100%;therefore,the weed seedbankmay be replenishedin continuouscroppingwhile being depleted in crop-fallowsequences.In this study,weed controlwas the same in all treatmentsand rangedfrom 60 to 95% dependingon the site and year (datanot shown). If densitiesbecome consistently

250 Todmore 200

Z rCC EN UTCC

150

m ZTCF

100

CC

1988 Figure 1. Totalweed densities (? standarderror)in mid-Julyat Ituna,Tadmore, andWaldronwithinzero-tillagecontinuous-cropping(ZTCC),zero-tillagecropfallow (ZTCF),minimum-tillagecontinuous-cropping(MTCC),minimum-tillage crop-fallow (MTCF), conventional-tillagecontinuous-cropping(CTCC), and conventional-tillagecrop-fallow(CTCF)treatmentsfrom 1988 to 1990.

greaterin continuouscropping,the potentialfor yield loss may be greater,especially in yearswhen crop competitionis reduced by factors such as drought,but the potentialfor crop yield loss mustbe weighed againstthe potentialagronomicand economic benefits of annually harvested crops. Agronomic information and analysis pertainingto crop establishment,moistureusage, andyield for this researchhas been summarizedby Lafondet al. (15).

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DERKSENET AL.: IMPACTOF AGRONOMICPRACTICESON WEED COMMUNITIES Table3. WSSA-approvedcodes, scientific names, common names, and life cycle for weed species found at Ituna,Tadmore,andWaldronfrom 1986 to I990.a Codeb

Scientific name

Commonname

Life cyclec

AGRRE AMABL AMARE ANSSE ARTBI AVEFA BROIN BRSNS* CAPBP CHEAL CHEGL CIRAR CMAMI CVPTE DESSO DRBNE EPHSP* EQUAR ERICA ERWGA GAETE HORJU HORVX KCHSC LEPDE LINUS* LPLSQ MALPU* MATIN MATMT MEDLU MEDSA MELNO MENAR MEUXX* NEAPA POACO POLAV POLCO POLLA ROSXX* SASKR SECCE SENVU SETVI SINAR SOLTR SONAR SOCCA TAROF THLAR TRZAS* TRZAW* TROPR VAAPY

Elytrigia repens(L.) Nevski Amaranthusblitoides S. Wats. AmaranthusretroflexusL. AndrosaceseptentrionalisL. Artemisiabiennis Willd. Avenafatua L. BromusinermisLeyss. Brassica napusL. Capsella bursa-pastoris(L.) Medicus ChenopodiumalbumL. ChenopodiumglaucumL. Cirsiumarvense (L.) Scop. CamelinamicrocarpaAndrz.ex DC. CrepistectorumL. Descurainia sophia (L.) Webb.ex Prantl Draba nemorosaL. EuphorbiaserpyllifoliaPers. Equisetumarvense L. Conyzacanadensis (L.) Cronq. Erucastrumgallicum (Willd.) 0. E. Schulz Galeopsis tetrahitL. HordeumjubatumL. HordeumvulgareL. Kochia scoparia (L.) Schrad. LepidiumdensiflorumSchrad. LinumusitatissimumL. Lappulaechinata Gilib. Malvapusilla Sm. MatricariaperforataMerat Matricariamatricarioides(Less). C. L. Porter Medicago lupulinaL. Medicago sativa L. Silene noctifloraL. Menthaarvensis L. Melilotus species Neslea paniculata (L.) Desv. Poa compressaL. PolygonumaviculareL. PolygonumconvolvulusL. PolygonumlapathifoliumL. Rosa species Salsola iberica Sennen and Pau Secale cereale L. Senecio vulgarisL. Senecio viridis (L.) Beauv. Brassica kaber(DC.) L. C. Wheeeler SolanumtriflorumNutt. Sonchusarvensis L. Solidago canadensis L. TaraxacumofficinaleWeberin Wiggers Thlaspiarvense L. TriticumaestivumL. TriticumaestivumL. Tragopogonpratensis L. VaccariapyramidataMedicus

Quackgrass Prostratepigweed Redrootpigweed Northernrockjasmine Biennial wormwood Wild oat Smoothbrome Canola Shepherdspurse Commonlambsquarters Oakleafgoosefoot Canadathistle Smallseedfalseflax Narrowleafhawksbeard Flixweed Wood whitlowgrass Thyme-leavedspurge Field horsetail Horseweed Dog mustard Commonhempnettle Foxtail barley Barley Kochia Greenflowerpepperweed Flax Europeansticktight Round-leavedmallow Scentless chamomile Pineappleweed Black medic Alfalfa Nightfloweringcatchfly Field mint Sweet clover Ball mustard Canadabluegrass Prostrateknotweed Wild buckwheat Pale smartweed Wild rose species Russianthistle Fall rye Commongroundsel Greenfoxtail Wild mustard Cutleafnightshade Perennialsowthistle Canadagoldenrod Dandelion Field pennycress Springwheat Winterwheat Meadow salsify Cowcockle

PG AD AD AD AD AG PG VC AD AD AD PD AD AD AD AD AD PC AD AD AD PG VC AD AD VC AD AD AD AD AD PD AD PD AD AD PG AD AD AD PD AD VC AD AG AD AD PD PD PD AD VC VC AD AD

Ituna

Tadmore

Waldron

x x x

x x x x

x

-

x x x x x x x x

x

x x x x x x x x x x

-

-

x

x

x x x x x x x

x

x x x

x x

x x x

x x x x x x x

x x x x x x

x x x x x

x x x x

x

x x x x x x x x x x x x x

x x

x x

x x x

x x

x x x x x x x x x

x x

-

-

x x x x x

x

aAnx denotes species presence. bWSSA-approvedcomputercode from CompositeList of Weeds, Revised 1989. AvailablefromWSSA, 1508 West UniversityAve., Champaign,IL 61821-3133. *Codes not in CompositeList of Weeds. CLifecycle groupingsbased on simplestperennation.AD = annualdicot, AG = annualgrass, PC = perennialcryptogram,PD = perennialdicot, PG = perennial grass, VC = volunteercrop (annual).

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WEED SCIENCE Table4. Mahalanobissquareddistances(M2) and level of significance betweentreatmentsfrom CDA.a M2 by treatmentb Treatmentb Ituna 1988: ZTCC ZTCF MTCC MTCF CTCC CTCF Ituna 1989: ZTCC ZTCF MTCC MTCF CTCC CTCF Ituna 1990: ZTCC ZTCF MTCC MTCF CTCC CTCF Tadmore1988: ZTCC ZTCF MTCC MTCF CTCC CTCF Waldron1988: ZTCC ZTCF MTCC MTCF CTCC CTCF Waldron1989: ZTCC ZTCF MTCC MTCF CTCC CTCF Waldron1990: ZTCC ZTCF MTCC MTCF CTCC CTCF

ZTCC

P < 0.001 NS P < O.Q01 P < 0.05 P < 0.01

ZTCF

MTCC

MTCF

CTCC

CTCF

22.4

7.4 20.9

27.8 7.4 27.0 -

12.2 21.8 8.2 19.9

P < 0.01 P < 0.05

19.9 15.5 14.3 11.6 12.7

P < 0.05

P < 0.001 NS P < 0.001 P < 0.01 8.9

NS P< 0.10 P