Citrus Weed Management

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Citrus Weed Management
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FactSheetHS-164 TheInstituteofFoodandAgriculturalSciencesisanequalopportunity/affirmativeactionemployerauthorizedtoprovideresearch,educational informationandotherservicesonlytoindividualsandinstitutionsthatfunctionwithoutregardtorace,color,sex,age,handicap,ornational origin.Forinformationonobtainingotherextensionpublications,contactyourcountyCooperativeExtensionServiceoffice. FloridaCooperativeExtensionService/InstituteofFoodandAgriculturalSciences/UniversityofFlorida/ChristineTaylorStephens,DeanJune1993CitrusWeedManagement1 D.P.H.TuckerandM.Singh2INTRODUCTIONFlorida'swarmclimateandrelativelyhighrainfall isconducivetointensivepest,disease,andweed pressures.Theenvironmentissuchthattherateof weedgrowthisveryrapid,andcroplands,including citrusgroves,canreverttoproverbialjunglesovera shortperiodoftimeifleftunmanaged.Weedcontrol accountsforabout20percentofthetotal productioncosts,andweedshavebeenestimatedto accountforabout150milliondollarsannuallyinlost revenuestothecitrusindustry.Suchlossesreflectthe costofweedcontroloperations,includingaquatic control,directcompetitiveeffectsandthereduced efficiencyofproductionandharvestingoperations.It isrecognizedthatmostofthecurrentcitrusacreage isundersomeformofintegratedweedmanagement programinvolvingchemicalandmechanicalcontrol methods. Therearevariousreasonsforfollowinggood weedmanagementpracticesincitrusgroves.Weeds competewithtrees,particularlyyoungtrees,for water,nutrientsandlight,withclimbingvineseasily coveringlargertreesifleftuncontrolled.Good vegetativegrowthandtheattainmentofearly productivityispartiallyattributedtothetimely eliminationofweedcompetition.Weedgrowth aroundtreetrunksandcanopycreatesfavorable conditionsforthedevelopmentoffungusdiseases suchasfootrotandbrownrot(onfruit).Rankweed growthkilledduringthewintercanbecomeafire hazardduringthedryspringmonths.Weedsreduce theefficacyofsoilappliedpesticidesdueto interceptionandalsocaninterferewiththeoperation oflowvolumeirrigationsystems. Soilandairtemperaturesarehigheringroves withbareground.Grovesundernontillagechemical weedcontrolprogramshavebeenshowntobe2Fto 4Fwarmerduringfreezesthanthoseunder mechanicalcultivationorsodculture. Asatisfactoryweedcontrolprograminvolvesan ongoingprocessofmonitoring,andtimelycontrol measuresinvolvingchemical,mechanical,biological methodsandoccasionalhandlabor.Withouta plannedapproachtoweedmanagementresultswillbe inconsistent.Sourcesofweedinfestation(seedor vegetative)identifiedandcontrolledpriorto disseminationwillgreatlyreducethecostofweed controloperationsandotherindirectlosses.New species,ifperceivedaspotentialproblems,should immediatelybeaddressedintheweedcontrol program.Asmanycurrentlytroublesomeweed speciesspreadfromoutsideareassuchasditchbanks, fencerowsandadjacentwoodlands,theseareas shouldalsobecloselymonitored. 1.ThisdocumentisFactSheetHS-164,aseriesoftheHorticulturalSciencesDepartment,FloridaCooperativeExtensionService,Instituteof FoodandAgriculturalSciences,UniversityofFlorida.Publicationdate:June1993. 2.D.P.H.Tucker,professor,ExtensionHorticulturist;M.Singh,professor,WeedScientist,CitrusResearchandEducationCenter,LakeAlfred FL33850,CooperativeExtensionService,InstituteofFoodandAgriculturalSciences,UniversityofFlorida,GainesvilleFL32611. Theuseoftradenamesinthispublicationissolelyforthepurposeofprovidingspecificinformation.Itisnotaguaranteeorwarrantyofthe productsnamed,anddoesnotsignifythattheyareapprovedtotheexclusionofothersofsuitablecomposition.

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CitrusWeedManagement Page2WEEDCLASSIFICATIONIN RELATIONTOCONTROLMEASURESFromthestandpointofcontrol,weedscanbe classifiedintwoways.Oneclassificationisbasedon lifecycleandtheotheronthesubclassoftheweed. Onthebasisoflifecycle,weedsaregroupedas annuals,biennials,andperennials.Mostannual weedsreproducefromseedandcompletetheirlife cycleinoneyearoroneseason.Weedsgerminating inthefallandproducingseedthefollowingspringor summerarecalledwinterannuals.Biennials germinateandproducevegetativegrowthduringthe firstseason,produceseedafterwinterdormancy,and dieduringthesecondseason.Fromacontrol standpoint,manybiennialsmaybegroupedwith annualssincebothhaveadeterminatelifespan,are self-destructive,andpropagatebyseed.Control objectivesinclude:(1)thecontrolofgrowthofthe above-groundportionofweeds,and(2)the preventionofseedproduction. Perennialweedslivefor3ormoreyears.Most noxiousperennialsarecomplexandreproduceby seedandvegetativelybymeansofroots,stolons,and rhizomes.Controlmethodsmust,therefore,be designedtodestroytheunderground,vegetative organs.Manyannualweedsarequiteprolificseed producers,butseedproductionbyperennialspecies rangesfromessentiallynoviableseedtomany thousandperplant.Someweedseedsliedormantin thesoilformanyyearsbeforegermination,thereby beingalwayspresentforreinfestationundertheright conditions. Weedsarealsosometimesclassifiedbysubclass intomonocotyledons(grasses)anddicotyledons (broadleafspecies).Theseclassificationsareuseful whereselectivechemicalcontrolisbasedon morphologicaldifferences. Ithasbeenestimatedthatofthemorethan100 weedspeciesthatcommonlyoccuringroves,only about30areconsideredveryundesirable.Ofthese perhaps20arecapableoforhavebecomeserious pestsasevidencedbytheirundesirablegrowth behavior,competitiveeffectsandcostofcontrol. SomeoftheseincludeMilkweedorStranglerVine (Morreniaodorata ),BalsamappleVine( Momordica charantia),Morningglory(Ipomeasp.)(scarlet,pitted, cypressvine,sharppod),VirginiaCreeper (Parthenocissusquinquefolia ),Briars(Similaxsp.), Cat'sClawVine( Bignoniaunguis-cati),Narrow-Leaf MilkweedVineorWhiteVine( Sarcostemma clausum),LoveVineorWoevine( Cassythafiliformis), Lantana(Lantanacamara),Goatweed(Scoparia dulcis),Saltbush(Baccharishalimifolia ),Teaweed (Sidaacuta ),Torpedograss( Panicumrepens), Vaseygrass(Paspalumurvillie ),Guineagrass( Panicum maximum),Peppervine(Ampelopsisarborea),Air Potato( Dioscoreabulbifera),andWildGrape( Vitis rotundifolia).METHODSOFWEEDCONTROLMowing,tillage,andchemicalweedcontrolare theprimarymethodsofweedcontrolusedwidelyin Floridacitrusgroves.Eachmethodhasitsadvantages anddisadvantages.Thechoiceofwhichmethodto usedependsongrovelocationandplantingsystem, theweedprobleminthegroveandtoanincreasing extentontheenergyrequirementsandassociated costsinvolved.Frequently,acombinationofmethods isthemostappropriate.CultivationCultivationisnolongeraswidelyusedasa controlmethod.Timingofcultivationofannualweed controlisnotascriticalaswithsoil-activeherbicides. Annualweedsarekilledefficientlyandeconomically byseveringthestemsfromtherootswhiletheweeds areyoung.Eachcropofweedsmustbekilledin ordertopreventcompetitionwiththetreesand productionofseeds.Whenseedsareproduced, however,theyareincorporatedintothesoilduring thenextcultivationwheretheyremainviablefor manyyears.Eachcultivationalsobringsseedstothe soilsurfacewheretheycangerminate.If environmentalconditionsareright,anewcropof weedsisproduced.Thesequenceofcultivation followedbyweedseedgerminationmaycontinue indefinitely.Thenumberofcultivationsrequired shouldbedeterminedbythenumberofflushesof weedemergence.Cultivationatpropertimeintervals canbeusedtoultimatelykilldeep-rootedperennial weedsbyexhaustionoftheirundergroundfood reserves.Storedfoodisconsumedintheprocessof sendingnewshootstothesurfaceaftereach cultivation.Withdrawaloffoodreservesmay continueforaweekormoreafternewshootsemerge. Whensufficientfoliagehasdeveloped,foodreserves arereplenishedbyproductsofphotosynthesis. Cultivationismosteffectiveatthetimeofmaximum netfooddepletion. Controlofmanyperennialsincitrusgrovesis rarelyachievedbycultivation.Thus,although

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CitrusWeedManagement Page3infrequentcultivationprovidestemporaryweed control,itspreadsandinvigoratesperennialweedsby increasingthenumberofburiedweedseedsandby widelydistributingrhizomeandstoloncuttings, tubers,andbulbs.Theresultofinfrequenttillageis theestablishmentofsolidstandsofaggressive, resistantspeciessuchasguineagrass,bermudagrass, torpedograss,andnutsedge. Constantcultivationresultsinthecontinual destructionofsurfacecitrusfeederrootswhich normallygrowintheundisturbedportionofthesoil. Forallpracticalpurposes,thecultivatedportionof thesoilisofnousetothetree.MowingMowingisnowmorewidelyusedingroveswhere cultivationisnotrecommendedduetoshallowroot systemsoftreesonbeddedflatwoodssoils.Itis appropriatewhereacovercropisdesiredinbedded grovesoronslopingterraintopreventsoilerosion. Mowingiseasytoperformandpresentsapleasing groveappearance,buthasahighenergydemand. Weedscanalsobespreadbyseedandvegetatively duringmowingoperations. Whileweedcontrolbymowingisusefulagainst tall-growingweedspecies,low-growingprostrate speciesaregivenanoverwhelminglycompetitive advantage.Mowedgrovesinmanyareasrapidly developasolidstandofperennialgrasseswhich becomesthestablevegetation,andthegroveisthus convertedtoasodculture.Evenfrequentmowing, however,willnotsufficientlylessenwater consumptionbyperennialgrass,especiallyunder droughtconditions.Yieldandgrowthoftreesare reducedbysodcultureincomparisonwithclean cultivationandchemicalweedcontrol.Mowingis practicedbetweenthetreerowsandawayfromthe treesincombinationwithherbicideapplicationsinthe treerowovertherootzoneoftrees.ChemicalMowingWiththefrequencyofmechanicalmowing requiredtomaintainrowmiddlesunderFlorida conditionsanditsincreasingcost,chemicalmowing andwipingwithlowratesofglyphosateinlowvolume applicationshasbecomeincreasinglypopular. Middlesmanagement,astheprogramiscalled, utilizesfewermechanicalmowingsatareducedcost andeventuallyresultsintheeliminationoftall growingspeciesandestablishmentofamore manageablesodofbermudaorbahiagrasses. Advantagesofestablishedintegrated mechanical/chemicalmiddlesmanagementprograms includelowercosts,reducedweedcompetitionwith treeswhererootsystemsextendintorowmiddlesand goodseedheadsuppressionleadingtoreducedweed proliferationthroughseeddissemination.ChemicalWeedControlChemicalweedcontrolisnowwidelyaccepted andusedintheFloridacitrusindustry.Thesuccess ofsuchaprogramisfarmorelikelytobeassuredif particularattentionispaidbytheuserto: thelabelinformationontheherbicide,regarding itsproperties,advantages,limitations,and precautions choosingtheappropriateherbicidesforthe particularweedproblem selectingthelowestrateofapplicationwithin labelrecommendationsthatwillbeeffective againstthespeciesonthesoiltypeinquestion calibrationoftheequipmentfortheapplication raterequired selectionofanozzlearrangementontheboom andboomheightforuniformcoverageofthe surfacetobesprayed thechoiceofapplicationtimingandfrequencyto ensuremaximumeffectivenessagainsttarget species. Itisimportanttounderstandthedirectionsonthe productlabelsothatthechancesofpoorweed controlresults,treedamage,illegalresiduesinfruit, andsoilaccumulationwillbeminimized. Herbicidesmaybeclassifiedasfoliar-orsoilappliedandmayhavesystemicorcontactactivity. Systemicherbicidesarethosethatareabsorbedby eitherrootsorabove-groundplantpartsandare translocatedthroughouttheplant.Contactherbicides killallplantpartsactuallysprayedwithlittleifany translocation.Forthecontrolofwellestablished perennialweeds,apostemergenceherbicidewith systemicactivitymaybeusedwithapreemergence material.Forherbiciderecommendationsrefertothe current FloridaCitrusPestManagementGuide:Weeds.

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CitrusWeedManagement Page4HerbicidesandRatesofApplicationThechoiceofherbicidesandratesdependson: Labeldirections Scionvariety Treeage Weedspecies Stageofweedgrowth Soiltype(organicmattercontent)andthe recognizedpotentialforleaching.TimingofApplicationsTimingofapplicationsisveryimportantassoilactiveherbicidesaremostactiveagainstgerminating weedseedlings.Asweedsbecomemorematureand weedcovermoredense,higherratesincombination withcontactorsystemicherbicidesarerequiredto achievesatisfactorycontrol.Althoughsomesoilactiveherbicideshavepostemergencecontactactivity also,itshouldberecognizedthattheirpreemergence residualactivityistheirgreatestasset. Ifaparticularweedspeciessuchasbalsamapple vineisthemajorproblem,forexample,anapplication shouldbetimedtocoincidewithpeakemergenceof thevineseedlings.Ifappliedtooearly,theherbicide concentrationinthesoilmayhavedecreasedbelow aneffectivelevelinthegerminationzonedueto breakdownand/orleaching.Laterapplicationswhen vinesarealreadyintreecanopiesarealso unsatisfactory.Lantanaoffersasecondexampleofa specieswhichproliferatesduetopoorapplication timing.Initsearlyseedlingstages,lantanais susceptibletosomeofthecommonlyusedsoil residualherbicides,however,largerseedlingsare resistant.Milkweedvineoffersanotherexample,in thatasthevinesbecomeestablishedtheybecome moretoleranttorecommendedratesofpreemergence herbicides.Repeatedapplicationsmustbeusedto eventuallyeliminateestablishedplants. Glyphosateisactuallymoreeffectivewhen appliedtoperennialgrassesatseedheadstagedueto bettertranslocation.Similarly,fallapplicationsare moreeffectiveonwoodyperennialspeciesvineand shrub,speciessuchasVirginiaCreeperandLantana. However,assuchweedspeciesmature,densegrowth andheightmakegoodspraycoveragedifficultto achieve.FrequencyofApplicationUnderFloridaconditions,eventwoapplications ayearmaynotbeenoughforsatisfactoryweed control.Thisisparticularlytrueinthesouthern locationswhereyear-roundgrowingconditionsare morelikely.Twoapplicationsormoreayearare required,particularlyattheonsetofachemicalweed controlprogramwhendifficult-to-controlspecies includingvines,shrubs,andperennialgrassesarewell established.Asasatisfactorydegreeofweedcontrol isestablishedwithtime,residualherbicideratesmay bereduced,particularlyiftwoormoreapplicationsa yeararemade. Whereacontinualtree-resettingprogramisin effect,asiscurrentlythecaseinmostFloridagroves, itwouldbeprudenttolowertheratesandincrease thefrequency.Morefrequent,lowerratesof applicationarelesslikelytoinjurethenewlyplanted trees,particularlythoseonpoorersoils.Although increasedapplicationfrequenciesresultingreater equipmentoperationcosts,thismaybeminimizedby includingsplitratesofherbicideinfluidandliquid fertilizerbandapplications.Withthecurrent emphasisongroundwatercontamination,thetrend towardmorefrequentapplicationoflowerratesof soilresidualherbicidesisappropriate.BiologicalControlSuchcontroloccurstoalimitedextentnaturally inFloridacitrusgroves.Keenobserversingrovescan frequentlyobserveweedsunderstress,perhaps showingfoliardiscolorationordistortionsymptoms causedbyoneormoreoftheseagents.Themost commonlyknownbiologicalcontrolagentnowisthe fungalpathogen, Phytophthorapalmivora ,whichwas developedintothemycoherbicide,DeVine.This endogenouspathogenwasoriginallyfoundattacking themilkweedorstranglervineingroves. ThelarvaeofNoctuidsincludingarmyworms, cutworms,loopers,andothershavebeenobservedto effectivelydecimatestandsofvariousgrassspeciesin groves.Anumberofleaf-miningbeetles (Chrysomelidae)havebeenusedsuccessfullyin AustraliaandHawaiiincontrollinglantana, Lantana camara,andcouldbeintroducedintoFlorida. However,suchintroductionsarefrequentlydelayedor deniedbyvariousfederalorstatecommitteeswhich

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CitrusWeedManagement Page5mustevaluatetherisk-benefitpicture.Therehas beensomeoppositiontotheintroductionofbiological controlagentsduetothefearthattheintroduced insectsorpathogenswillattackotherplantsonce theirprimaryfoodsourcehasbeenconsumed.Also, aweedofeconomicimportancetoonemaybea desirableplantofvaluetoothers.Inresolvingsuch conflicts,theeconomicimpactoftheweedin croplandsmustbecomparedwiththenegativeaspects indicatedbythosewhoopposeitscontrolbysuch introductions.AllelopathyinCitrus WeedManagementOneareaofinnovativeresearchisallelopathyand itsapplicationinagriculture.Allelopathy,the regulationofgrowthofoneplantspeciesbychemicals releasedbyanother,occurswidelyinplant communitiesandisbelievedtoregulatespecies densityanddistribution.Isolationandidentification ofnaturalcompoundsimplicatedinallelopathyhave receivedconsiderableattentionasitispossiblethat relatedcompoundsmaybesynthesizedthatcan producesimilareffectsaspesticides.Severalplant productshaveprovenusefulincontrolofinsects,and thepotentialexistsforthedevelopmentofmany othernaturalinsecticides,herbicidesandgrowth regulators. Lantana,arapidlygrowingperennialwoody shrub,isaneconomicweedpestinFloridacitrus groves.Itsproliferationandspreadareattributedto severalfactorsincludingnaturalpropagation,reduced useofmanualweeding,tolerancetomanycurrently usedherbicidesandlittlecompetitionfromother weedspecies.Theinterferencewithgrowthofplants growinginthevicinityoflantana( Lantanacamara L.) hasbeenattributed,atleastinpart,tothe allelopathiceffectsoftheweed.Theabsenceof milkweedvine(Morreniaodorata Lindl.)infestations nearlantanaplantswasobserved. Allelopathiceffectsoflantanaresidues(root, shoot),foliarleachates,andthesoil(wherelantana wasgrown)onmilkweedvineseedgerminationand growthovera30-dayperiodwereexamined.Foliar leachatesorthesoilcollectedfromthefieldwhere lantanahadbeengrowinghadnoeffectonthefinal germinationpercentageortheseedlinggrowthof milkweedvine.Incorporationofdriedlantanashoot orrootmaterialintosoilhadnoeffectonthefinal percentagegerminationbutcausedsignificant reductionsinmilkweedvinegrowthovera30daytest period.Rootsweremoreinhibitorythanshoots. Fiftypercentofmilkweedvineseedlingsdiedwithin15 daysaftergerminationat1%(w/w)driedlantanaroot incorporationintothesoil,andhigherconcentrations increasedseedlingdeath.Lantanarootsincorporated intothesoilproducedfoliarsymptomssuchaswilting anddesiccation,whereaslantanashootsincorporated intothesoilproducedyellowingofthefoliageof milkweedvine.Allelopathicactivityoflantana residueswasstillstrongevenafterdecompositionof lantanaresiduesfor4weekspriortotheplantingof milkweedvineseeds.WEEDCONTROLPROGRAMSWeedcontrolprogramsvarywithlocationinthe statedependingonsoiltype,plantingsystemand rootingdepth.Programsforridgegrowingconditions withdeep,welldrainedsandysoilincludeherbicide applicationsintreerowsandmechanicallycultivated ormowedrowmiddles.Somechemicalmowingis nowpracticed.Programsforflatwoodsgrowing conditionsinvolvingshallow,inherentlypoorly drainedsoilswhicharedisturbedintheprocessof beddingandditching,alsoincludeherbicidesintree rowsandmechanicallyorchemicallymowedmiddles. Nocultivationisusedbecauseofpossibledamageto shallowrootsystems.Inbothgrowingareasand plantingsystemssupplementalweedcontrol applicationsincludeherbicideinjectionthroughlow volumeirrigationsystemsandlocalizedspot treatmentsforweedsescapingthestandardstrip applications.THEINFLUENCEOFWEEDCONTROL METHODSONTHEENVIRONMENTWeedcontrolinfluencesandisinfluencedby othergrovepractices.Astheindustryhasconverted fromhighvolumeoverheadsprinklerirrigationtolow volumesystems,lessofthegrovefloorsurfaceis wettedtherebyreducingweedgrowthinrowmiddles andareasoutsidetheemitterzones.Infact,during extendeddryweatheritnowbecomesdifficultto establishcovercropsinnewlydevelopedgroveswhich inturncanaffectwinderosion.Theintensiveusage oflowvolumeirrigationhasledtothemorerapid leachingofherbicidesintheemitterzones,resulting inearlierresumptionofweedgrowth.Fertilizer appliedthroughthesesystemsalsocontributesto greaterweedgrowth.Tocontrolsuchreturngrowth moreeffectivelyandminimizeleaching,lowerratesof herbicidesarebeingappliedmorefrequentlyalong

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CitrusWeedManagement Page6withherbicideapplicationsthroughirrigationsystems andspottreatmentswithpostemergenceherbicides. Asplantingdensitiesincrease,particularlyin rows,thegrovefloorsurfacebecomesshadedmore rapidlybytreecanopiestherebysuppressingweed growth.Conversely,thehighincidenceoftreeloss duetocitrusblight,citrustristezavirus,soilinsects andnematodesandfreezes,resultsinthe reestablishmentofweedgrowthasthegrovefloor surfacebecomesreexposedtosunlight.ENVIRONMENTALCONSIDERATIONSMisapplicationofherbicidescandamageorkill desirablevegetationandcitrustrees,ultimately causingundesirableenvironmentaleffectsand possiblyresultinlegalproceedings.Toreducethe impactofherbicidesontheenvironmentoutsidethe immediatetargetareaeffortsshouldbedirected towards: minimizingapplicationrateswithinrecommended rangesforweedcontrol avoidingtheuseofherbicideswithhighleaching potentialsinareaswithahistoryofgroundwater contaminationproblems avoidingapplicationclosetodesirableplantsand treessusceptibletotheparticularherbicide(s) minimizingdriftbynotapplyingherbicidesunder windyconditionsandbyusinglowpressuresand, ifnecessary,driftcontroladditives betterirrigationmanagementtoreduceleaching monitoringdrainageoutflowstodeterminelevels ofherbicidesinwater bettererosioncontroltoavoidmovementof excessiveamountsofsurface-appliedchemicals intoponds,lakes,andwaterways bettermixingandloadingproceduresand thoroughon-sitecleanuptoavoidtreedamage andcontaminationofadjacentwatersources maintainingcropresiduelevelswithinthe approvedrangesbykeepingapplicationrates withinrecommendedrangesandavoiding excessiveherbicidespraycontactwithtrees.PROCEDURESFORREDUCING HERBICIDEMOVEMENTINSOILIdeallyaherbicide,whenappliedtothesoil, shouldreachitstargetintheplantrootzonein sufficientconcentrationtocontroltargetweedsfora desiredperiodoftime,andthendissipaterelatively rapidlywithminimalnegativeenvironmentalimpact. Conventionally,manyherbicideswereappliedatrates higherthanthoseactuallyrequiredforweedcontrol underidealconditionstooffsetlossesthatoccurfrom theirsiteofactionintheplantrootzone. Alternativesavailableforreducingherbicide movementthroughthesoilprofileincludethe selectionoflessmobileherbicideswithhigh adsorptioncapacity,lowwatersolubilityandashort persistenceinthesoil.Moresolubleproductsshould beappliedatlowerratesatmorefrequentintervals alongwithgoodirrigationmanagement.Newly developedherbicidesaremostlikelytobeeffectiveat muchlowerratesandberelativelyinsoluble. Adjuvantshavebeenusedextensivelytomodify theactivityofherbicidesonplantsurfaces,however littleinformationisavailableconcerningtheireffects onthebehaviorofpesticidesinsoils.Arecent approachbywhichcertainadjuvantsbindthe herbicidemoleculesandthenreleasethemslowly overtimeholdsconsiderablepromiseforminimizing herbicidelossesthroughthesoilprofile.Therelease inacontrolledmannernotonlymayreduce groundwatercontaminationbutmayalsoenhancethe efficacyofherbicidesbyretainingthemintheweed germinationzone. Adjuvantscaneitherbeformulatedwiththe herbicidesoraddedtospraysolutionsjustbefore application.Aherbicidecaneitherbechemically attractedto,orphysicallyentrappedwithinanother substancesuchasapolymer.Theresultingcomplex containstheherbicideinamannercapableofbeing releaseduponhydration,hydrolysis,erosion, biodegradation,diffusion,osmosis,mechanicalrupture orothermeans. TheeffectofthepolymerSTAY-TEConthe efficacyoffourherbicides(bromacil,diuron, norflurazon,simazine)wasevaluatedingreenhouse andfieldexperiments.STAY-TEC,whenaddedto herbicidesolutions,increasedtheefficacyofbromacil, norflurazon,andsimazine,whichprovidedlonger lastingweedcontrolinthefieldthantheherbicide appliedwithoutSTAY-TEC.Ingreenhouse experiments,simazineappliedwithSTAY-TEC providedlesscontrolofweedsthantheherbicide appliedaloneinthefirst6weeksafterapplication. Herbicidalactivitywashigher,however,intrays treatedwithsimazineappliedincombinationwith STAY-TECthanintraystreatedwithsimazinealone 12weeksaftertreatment.Inlaboratoryexperiments, STAY-TECincreasedtheadsorptionanddecreased

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CitrusWeedManagement Page7themobilityofsimazine,butitdidnotaffectthe adsorptionorleachingofbromacil,norflurazon,or diuroninsoil. Asseenfromthegreenhouseexperiment,one problemwithcontrolled-releaseformulationsisthat theamountofherbicidereleasedfromtheherbicidepolymercomplexinitiallyisnotsufficienttoprovide effectiveweedcontrol.Therefore,theratesofthe herbicidesshouldbeadjustedtoprovide concentrationshighenoughforweedcontrol,butwell belowthetoleranceleveloftheassociatedcrop. Successfulreleaseoftheherbicidefromtheherbicidepolymercomplexindesiredconcentrationswill dependuponseveralfactors,suchasrainfall,pH,soil type,etc.Thepolymersusedascontrolled-release agentsshouldbebiodegradableandshouldnotalter thesoilphysicalcharacteristicspermanently.