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Agronomy2020,10,257;doi:10.3390/agronomy10020257www.mdpi.com/journal/agronomy
Review
Non‐ChemicalWeedManagementinVegetablesby
UsingCoverCrops:AReview
HusrevMennan
1,
*,KhawarJabran
2
,BernardH.Zandstra
3
andFiratPala
4
1
DepartmentofPlantProtection,AgricultureFaculty,OndokuzMayisUniversity,55139Samsun,Turkey
2
DepartmentofPlantProductionandTechnology,FacultyofAgriculturalScienceandTechnologies,Nigde
OmerHalisdemirUniversity,51240Nigde,Turkey;khawarjabran@gmail.com
3
DepartmentofHorticulture,MichiganStateUniversity,EastLansing,MI48824,USA;zandstra@msu.edu
4
DepartmentofPlantProtection,AgricultureFacultySiirtUniversity,
56100Siirt,Turkey;
firatpala@gmail.com
*Correspondence:hmennan@omu.edu.tr;Tel.:+90‐362‐3121919
Received:04December2019;Accepted:05February2020;Published:11February2020
Abstract:Vegetablesareasubstantialpartofourlivesandpossessgreatcommercialand
nutritionalvalue.Weedsnotonlydecreasevegetableyieldbutalsoreducetheirquality.
Non‐chemicalweedcontrolisimportantbothfortheorganicproductionofvegetablesand
achievingecologicallysustainableweedmanagement.Estimateshaveshownthattheyieldof
vegetablesmaybedecreasedby45%–95%inthecaseofweed–vegetablecompetition.
Non‐chemicalweedcontrolinvegetablesisdesiredforseveralreasons.Forexample,thereare
greaterchancesofcontaminationofvegetablesbyherbicideresiduecomparedtocerealsorpulse
crops.Non‐chemicalweedcontrolinvegetablesisalsoneededduetoenvironmentalpollution,the
evolutionofherbicideresistanceinweedsandastrongdesirefororganicvegetablecultivation.
Althoughthereareseveralwaystocontrolweedswithouttheuseofherbicides,covercropsarean
attractivechoicebecausethesehaveanumberofadditionalbenefits(suchassoilandwater
conservation)alongwiththeprovisionofsatisfactoryandsustainableweedcontrol.Severalcover
cropsareavailablethatmayprovideexcellentweedcontrolinvegetableproductionsystems.
Covercropssuchasrye,vetch,orBrassicaceaeplantscansuppressweedsinrotations,including
vegetablescropssuchastomato,cabbage,orpumpkin.Growersshouldalsoconsiderthenegative
effectsofusingcovercropsforweedcontrol,suchasthenegativeallelopathiceffectsofsomecover
cropresiduesonthemainvegetablecrop.
Keywords:covercrops;weeds;vegetables;non‐chemicalweedcontrol;allelopathy;physicalweed
control
1.Introduction
Severalproductionproblemsdiminishthequalityandyieldofvegetables.Salinity,drought
stressandclimatechangesareafewabioticfactorsthatdecreasetheyieldofvegetableswhilethe
majorbioticfactorsthatreducetheyieldincludediseasepathogens,insectpests,viruses,andweeds[1].
Theyieldlossescausedbyuncheckedweedsinvegetableproductionsystemsmaybebetween45to
95%dependingontheproductionenvironment[2].Despitethecommonuseofherbicidesinthe
USA,thelossinvalueofcertainvegetablesduetoweedsrangedbetween8%to13%[3].Weedsnot
onlyreducetheyieldofvegetablesbutalsodecreasetheirqualityandmarketvalue[4].
Chemicalweedcontrolhasbeenagreatoptioninconventionalvegetableproductionsystems
becauseitprovideseffectiveandsustainableweedcontrol.However,overuseofherbicidescauses
environmentalconcernsbecauseherbicideshavenegativeeffectsonnon‐targetorganisms
Agronomy2020,10,2572of16
(beneficialspecies),maypollutethefoodandgroundwaterwiththeirresidue,andcausetoxicityin
mammalians[5,6].Therehasbeenagreaterinterestinnon‐chemicalweedcontrolafterpeople
becamemoreawareofthedamagecausedbythemisuseofherbicides[7].
Weedsineitherorganicorconventionalvegetableproductionsystemscanbecontrolled
throughhandhoeingormechanicalcultivation.Inter‐rowcultivationhasaprovenusefulnessfor
controllingweedsinvegetableproductionsystemsbutthehighcostoffuelmayconstrainthis
practice.Moreover,thepracticeofinter‐rowcultivationmayacceleratesoilerosion[8–10].
Programmedweed‐controllingrobotshavepotentialfortheautomationofweedmanagement
operations.However,becausethetechnologyisexpensive,itstillneedssometimetobeadoptedby
thevegetablegrowers.Therootsystemofvegetablesmaybedamagedbythehand‐hoeing,andthis
practiceislabor‐intensiveandcostly[7,11,12].
Organicandconventionalvegetableproductionsneedeffectiveweedcontrolstrategies.Theuse
ofcovercropsforallelopathicandphysicalweedcontrolisamongtheeffectivestrategies[13–16].
Covercropscanbeconsideredaseffectivetoolstosuppressweedsinvegetablefields[14,17].Cover
cropscanactasanalternativetotillageforcontrollingweedsandreducingyieldlossescausedby
weeds[18].Covercropsarenotgrownasacashcropbutratheraregrownforseveraloftheir
ecologicalbenefits.Someofthesynonymsusedforcovercropsarelivingmulch,catchcrop,smother
cropandgreenmanure.Thesecanbegrownduringfallowperiods,alongwiththemaincashcropor
duringapartofthegrowingseasonofacashcrop.Theecologicalbenefitsofcovercropsarenot
limitedtoimprovementsinsoilfertility,soilconservationorpestsuppression.Covercropsmodify
themicroclimateofthecompanioncropbyinterceptingsunradiation,provideahabitatfor
beneficialbiologicalagents,suchasinsectsthatpreyonweedsorotherdetrimentalinsects,modify
theenergyofraindropsandplayaroleintheuniformdistributionofprecipitationinthesoil.
Besidesthosefeatures,covercropsmaydecreaseerosion,conservesoilmoisture,increasesoil
fertilityandimproveitsstructure.Negatively,sometimescovercropsmayalsofacilitate/promote
somepestsand,ifnotremovedpriortothecriticalperiodforweedcontrolofthecompanioncrop,
canthemselvesactasweedsandreducecropyield[19].
Previousresearchshowsthatcovercropscanbegrownforweedcontrolinvariousagricultural
systems[20–22].Manycovercropshavebeeninvestigatedindifferentpartsoftheworldandhave
showngoodresults.Forexample,covercropssuchasfodderradish(RaphanussativusL.),vetch
(Viciaspp.),rye(SecalecerealeL.)ortheirappropriatecombinationscanprovideeffectiveweed
controlinno‐tillorganiccropproductionsystems[23].S.cerealewithallelopathicpotential
decreasedthebarnyardgrass(Echinochloacrus‐galli(L.)P.Beauv.)leafcountanddrybiomass[24].A
numberofreviewarticlesareavailablethataddressvariousaspectsofcovercrops.Nevertheless,to
thebestofourknowledge,noreviewarticleaddressestheuseofcovercropsforachieving
non‐chemicalweedcontrolinvegetablecrops.Hence,inthisreviewpaper,wehavereviewedthe
literaturetoexplaintheyieldlossescausedbyweedsinthevegetables,theneedfornon‐chemical
weedcontrolinvegetables,theuseofcovercropsforweedcontrolinvegetablesandthe
disadvantagesofthecovercrops.
2.YieldLossesCausedbyWeedsinVegetableCrops
Weedsareasignificantissueintheproductionofvegetables.Severalfactorsaffectthedegreeof
vegetableyieldreductionanddamagetoqualitycausedbyweeds.Themostimportantoftheseare
thecompetitivenessofthevegetableplantswiththeinfestingweeds,therelativedensitiesofweeds
andvegetableplants,weedemergencetime,andthecompetitionduration[25–27].Vegetables
plantedeitherasseedsorseedlingsareweakcompetitorsbecauseoftheirshallowrootsystemand
sluggishgrowth,especiallyduringtheearlystages.Hence,thevegetablesarehighlysensitiveto
weedcompetitionandneedtobekeptweed‐freeduringthatearlygrowthstage.Theyieldlosses
causedbyweedsinvariousvegetableshavebeenreportedinpreviousstudies.Forexample,weed
coverinpea(PisumsativumL.)was73%ifitwasgrownwithoutanycovercrop,i.e.,withoutany
weedcontrolpractice[28].Decreaseinyieldoflettuce(LactucasativaL.)becauseofweed
competitionhasbeenreportedinpreviousstudies[29].InastudyfromEngland,adensityof65
Agronomy2020,10,2573of16
weedsm−2,includingamixtureofbroadleafandnarrowleavedweeds,couldcompletelydestroythe
lettucecrop[29].Similarly,astudyfromCaliforniashowedthatlettuce–weedcompetitionforthe
wholegrowingseasondecreasedtheyieldby50%[30].InFlorida,aseason‐longlettuce–weed
competitionreducedtheyieldoflettuceby56%[31].Besidestheyieldreduction,thequalityof
lettuceisalsoreducedduetoweedcompetition[32].Similarly,weedcompetitioninsoybean
(Glycinemax(L.)Merr.)duringthefirst(v1)andthirdfoliates(v3)stages(accordingtoBBCH)was
criticalindamagingthecropplantdevelopment[33].
Similarly,inpepper(CapsicumannuumL.),weedssuchaspurplenutsedge(CyperusrotundusL.)
coulddecreasethefruityieldbyupto44%[34].Inanotherstudy,thefruitnumberofthebellpepper
(C.annum)wasdecreasedby94%ifitsgrowthwasinterferedwithbytheweedPalmeramaranth
(Amaranthuspalmeri(S.)Wats.)[16].Moreover,aweed‐freeperiodof12.2weekswasneededbychili
pepper(C.annum)toavoidayielddecreaseofmorethan5%[35].Yieldofthetomato(Solanum
lycopersicumL.)wasdecreasedby48%–71%whenthevegetableplantsweregrowingincompetition
withweedssuchaslargecrabgrass(Digitariasanguinalis(L.)Scop.),tallmorningglory(Ipomoea
purpurea(L.)Roth),commoncocklebur(XanthiumstrumariumL.),andjimsonweed(Datura
stramoniumL.)[36].
3.NeedforNon‐ChemicalWeedControlintheVegetableCrops
Non‐chemicalweedcontrolisdesiredforseveralreasons.Thedevelopmentofmodern
vegetableproductionsystemsislikelytobeslowedifweedmanagementoptionsarelimited[37].
Theavailabilityofnon‐chemicalweedcontrolmethodsalongwithweedcontrolwithherbicideswill
increasethelikelihoodofachievingsustainableweedcontrolinvegetableproductionsystems.
Furthermore,thereisastrongdemandfororganicfoods,particularlyforvegetablesglobally,and
thisorganicvegetableproductionisnotpossiblewithoutnon‐chemicalweedcontrol[38].
Herbicide‐resistantweedsinvegetableproductionsystemsalsostresstheneedfor
non‐chemicalweedcontrol[39].Similarly,themisuseofherbicidescausesenvironmentalpollution,
andnon‐chemicalweedcontrolshouldbeavailableasanadditionalchoiceforsustainableweed
control[7,40].
4.TypesofCoverCrops
Covercropscouldbeclassifiedintofourgroupsbasedontheirtaxonomy.Table1givesa
comprehensivelistofcovercropsthathavebeenusedforcontrollingweedsinvegetablesandother
crops,andforachievingotherecologicalbenefits.Accordingtothisclassification,therearegrasses,
legumes,Brassicaceaeplants,andothers.Importantexamplesofgrasscovercropsareryegrass
(LoliummultiflorumLam.),barley(HordeumvulgareL.),andsorghum(Sorghumspp.).R.sativusand
mustards(Sinapisspp.)areimportantintheBrassicaceaeplants.Similarly,beans(Phaseolusspp.),
vetches(Viciaspp.)andpeas(Pisumspp.)areimportantpulsesthataregrownascovercrops(Table1).
Growersmayselectafewofthesecovercropspecies,consideringtheirspecificfarmsituations.For
example,theymayavoidusinglegumesandprefercerealcovercropsifthesoilisrichinnitrogenor
hasnitrogenresiduefromthepreviouscrop.Ontheotherhand,legumecovercropsmaybe
preferredifthesoilhaslessnitrogen[41].
Table1.Alistofimportantcovercropsusedforweedcontrolandotherecologicaladvantages.
CoverCropTypeNameofCoverCropReference
Cereals
Bristleoat(Avenastrigosa)[42]
Winterrye(Secalecereale)[43]
Oat(Avenasativa)[43]
Sudangrass(Sorghum×sudanense)[44]
Wheat(Triticumaestivum)[44]
LegumesPea(Pisumsativum)[45]
Cowpea(Vignaunguiculata)[44]
Agronomy2020,10,2574of16
Subterraneanclover(Trifoliumsubterraneum)[45]
Crimsonclover(Trifoliumincarnatum)[45]
Egyptianclover(Trifoliumalexandrinum)[45]
Redclover(Trifoliumpratense)[45]
Sunnhemp(Crotalariajuncea)[45]
Velvetbean(Mucunapruriens)[45]
Soybean(Glycinemax)[45]
Fababean(Viciafaba)[45]
Hairyvetch(Viciavillosa)[43]
Commonvetch(Viciasativa)[46]
Brassicaceaeplant
Forageradish(Raphanussativus)[42]
Rapeseed,canola(Brassicanapus)[45]
Whitemustard(Sinapisalba)[45]
Non‐legumes
Buckwheat(Fagopyrumesculentum)[42]
Flax(Linumusitatissimum)[42]
Niger(Guizotiaabyssinica)[42]
5.HowCoverCropsSuppresstheWeeds?
Suppressionofweedsthroughcovercropsisdependentonseveralfactorsandaselectiveweed
controlisofferedbysomecovercrops[13,42–46].Therefore,cereal,legumeandbrassicaceaecover
cropsarewidelyusedinvariouscroppingsystems.
Weedsuppressionthroughcovercropsisachievedbymorethanasingleaction.Importantly,
bothcompetitionandallelopathyhavebeenassumedasmechanismsofweedsuppressionbycover
crops[46,47].Oneimportantmechanismofactionofacovercropisitsphysicaleffectonweeds.
Mostofthestudiesontheeffectsofcovercropsonweedsessentiallyconsiderthequantityof
accumulatedcovercropbiomass[48].Acovercropwithhighbiomassproductionislikelyto
produceagoodphysicaleffectonweeds,andhenceresultineffectiveweedsuppression.
Early‐seasontotalbiomassaccumulationbycovercropsreducestheriskofweedemergence
(Figures1and2)[49,50].ThegrasscovercropsL.multiflorum,A.sativa,andS.cerealehadahigher
biomassthantheothercovercropsandsuppressedtheweedseffectively(Figures3and4).
Figure1.Biomassproduction(kgha−1)ofcerealcovercropsindifferentvegetableproduction
systemspriortoterminationin2005(graybars)and2006(whitebars).Verticallinesrepresent
standarderrorsofthemeans(p<0.05)[49,50].
0
500
1000
1500
2000
2500
3000
3500
4000
L.multiflorum A.sativa S.cereale S.bicolor T.aestivum S.vulgare
Biomass(kgha‐1)
Covercrops
Agronomy2020,10,2575of16
Figure2.Biomassproduction(kgha−1)ofcovercropsindifferentvegetableproductionsystemsprior
toterminationin2005(graybars)and2006(whitebars).Verticallinesrepresentstandarderrorsof
themeans(p<0.05)[49,50].
Figure3.Effectsofvariouscerealcovercropsindifferentvegetableproductionsystemsonthedry
biomassproduction(gm−2)ofweedspeciesatthetimeofcovercropterminationin2005(graybars)
and2006(whitebars).Verticallinesrepresentstandarderrorsofthemeans(p<0.05)[49,50].
0
500
1000
1500
2000
2500
3000
3500
4000
4500
P.sativum T.
meneghinianum
T.aestivum T.alexandrinum V.sativa V.villosa
Biomass(kgha‐1)
Covercrops
0
100
200
300
400
500
600
700
L.multiflorum A.sativa S.cereale S.bicolor T.aestivum S.vulgare Control
Weeddrybiomass(gm‐2)
Covercrops
0
100
200
300
400
500
600
700
P.sativum T.meneghinianum T.alexandrinum V.sativa V.villosa Control
Weeddrybiomass(gm‐2)
Covercrops
Agronomy2020,10,2576of16
Figure4.Effectsofvariouslegumecovercropsindifferentvegetableproductionsystemsonthedry
biomassproduction(gm−2)ofweedspeciesatthetimeofcovercropterminationin2005(graybars)
and2006(whitebars).Verticallinesrepresentstandarderrorsofthemeans(p<0.05)[49,50].
ThebiomassesofL.multiflorum,A.sativa,andV.sativaspecieswererecordedashavingthe
highestvalues(Figure1).Ontheotherhand,thebiomassesaccumulatedbyP.sativum,S.bicolor,and
ballclover(TrifoliummeneghinianumClementi)specieswererecordedasthelowestvalues,
respectively.Covercropshadastrongerimpactonthebiomassofweedsthanontheirdensity.Even
ifweedswerefoundinahighnumberafteracovercrop,therewasagreaterdecreaseintheir
biomass[51].Sometimes,amixtureofcovercropsmayperformbetterthanasinglecrop,butthis
maynotalwaysbethecase.AcomparisonofS.cereale,hairyvetch(ViciavillosaRoth),andaS.
cereale–V.villosacombinationshowedthatS.cerealewasthebestweedsuppressorcovercropandthe
mixtureofS.cereale–vetchwassimilartotheS.cerealecovercrop[51].
Astatisticalrelationship(correlation)hasalsobeenconsistentlydescribedforthequantityof
covercropbiomassandtheextentofweedsuppression(Figures3and4).Acovercropbiomassof8t
ha−1orhighermaybeenoughtoachieveasatisfactoryandsustainableweedsuppression[48].For
instance,forageradishisanimportantcovercropanditsweed‐suppressingmechanismhasbeen
reportedpreviously[52].Rapidcanopydevelopmentandotherweed‐competitivetraitsofforage
radishwerefoundtobeacauseofweedsuppressionandallelopathywasfoundtohavenorole[52].
Generally,earlysoilcoverageandahighseedingrateofcovercropscanincreasepressureonweeds.
Growingacovercropwithahigherseedingdensitymeansproducingmorebiomassofcovercrop;
thiswillultimatelysuppressmoreweeds[53].S.cerealecovercropseedingratesdidnotaffectthe
weedemergence,butahighcovercropdrybiomassproducedahighseedingratethatdecreasedthe
weedbiomass[54].
Weeddrybiomassesbecauseoftheapplicationofcovercrops,includingP.sativum,T.
meneghinianum,andT.alexandrinumspecies,wererecordedashavingthehighestvaluesafterthe
valueofcontrolplots(Figures3and4).Ontheotherhand,thedrybiomassofL.multiflorum,S.
cerealeandT.aestivumspecieswererecordedashavingthelowestvalues,respectively.
Thedensityofweedsinsudangrass(Sorghumvulgare),S.bicolor,andT.aestivumwererecorded
ashavingthehighestvaluesafterthevalueofcontrolplots(Figure5).Ontheotherhand,theweed
densityofL.multiflorum,A.sativa,andS.cerealewererecordedashavingthelowestvalues,
respectively.
Figure5.Effectsofvariouscerealcovercropsindifferentvegetableproductionsystemsonweed
densityafterdifferenttimeofcovercroptermination.Verticallinesrepresentstandarderrorsofthe
means(p<0.05)[49,50].
0
5
10
15
20
25
14 28 56
Weeddensity(plantm‐2)
Daysafterterminationcovercrops(DAT)
L.multiflorum
A.sativa
S.cereale
T.aestivum
S.bicolor
S.vulgare
Control
aa
a
a
b
a
a
a
bb
c
c
bc
b
b
b
a
b
b
bc
b
a
a
Agronomy2020,10,2577of16
Severalothermechanismsexistthathelpthecovercropstosuppressweedsinvegetablecrops.
Forinstance,competitionforspacebetweenlivingcovercropsandweedsreducesthespacefor
weeds.Shadingistheothermechanismthroughwhichcovercropsarelikelytosuppressweedsin
vegetablecrops.Further,thecovercropsalterthelightqualitythatislikelytoinfluencethe
developmentofweeds[55].Forexample,far‐redlightreflectioncausesdevelopmentalchangesin
weedsthatreducestheirbiomassandseedproduction[56].Somecovercropsinducethe
germinationofweedseeds,andhencecauseadepletionoftheweedseedbank;e.g.,amixtureofS.
cereale,buckwheat(FagopyrumesculentumMoench),andyellowmustard(Brassicajuncea(L.)Czern.)
grownasacovercrophelpedtocompletelydepleteSetariaspp.intheweedseedbank,andcaused
an80%–85%decreaseinthegerminableseedbankofvelvetleaf(AbutilontheophrastiMedik.)and
commonlambsquarters(ChenopodiumalbumL.)[57].Thelong‐termpracticingofcovercropping
resultsinareductionintheweedseedbankinthesoil[58].Earlieremergenceofacovercropmay
enableittocauseahighsuppressionofweeds[59].Theweeddrybiomassinorganicallygrown
vegetableswasinverselyproportionaltotheseedingratesoflegumes(e.g.,V.faba,P.sativum)and
cereal(A.sativa)[60].
Theallelopathicpotentialofcovercropsistheotherimportantmethodthroughwhichcover
cropssuppressweeds(Table2).S.cerealeisamongthemosteffectiveandimportantcovercropsand
itsallelopathicpotentialhasbeenreportedconsistentlyintheliterature[61–63].S.cerealevariesin
theconcentrationofallelochemicalsand,hence,theeffectonweeds.Thecultivarsvariedintheir
contentofbenzoxazinoides,whileweedshadavariableuptakeofallelochemicalsthatcausedsome
weedstobemoresensitivetocovercrops.Forexample,ingreenhouseandfieldexperiments,
redrootpigweed(AmaranthusretroflexusL.)andcommonpurslane(PortulacaoleraceaeL.)showeda
greaterresponsetoS.cerealecovermulchesthanC.album[64].AllelochemicalssuchasDIBOA,BOA,
DIBOA‐glucoside,andAZOBhavebeenreportedfromS.cerealeplants,andthepersistenceofthese
allelochemicalsinthesoilhadacorrelationwiththeaccomplishedlevelofweedcontrol[47,65,66].
PlantsfromtheBrassicaceaefamilyareanimportantcovercropgroupforweedsuppression.
Glucosinolatesareimportantallelochemicalsthatarepresentinbrassicaspeciesandmayhavearole
intheweedcontrolactivity[67].Brassicacovercropsexudetheseglucosinolatestothesoil
environment,wheretheyareconvertedtotheiractiveform,i.e.,isothiocyanates.Forinstance,six
allelochemicals(isothiocyanates)wereidentifiedinfieldmustard(BrassicarapaL.);thesewere
2‐phenylethyl‐ITC,n‐butyl,3‐butenyl,benzyl,allyl,and4‐pentanyl[68].Previousstudieshave
describedtheallelopathiceffectsofcovercropsonweeds.WeedssuchasC.albumandwildfoxtail
millet(Setariamedia(L.)Vill.)weresuppressedby60%bycovercropssuchaswhitemustard(Sinapis
alba),R.sativus,V.sativaandthemixtureofthesecovercropssuppressedtheweedsby66%[69].In
manyinstances,itmaybedifficulttoquantifytheexactmechanismofweedsuppressionbycover
crops.Insuchcases,bothcompetitionandallelopathy,oranyofthese,maybeinvolved.For
example,thecovercropsdecreasetheemergenceofweedseedlings,expansionoftheweedcanopy
andbiomassproductionbyweeds,andthiswasachievedeitherthroughphysicaleffector
allelopathy[70].
Table2.Allelochemicalsreportedinvariouscovercrops.
CoverCropAllelochemicalsReference
Rye(Secale
cereale)MBOA,BOA,HMBOA,DIBOA[71]
Barley(Hordeum
vulgare)
Gramine,hordenine,p‐hydroxybenzoicacid,vanillicacid,
p‐coumaricacid,syringicacid,ferulicacid[72,73]
Sorghum
(Sorghumbicolor)Sorgoleone,m‐coumaricacid,caffeicacid,chlorogenicacid[74]
Wheat(Triticum
aestivum)
DIMBOA,2,4‐Dihydroxy‐1,4‐benzoxazine‐3‐one(DIBOA)[75]
DIMBOA,syringicacid,vanillicacid,p‐hydroxybenzoicacid,
cis‐ferulicacid,trans‐ferulicacid,trans‐p‐coumaricacid,
cis‐p‐coumaricacid
[76]
Agronomy2020,10,2578of16
Fieldmustard
(Brassicarapa)2‐Phenylethyl‐isothiocyanate[67]
Rapeseed
(Brassicanapus)
Glucosinolates,2‐Phenylethyl‐isothiocyanate,benzyl
isothiocyanate,allylisothiocyanate,3‐butenylisothiocyanate,[77,78]
Fieldmustard
(Brassica
campestris)
Benzylisothiocyanate,allylisothiocyanate,3‐butenyl
isothiocyanate[78]
Thereisparticularsignificanceinusingcovercropsforweedcontrolinno‐tillsystems.In
conventionalvegetableproductionsystems,theweedsarecontrolledbyusingtillageasatoolalong
withotherweedcontrolmethods,suchasherbicideapplications.However,no‐tillvegetable
productionhasbeenpopularinrecentyearsduetoassociatedbenefitssuchasenvironmentaland
resourceconservation,andcost‐effectiveness.Growingvegetablesintheno‐tillsystemlimitsthe
weedcontroloptionsasitrestrictstheuseoftillage,whichisapracticeprimarilycarriedoutfor
seedbedpreparationandweedcontrol.Covercropsareanexcellentoptiontocontrolweedsin
no‐tillvegetablesystems.Covercropsalsoconservethesoilandwaterandpromoteahealthy
environmentforsustainablevegetableproduction.
Anotherimportantquestionthatarisesintheuseofcovercropsisthepersistenceofallelopathic
substancesinsoilandaccordingactivitylevels.Soilfactorscanmodifytheallelopathicactivityof
covercropsinthesoil;themostimportantofthesefactorsaretheion‐exchangecapacityofthesoil,
theconcentrationoforganicmatterinitandthebioticcontent[79].Forexample,phenolicacid
concentrationsmayreachamaximumafter10–15daysofcovercropincorporationsandmayremain
effectiveuntil20–25days[80].Thiscovercropresidueinhibitsthegerminationofweedseedsand
decreasestheweeddensity,buttheireffectsareactiveforashorttimeandtheweedsoftenreemerge
[81].
Table3.Covercropswithanallelopathicpotentialandtheweedssuppressedbycovercrops.
CoverCropWeedsSuppressedReferences
Wheat(Triticumaestivum)Ipomoealacunose,Eleusineindica
,
Amaranthuspalmeri[82]
Rye(Secalecereale)Eleusineindica
,
Amaranthuspalmeri
,
Ipomoealacunosa[82]
Rye(Secalecereale)Chenopodiumalbum
,
Abutilontheophrasti[83]
Annualryegrass(Loliummultiflorum),
rye(Secalecereale),bristleoat(Avena
strigosa),commonvetch(Viciasativa),
radish
Brachiariaplantaginea,Bidenspilosa,
Euphorbiaheterophylla[23]
Hairyvetch(Viciavillosa),oat(Avena
sativa)
Digitariasanguinalis
,
Eleusineindica
,
Amaranthusretroflexus,Datura
stramonium
[84]
Sorghumsudangrass(Sorghum
bicolor×Sorghumsudanense)Broadleavedweeds[85]
Bristleoat(Avenastrigosa)
,
hairy
vetch(Viciavillosa)
Amaranthuspalmeri
,
Portulacaoleracea
,
Helianthusannuus[86]
Rye(Secalecereale),hairyvetch(Vicia
villosa),barley(H.vulgare)×triticale,
Austrianwinterpea(Pisumsativum)
Chenopodiumalbum
,
Amaranthushybridus
,
Thlaspiarvense,Taraxacumofficinale,
Stellariamedia,Elymusrepens,Panicum
crus‐galli,Setariaglauca
[87]
Whitemustard(Sinapisalba)Amaranthusblitoides,Chenopodiumalbum[88]
Agronomy2020,10,2579of16
Hairyvetch(Viciavillosa)
,
subterraneanclover(Trifolium
subterraneum),oat(Avenasativa)/hairy
vetch(Viciavillosa)
Amaranthusretroflexus,Chenopodium
album[89,90]
6.DisadvantagesofUsingCoverCropsforWeedControlinVegetableProductionSystems
Althoughcovercropscanrepressweedsandhaveadditionalecosystembenefits,somenegative
aspectsofusingcovercropsinvegetableproductionsystemshavealsobeennoted.Forexample,
someofthecovercropsmaypromotesomeinsectpestsordiseasepathogens[91].Moreover,as
vegetableshaveadifferentmorphologyandgrowthhabittootherarablecrops,therearechances
thatcovercrops,alongwithweeds,mayalsocompetewithvegetablecrops.Hence,acareful
managementplanisrequiredwhencovercropsaretobeusedforweedcontrolinvegetables[92].
Covercropsmaynotalwaysprovideeffectivecontrolofweedsinvegetables,i.e.,negativeor
noeffectsofcovercropsinvegetableshavealsobeennoted.Forexample,foragesoybeanwasused
asacovercropincollard(BrassicaoleraceaL.)vegetables;thecovercropwasineffectiveincontrolling
weeds[93].Moreover,covercropsmayoccasionallyhaveapositiveeffectongerminationor
seedlinggrowthofweeds[94].Additionally,someshortfallsofcovercropsasaweedcontrol
techniquearealsoonrecord.Forexample,covercropsmaynotbeeffectiveincontrollingintra‐row
weeds;thisisbecauseneithertheirallelopathiceffectsnorshadingeffectreachestheintra‐rows.This
mayalsobethereasonthatcovercropsusuallydamageonlytheweedsandnotthecrops[95,96].
Throughtheuseofcovercropsinvegetables,annualweedsareeasilycontrolled,butperennials
generallyreceivealowerimpactwhilesedgesaredifficulttocontrol[97].Otherresearchworkalso
showedthatcovercropsmayhaveweakeffectivenessagainstperennialweedssuchasnutsedges
(Cyperusspp.)[47,58]
Anotherimportantaspectisthathighbiomassproduction(inordertogaingreaterweed
suppression)maydisturbtheseedingoperation,cropestablishmentandothercropmanagement
operations[98].Similarly,allelochemicalsfromcovercropscanposeanegativeeffectonthemain
vegetablesalongwiththeireffectsonweeds[64].Awell‐establishedpracticeistogrowcovercrops
andthendesiccatethesethroughsomemeans(e.g.,herbicideapplication,mechanical
incorporation).Theresiduecovercropsuppressestheweedsinthenextseasoncrop;however,there
isalikelinessthatthiscovercropresiduewillalsonegativelyaffectthegerminationandseedling
growthofthemaincrop.Forexample,thegermination,seedlingemergenceorestablishmentof
spinach(SpinaciaoleraceaL.)andlettucewerenegativelyaffectedbytheallelopathicactivityoftwo
importantcovercrops,S.cerealeandwinteroilseedrape(BrassicanapusL.)[94]
7.CoverCropsforWeedControlinVegetableCrops
Covercropsarenowcommonlyusedinorganicvegetableproductionsystemsinsomepartsof
theworld[97].Forexample,growinglegumesandgrasscovercropsinthemixture(e.g.,growingS.
cerealeinmixturewithsoybean,usingbothascovercrops)islikelytodecreasetheseedproduction
byweeds,andhenceareducedweedseedbankisexpectedwiththiskindofcovercropapplication
[98].
CovercropmulchesinvegetableproductionsystemsoftheUnitedStatescouldprovidefields
freeofweeds:importantcovercropswereoilseedradish(RaphanussativusL.),S.albaandB.juncea[99].
Similarly,legumecovercropsvelvetbean(Mucunadeeringiana(Bort)Merr.),jumbiebean(Leucaena
leucocephala(Lam.)deWit),jackbean(Canavaliaensiformis(L.)DC.)]aretraditionallyusedinMexico
forcontrollingweedsinfieldcropsandachievingotherecologicalbenefitsofcovercrops,suchas
improvedsoilfertility[100].
Anumberofmulches—V.villosa,Egyptianclover(T.alexandrinumL.),A.sativa,T.
meneghinianum,wheat(TriticumaestivumL.),S.cereale,V.sativa,L.multiflorum—weretestedfortheir
weedcontrolefficacyintomato[49].Thecovercropswerekilledbeforetransplantingtomato
seedlings.Outofthetestedmulches,twomulches,i.e.,L.multiflorumandS.cerealedecreasedthe
weedbiomassbymorethan75%.Twooftheotherofmulches(V.sativaandV.villosa)werealso
Agronomy2020,10,25710of16
effectiveforweedcontrolintomato[49].Similarly,V.villosaasacovercropintomatoproduceda
biomassof7.49tha−1,andthemulchfromthiscovercropdecreasedthebiomassofsomeweed
species(A.retroflexus,D.sanguinalis,P.oleracea)by40%,andtheirdensityby>70%intomato[89,90].
Inanotherstudy,S.cerealewasgrownasacovercropanddesiccatedtostayasmulchoneweek
beforeplantingtomato.However,inthisstudy,additionalweedcontrolmeasureswererequiredto
achievesatisfactoryweedcontrol[101].Inanotherstudy,covercropssuchasT.incarnatumandV.
villosaeffectivelycontrolledtheweedsintomatoproductionbutdidnotimprovethetomatoyield
[102].
Amongseveralcoverspecies,themosteffectiveforweedcontrolincabbage(Brassicaoleracea
var.capitata)wereS.bicolor,sudangrass(Sorghumsudanense(P)Stapf.),andV.villosa,whichcaused
thehighestdecreaseinweedbiomassanddensity[50].Covercropswereeffectiveinsuppressing
weedssuchasA.retroflexus,Europeanheliotrope(HeliotropiumeuropaeumL.),P.oleraceae,field
pennycress(ThlaspiarvenseL.),annualsowthistle(SonchusarvensisL.),blacknightshade(Solanum
nigrumL.),shepherd’s‐purse(Capsellabursa‐pastoris(L.)Medik),C.album,sunspurge(Euphorbia
helioscopiaL.),wildmustard(SinapisarvensisL.),etc.,foralmosttwomonths.WeedcontrolbyV.
villosadoubledthekale(BrassicaoleraceaL.var.acephalaDC)yieldcomparedtountreatedcontrol
[50].Incucumberproduction,thephysicalpressureandallelopathiceffectsof(S.bicolor×S.
sudanense.)andS.cerealehelpedtocauseamorethan80%decreaseinweeddensities[14].V.villosa
wasalsoeffectiveincontrollingweedsbutityieldedsimilartocontroltreatment.Nevertheless,S.
cerealeandS.bicolor×S.sudanensesignificantlyincreasedthecucumberyieldovercontrol[14].
Lettuceisextensivelyusedasasaladaswellasinthemakingofrolls,wraps,sandwichesand
severalotherrecipes.Aslettuceismostlyconsumedasfreshleaves,non‐chemicalpestcontrolis
moreimportantforthisvegetablecrop.Alongwithothernon‐chemicalmethods,covercropshave
alsobeeninvestigatedforweedcontrolinlettucefields[17].Forexample,thecovercropofcowpea
(V.unguiculata(L.)Walp.)(growninsummerandtheneitherkeptasmulchorincorporatedinthe
soilduringfall)hadveryfewweedspeciesandhigherlettuceyieldscomparedtotheothercover
crop,S.vulgare,orthefallowtreatment[17].Inpepperproduction,thecovercropspecieswiththe
highestcompetitivenessagainstweedswereoat(AvenasativaL.),L.multiflorum,commonvetch
(ViciasativaL.)andV.villosa[103].Bytheendofthefirstandsecondmonthsofcovercrop
incorporation,theV.villosahadthehighestweedsuppression(70%orhigher)andtheplotswiththe
samecovercrophadthehighestpepperyield[103].Similarly,cowpeaasacovercropwasalso
effectiveinweedcontrolinpepperproduction,alongwithanincreaseinthegrowthandbiomass
productionofpepperplants[104].
Ladinoclover(TrifoliumrepensL.)asacovercropresiduewaseffectivetosuppressweedsin
squash(CucurbitamaximaDuch.)atfourweeksafterplanting;however,theeffectivenessofthecover
croptoaffectweedswasdiminishedbytheendofthesecondmonthofplanting[105].Inanother
study,S.cerealeandH.vulgareascovercropsinsugarbeet(BetavulgarisL.)suppressedweedssuchas
D.sanguinalis,hookedbristlegrass[Setariaverticillata(L.)P.Beauv.],andE.crus‐galli.Thisweed
suppressionwaslikelyduetotheallelopathicpotentialofH.vulgareandS.cereale[106].V.villosa,S.
cereal,oramixtureofthesetwocovercropswasevaluatedfortheirweedcontrolabilityinsweet
corn.Eitheraloneorinamixture,thetwocovercropssuppressedtheweedsandincreasedtheyield
ofsweetcorn[107].Similarly,inanotherstudy,covercropssuchasS.cereale,V.villosa,andwheat
suppressedtheearlyseasonweedsbynearly50%insweetcorn.However,thecovercropswerenot
effectiveincontrollingsedgeweeds.WheatandS.cerealeasacovercroporamixtureofthesewith
V.villosahadanegativeeffectontheyieldofsweetcorn[108].
Generally,itisassumedthatweedcontrolthroughcovercropsinvegetablescanbeimproved
byusingamixtureofcovercropsbelongingtodifferentfunctionalgroups[109].However,thismay
notalwaysbetrue.Forexample,thirteenmixturesofcovercropswereevaluatedfortheirweed
suppressioninvegetables[110].Mixturesofanycovercropspoorlysuppressedweedsifthemixture
wasapoorweedcompetitororhadpoormechanicaldesiccation.Importantly,individualcover
cropsthatperformedwellwithgoodestablishmentandweedcompetitionalsoperformwellwhen
growninmixtures.Covercropsthatperformedwellbothasindividualcropsandinamixturewere
Agronomy2020,10,25711of16
H.vulgare,S.cereale,V.villosa,andT.incarnatum,andtheseweresubsequentlyusedforweedcontrol
intomato[110].Inanotherstudy,theuseofcovercropsinno‐tillplantedvegetablescouldhighly
decreasetheemergenceofhairynightshade(SolanumsarrachoidesSendtn.)andPowell’samaranth
(AmaranthuspowelliiS.Watson)[111].
Incontrasttoconventionalstudiesthatdeterminetheeffectofcovercropsonweedcontrolin
vegetables,someinnovativetechniqueshavealsobeeninvestigated.Forexample,S.cerealecover
cropinoculatedwiththeplantbeneficialfungusTrichodermavirenscouldcontrolweedsin
transplantedvegetables[112].However,thenumberofsuchstudiesisverylimited.
8.IntegrationofCoverCropswithVegetableCrops
Thereisagoodopportunitytogrowcovercropsduringthefallowperiod.Thecovercrops
grownduringthefallowperiodrestrictthegrowthofweedsandseedestablishment,hence
providingweed‐freefieldsfortheupcomingvegetablecropseason.Forexample,S.cereale,R.sativus
andB.napuswerestronginhibitorsofweedsduringthefallowperiod[113].Covercropsaresown
onetotwomonths(orafewweeks)priortothesowingofthemaincropandthendesiccatedor
killed.Insomeinstances,covercropsandvegetablescangrowtogetherforaperiodofafewweeks.
Alongwiththeuseofglyphosate,theuseofmechanicalkillingofcovercropshasalsobeen
proposed.Desiccationofcovercropsmaybeeasywhenitisperformedduringflowering,or
immediatelybeforeorafterthisperiod.EasymechanicalmanipulationofcovercropssuchasH.
vulgare,bigflowervetch(ViciagrandifloraScop.),S.cereale,T.subterraneum,crimsonclover(Trifolium
incarnatumL.),andV.villosawasachievedwhencropswereatbloomorpost‐bloomstage[114].
CovercropswereplantedattheendofMarchandharvestedintwomonths(endofMay),andthe
maincropswereplantedwithinaweek.Undercutterwasabetterterminatorthanfielddisksin
reducingthebiomassofgrassweedsandincreasingsoybeanandmaizeyields[115].Covercrop
managementshouldbedonewithregardtotheactionmechanismofcovercropsagainstweeds.For
instance,ifacovercrophasahighallelopathicactivity,itmaybemixedintothesoilafteritskilling.
However,ifthecovercropsuppressesweedsthroughitsshadingoraphysicaleffect,thenitmaybe
leftscatteredonthesoilafterdesiccation[116].
9.Conclusions
Non‐chemicalweedcontrolisdesiredanditisimportantinvegetablecropsforseveralreasons.
Withrisingconcernsregardingherbicideevolutioninweedsandherbicideresidueissuesinthe
ediblepartsofvegetables,itisnecessarytoattemptweedcontrolinvegetablesthroughtechniques
suchascovercrops.Thediscussioninthisreviewshowsthatseveralcovercropssuppressweedsin
vegetablecropsthroughtheirphysicalorallelopathiceffects.Nevertheless,widespread
implementationofcovercropsacrosstheworldislacking.Technologicalgapsandlackof
site‐specificexperimentationmaybereasonsbehindthis.Local‐scaleexperimentationandbridging
thetechnologicalgapscanaidinawidespreadutilityofcovercropsforsustainableweedcontrolin
vegetableproduction.
Funding:Thisresearchreceivednoexternalfunding
ConflictsofInterest:Theauthorsdeclarenoconflictsofinterest
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