Cotton is a slow-growing plant, and only a limited selection of herbicides can be used for cotton weed control. These two factors sometimes make weed control difficult

DeFelice, Michael S.

Fishel, Fred

Kendig, Andy

University of Missouri: MOspace

CottonAGRICULTURALMU GuidePUBLISHED BY UNIVERSITY EXTENSION, UNIVERSITY OF MISSOURI-COLUMBIACotton Weed ControlAndy Kendig, University of Missouri Delta CenterFred Fishel, Integrated Pest ManagementMike DeFelice, Department of AgronomyCotton is a slow-growing plant, and only a limit-ed selection of herbicides can be used for cottonweed control. These two factors sometimes makeweed control difficult.Cotton is a semi-tropical, perennial plant. Althoughit is grown as an annual crop, cotton's growth habit ismore like trees than corn or soybeans. Its growth isespecially slow in the northern Cotton Belt, where it isoften planted early into cool soils. Early in the season,cotton often seems to "just sit there." Unfortunately,cool weather does not stop the growth of most weeds.Because herbicides are expensive to develop,most are developed for large-acreage crops such ascorn and soybeans. Compared to these crops, cottonacreage is small, which reduces incentive to developnew herbicides for cotton. This limits the availableherbicides. Those that are available often have narrowselectivity (safety) margins relative to crop tolerance.At this time there are no selective, over-the-top,broadleaf herbicides available for cotton. Post-emer-gence broadleaf control is accomplished with herbi-cides that can injure cotton but which are applied in adirected spray that misses most of the cotton plant.Because of the availability and good performance ofpost-emergence grass herbicides, weed control pro-grams should emphasize controlling broadleaf weeds.A selective broadleaf herbicide (Staple), a Buctril (bro-moxynil) tolerant cotton variety and a Roundup(glyphosate) tolerant cotton variety may be availablesoon as over-the-top, broadleaf control options.As with all crops, the weeds most difficult to con-trol in cotton are the ones most closely related to thecrop. Velvetleaf (often called wild cotton), spurredanoda and prickly sida (teaweed) often escape cottonherbicides. Botanists group these weeds and cotton inthe Mallow or Malvaceae family.How to control weedsThis publication provides an overview of stan-dard weed control practices and an explanation of theconcepts of cotton weed control. Most weed controlpractices are eventually customized for individual"Tips for successful cotto.n weed controlII' Cotton should ndtpe planted .if'ltoextremely weedyfields.II' Cotton grows slowly" weeds grow fast. Weeds cannotbe allowed to grow taller than cotton. .II' Use a solid soil-applied herbicide program withfluometuron and a grass/small-seeded broadleaf her-"bicide (Trifluralin, Prowl or Dual).II' Apply Command or Zorial' if velvetleaf, prickly sida orspurred anoda i~, present.t/ PI~r:1 to apply two PQst-ellJergenc!idirected appUca-tid~ns; adjust a~ needed.tI S~out carefully, cultivate often.II' Consult MU publication MP 575, "Weed Control GuideFor Missouri Field Crops or your state's weed controlgUide for further details. Read the herbicide label!farms and fields. This publication is intended to serveas a starting point for new cotton growers.Cotton requires intensive herbicide applications,intensive cultivation and hand hoeing. Because weedcontrol is difficult, cotton should not be planted intoextremely weedy fields.A solid program of pre-plant incorporated andpre-emergence herbicides, followed by post-emer-,gence directed herbicides, is needed f<?r successfulcotton weed control. Scouting for weeds the seasonbefore can help you plan a weed control program foroptimal performance and minimal cost.Consult Table 1 to compare herbicides for theircontrol of particular weeds. Additional informationmay be found in MU publication MP 575, WeedControl Guide for Missouri Field Crops or an equivalentpublication for your state. These Extension guides areupdated annually and contain much detailed infor-mation regarding herbicide mixtures, effectiveness onindividual weeds, effects on rotational crops, adju-vants and rainfall effects. Weed control guides do notsubstitute for herbicide labels. Always read the labelbefore applying a pesticide. Labels change regularly.$.75 G 4251 Printed with soy ink on recycled paper"'UQ)(QCDI\.)G)~I\.)01-lo.Table 1" Weed response to cotton herbicides"'t- 0co0 0 Q) ...0 0 Q)0 1: 1::0 co co 8- 1U... ... coco C) C) ~ ~ 'C... c: c: c. c:C) C'" I1U 0 0 Q) 0 0 c:0 0 C) co 0 .c ct Q) co0 0 c: c: c: 'C "2 E C) 'C0 0 C) E J: J: Q) c: ~ C) ...co co en 0 'C co co co co ~ 0... ... ~ 0 0 0 0 Q) .c Q) ..J 0 co a: 'C c. c:C) C) 0 ""') ""') ... ... ~ 0 .c m 'C ct't- (,.) co ~ m en 't-'C co co 0 "2 ... C) Q) Z ~ Q) c: c: 'C en Q) c: co... 'C Q) f! 0 C) c: E .c c: en ... 0 C) m c: 0 ~ 'C 'C Q)co ~ is co Q) ;: Q) 0 E c: ~ co E Q) ! ;:;~ E C) D. "cu 0 is 0 :2 0 c.' J: "2 :2 ii = Q)c: co .c ... N 0 0 E E ;: E co ...... ... 0 co >< 0 m (,.) E c. ... (,.) ... 0 ~ >co Q) ... ... 1U 0 0 :E ~ 0 Q) 0 0 0 C) "i: ~ 0 E c. c. ~m m m 0 LL LL Cl en a: 0 ::; J: J: 0 ::E ii: D. D. 0 en en enHerbicidePre-plant incorporate~Prowl 9 0 9 9 9 9 9 9 4 0 0 3 0 0 8 2 9 0 9 3 2 2 0 1Treflan/trifluralin/others 9 0 9 9 9 9 9 9 5 0 0 3 0 0 8 2 9 0 9 3 2 2 0 1Zorial 9 2 9 9 9 9 9 9 2 4 5 7 3 9 9 6 9 9 9 8 6 7 9 7Pre-emergenceCommand* 9 3 9 9 9 10 9 9 2 - 6 9 4 3 9 5 5 9 - 8 7 8 8 10Cotoran/Meturon 7 0 8 9 8 8 8 7 0 0 7 7 6 9 9 7 9 8 9 9 7 3 6 5Karmexlothers 7 0 8 9 8 8 8 7 0 0 6 6 4 9 9 7 9 7 9 8 7 6 4 5Zorial 8 1 8 8 8 8 8 7 1 4 4 7 3 9 9 5 9 8 9 8 6 7 9 7Post-emergence - overtopAssure II 9 - 9 9 9 10 9 10 9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0Fusilade 8 9 8 8 8 9 9 9 9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0PoastlPoast Plus 9 7 10 9 9 10 9 9 8 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0Select - - 10 9 9 9 9 10 9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0Post-emergence - directedBladex + MSMA 9 0 9 9 9 9 9 9 6 6 9 9 5 9 9 9 9 9 8 9 6 7 7 7Caparol/others + MSMA 9 0 9 9 9 9 9 9 6 6 9 9 5 9 9 8 9 8 8 8 6 5 6 6Cobra 0 0 0 0 0 0 0 0 0 2 9 9 9 9 5 7 10 7 9 9 7 8 7 7Cotoran/Meturon + MSMA 8 0 8 9 9 8 7 8 6 6 9 9 5 9 9 7 9 7 6 8 8 5 7 6DSMA or MSMA 8 0 8 8 8 8 5 8 6 6 9 5 2 3 7 3 3 2 3 5 2 0 1 1Goal 4 0 4 4 4 4 6 4 2 2 8 9 7 9 9 9 9 8 9 9 9 7 5 1Karmexlothers + MSMA 9 0 9 9 9 9 9 9 6 6 9 9 5 9 9 8 9 8 8 8 6 5 5 5Probe + MSMA 8 0 8 9 9 8 7 8 6 6 9 9 - 9 9 8 9 8 7 8 8 8 8 8Loroxlothers 6 0 6 6 6 7 6 6 1 1 7 7 8 9 9 8 9 8 9 8 7 7 6 7Weed control: 8 to 10 =Good 6 to 7 =Fair** Less than 6 =Poor - =No data available*Command by itself is extremely injurious to cotton. Disyston or Thimet insecticides must be used in-furrow at planting to prevent cotton injury.**A weed control rating of 6 to 7 indicates partial control or suppression.Use this table as a guide for comparing the relative effectiveness of herbicides on individual weeds. Herbicides may perform better or worse than indicated due toextreme weather conditions and other variables. If you are obtaining satisfactory results under your growing conditions, changing products as a result of informationin this table is not necessarily recommended.~substitute for herbicide labels. Always read the labelbefore applying a pesticide. Labels change regularly.Incorporated andpre-emergence herbicidesThe herbicide fluometuron (Cotoran or Meturon)is essential to a cotton weed control program.Fluometuron in cotton is much like atrazine in corn:• It is a standard treatment• It controls many but not all of the weeds inthe crop• It provides excellent control of manybroadleaves and provides fair control ofgrasses.Fluometuron also supplies some suppression ofvelvetleaf, spurred anoda and prickly sida. Like mostsoil-applied herbicides, the fluometuron rate dependsupon soil texture. Cotton has fair tolerance to fluome-turon, but injury can occur if rates are excessive or ifthe soil texture is coarse. Injury can be a problem inmixed soils with sandy areas.It is important to remember that if pre-emergencebroadleaf control is inadequate and weeds grow astall as or taller than the cotton, post-directed herbi-cide applications will be ineffective. A small amountof fluometuron injury may be preferable to poorweed control. Most cotton growers apply fluometur-on pre-emergence, after planting, in a band over therow. Banding reduces herbicide costs but requiresearly cultivation to control weeds in the row middles.A pre-plant-incorporated grass / small-seededbroadleaf herbicide is also essential for the weedpressure in most fields. These products include tri-fluralin (Treflan, Trilin, Tri-4, Trifluralin) and Prowl.Prowl or Dual (metolachlor) may be used pre-emer-gence for no-till cotton; however, Dual occasionallyinjures cotton.If velvetleaf, spurred anoda or prickly sida is pre-sent in the field, the herbicides norflurazon (Zorial) orclomazone (Command) should be used in addition tofluometuron. Zorial is most effective in a split appli-cation where half is applied pre-plant incorporatedand half is applied pre-emergence (typically bandedbehind the planter). Command by itself can kill cot-ton. If Command is to be used, either Disyston orThimet insecticides must be used in-furrow at plant-ing. These insecticides act as an antidote to "safen"cotton and minimize the risk of Command injury.Command may be applied either PPI or PRE. LowCommand rates (4 to 8 ounces of product per acre)may be used for velvetleaf control.Command should be used with caution since it isextremely active on several vegetable, horticulturaland ornamental plants and tends to vaporize anddrift from the soil. -Because of this problem, incorpo-ration is required in Illinois and the northeasternUnited States. Consult the Command label for appli-cation restrictions. Lower rates and incorporation canbe used to reduce the drift problem while still provid-ing excellent velvetleaf control.With Command or Zorial, it is sometimes possi-ble to forego a dinitroanaline-grass/ small-seededbroadleaf herbicide. However, pigweed often escapessuch programs. Test strips should be used on a field-by-field basis to determine if the grass/small-seededbroadleaf herbicide can be eliminated.The insecticides Disyston and Thimet (whichsafen cotton to Command) can interact with the her-bicides fluometuron (Cotoran, Meturon), diuron(Karmex, Direx) and linuron (Lorox, Linex) to injurecotton. This injury is usually slight with fluometuron,but is usually severe with diuron and linuron.Post-directed herbicidesWith favorable environmental conditions, thesoil-applied herbicide program will give cotton a two-to four-week head start on weeds, after which theherbicides break down and weeds begin to emerge.Post-emergence directed herbicide applications aremade to control the emerging weeds. "Post-directed"sprayers are often mounted on cultivators so that thegrower bands the herbicide on the drill and cultivatesthe row middles at the same time. If post-directedsprays are not applied in a timely manner, weeds canquickly grow too tall and the cotton-to-weed heightdifferential will be lost. Plan to apply directed spraysapproximately two weeks after planting, but scout forweeds before spraying. Beware, an untimely rain caninterfer~withyour spray plans and allow weeds togrow too tall to be controlled!Cotton typically is treated with post-emergencedirected herbicides at a 3- to 8-inch height and at an8- to 14-inch height. Sometimes a third "lay-by"application is applied when the row middles are lap-ping and no further cultivation will be possible.Most directed treatments act as contact herbi-cides, although many provide residual control.Cotton leaves that are sprayed will be killed. If cottonterminals are sprayed, delayed maturity and yieldloss can occur.The most important factor for post-emergencedirected treatments is the adjustment of the sprayer.Figure 1 shows a diagram and photo of typicaldirected sprayers. The adjustment must balance goodweed coverage (for good weed control) with minimalcotton coverage (for minimal crop injury). Off-centerspray tips are often used and tips are aimed so thatthe spray is higher in the row middle but relativelylow on the cotton plant. Aiming the spray tip back-wards and down (rather than straight down) alsoreduces the exposure to cotton.G 4251 Page 3Early Post-Directed Later Season Post-DirectedRig Adjustment Rig Adjustment~[JI ~ilFigure 1. Drawings and photo of typical post-emergencedirected sprayers with shoes to control height of spray.(Illustrations from Recommended Chemicals for Weed andBrush Control, MP-44, University of Arkansas CooperativeExtension Service.)OverlapplowedgroundRear viewOverlapplowedgroundSide Rear(Flat fan tips)Angle tip back andtoward row untilspray overlaps drilland tilled soilOverlapplowedground4 inchesRear viewSide Rear(Off-center tips)Angle tip back andaway from row untilspray overlaps drilland tilled soilOverlapplowedground4 inchesGood coverage is important for adequate weedcontrol, but lower pressures (approximately 30 psi)should be used to keep spray mist from blowing intothe foliage and causing excessive injury. Taller cotton ismore tolerant of directed sprays - the terminal is high-er and farther away from the spray and lower leavesare less important. Any variability in cotton heightacross a field should be considered. Short cotton can beseverely injured by a sprayer adjusted for taller cotton.With cotton 8 inches and taller, directed sprays.can be applied as high as 2 to 3 inches up the cottonstalk. When cotton is 3 to 6 inches tall, it is best tokeep the spray off of the foliage. However, if weedsare almost as tall as the cotton, a "Sloppy Directed"application may be made. With "Sloppy Directed"applications, herbicides are applied higher and covermore of the cotton plant. MSMA, DSMA and fluome-turon are usually used because they cause less injuryto cotton, but crop injury will still be significant."Sloppy Directed" applications should be reservedfor cases where the need for weed control will out-weigh herbicide damage to the crop. Again, it isimportant to keep sprays off of cotton terminals.Most post-directed applications include the herbi-cide MSMA or DSMA mixed with a "broadleaf" herbi-cide. MSMA and DSMA provide good grass controland suppress nutsedge and broadleaves. MSMA andDSMA are particularly good for cocklebur control,although MSMA- and DSMA-resistant cocklebur areoccasionally found in the Cotton Belt. MSMA andDSMA can be carefully directed under cotton at least 3inches tall. Most, but not all, formulations contain sur-factants which improve the activity of MSMA/DSMAand of tank-mix herbicides. Consult herbicide labels todetermine if a surfactant or crop oil is needed.Directed broadleaf herbicides include Bladex,Cobra, Cotoran/Meturon, Goal and Karmex/DirexLorox/Linex. They also provide some grass control.These herbicides all require a weed-crop height dif-ferential and all cause injury if applied to cottonfoliage or terminals. Cotoran/Meturon may bedirected to cotton 3 inches or taller. Bladex, Cobra,and Goal require that cotton be at least 6 inches talland Karmex/Direx and Lorox/Linex require a mini-mum cotton height of 8 inches.Some growers use fewer directed applicationsthan others. Scouting and a well-planned weed con-trol program can optimize the use of directed sprays.Over-the-top, broadleaf salvageThe importance of good pre-emergence weedcontrol, followed by post-emergence directed appli-cations, cannot be overemphasized. However, unusu-al weather conditions (lack of activating rainfall orexcessive rainfall) can cause failures in pre-emer-gence broadleaf weed control and can eliminate thecrop-to-weed height differential. Cotton has slighttolerance to foliar applications of fluometuron(Cotoran, meturon) and MSMA/DSMA. These mate-rials are occasionally applied over the top of youngcotton as a salvage treatment to create a crop-to-weed height differential for directed sprays.Fluometuron (at 1/2 pound ai per acre) occasion-ally provides good salvage weed control and no cropinjury when applied over the top of cotyledon-stagecotton. However, this treatment occasionally killscotyledon- and larger-stage cotton and does notalways provide good weed control.MSMA and DSMA are also used regularly forsalvage. Cotton is temporarily injured and discol-ored, although it often recovers from over-the-topapplications. MSMA and DSMA provide little controlof velvetleaf.Although salvage treatments are routinely usedby some growers, they have the potential to reducecotton stands and yields. It is up to the individualG 4251 Page 4

Cotton weed control (1994)

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