Cotton Weed Control

Andrew Kendig
Delta Center
Fred Fishel
Integrated Pest Management
Michael S. DeFelice
Department of Agronomy

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.

Cotton is a semi-tropical, perennial plant. Although it is grown as an annual crop, cotton's growth habit is more like trees than corn or soybeans. Its growth is especially slow in the northern Cotton Belt, where it is often 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 as corn and soybeans. Compared to these crops, cotton acreage is small, which reduces incentive to develop new herbicides for cotton. This limits the available herbicides. Those that are available often have narrow selectivity (safety) margins relative to crop tolerance.

At this time there are no selective, over-the-top, broadleaf herbicides available for cotton. Post-emergence broadleaf control is accomplished with herbicides that can injure cotton but that are applied in a directed spray that misses most of the cotton plant. Because of the availability and good performance of post-emergence grass herbicides, weed control programs should emphasize controlling broadleaf weeds. A selective broadleaf herbicide (Staple), a Buctril (bromoxynil) tolerant cotton variety and a Roundup (glyphosate) tolerant cotton variety may be available soon as over-the-top, broadleaf control options.

As with all crops, the weeds most difficult to control in cotton are the ones most closely related to the crop. Velvetleaf (often called wild cotton), spurred anoda and prickly sida (teaweed) often escape cotton herbicides. Botanists group these weeds and cotton in the Mallow or Malvaceae family.

How to control weeds

This publication provides an overview of standard weed control practices and an explanation of the concepts of cotton weed control. Most weed control practices are eventually customized for individual farms and fields. This publication is intended to serve as a starting point for new cotton growers.

Cotton requires intensive herbicide applications, intensive cultivation and hand hoeing. Because weed control is difficult, cotton should not be planted into extremely weedy fields.

A solid program of pre-plant incorporated and pre-emergence herbicides, followed by post-emergence directed herbicides, is needed for successful cotton weed control. Scouting for weeds the season before can help you plan a weed control program for optimal performance and minimal cost.

Consult Table 1 to compare herbicides for their control of particular weeds..Weed control guides do not substitute for herbicide labels. Always read the label before applying a pesticide. Labels change regularly.

Table 1
Weed response to cotton herbicides. 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 to extreme weather conditions and other variables. If you are obtaining satisfactory results under your growing conditions, changing products as a result of information in this table is not necessarily recommended.

Weed control8 to 10 = Good6 to 7 = Fair2Less than 6 = Poor- = No data available
HerbicideBroadleaf and signalgrassBarnyardgrassBermudagrassCrabgrass
Pre-plant incorporated
Post-emergence — overtop
Assure II9-99
Poast/Poast Plus97109
Post-emergence — directed
Bladex + MSMA9099
Caparol/others + MSMA9099
Cotoran/Meturon + MSMA8089
DSMA or MSMA8088
Karmex/others + MSMA9099
Probe + MSMA8089
HerbicideFall PanicumFoxtailsGoosegrassJohnsongrass Seedling
Pre-plant incorporated
Post-emergence — overtop
Assure II910910
Poast/Poast Plus91099
Post-emergence — directed
Bladex + MSMA9999
Caparol/others + MSMA9999
Cotoran/Meturon + MSMA9878
DSMA or MSMA8858
Karmex/others + MSMA9999
Probe + MSMA9878
HerbicideRhizome JohnsongrassYellow NutsedgeCockleburJimsonweed
Pre-plant incorporated
Post-emergence — overtop
Assure II9000
Poast/Poast Plus8000
Post-emergence — directed
Bladex + MSMA6699
Caparol/others + MSMA6699
Cotoran/Meturon + MSMA6699
DSMA or MSMA6695
Karmex/others + MSMA6699
Probe + MSMA6699
HerbicideHemp SesbaniaHophornbean CopperleafCommon LambsquartersMorningglory, Annual
Pre-plant incorporated
Post-emergence — overtop
Assure II0000
Poast/Poast Plus0000
Post-emergence — directed
Bladex + MSMA5999
Caparol/others + MSMA5998
Cotoran/Meturon + MSMA5997
DSMA or MSMA2373
Karmex/others + MSMA5998
Probe + MSMA-998
HerbicidePigweedPrickly SidaPurslaneCommon Ragweed
Pre-plant incorporated
Post-emergence — overtop
Assure II0000
Poast/Poast Plus0000
Post-emergence — directed
Bladex + MSMA9989
Caparol/others + MSMA9888
Cotoran/Meturon + MSMA9768
DSMA or MSMA3235
Karmex/others + MSMA9888
Probe + MSMA9878
HerbicideSmartweedSpotted SpurgeSpurred AnodaVelvetleaf
Pre-plant incorporated
Post-emergence — overtop
Assure II0000
Poast/Poast Plus0000
Post-emergence — directed
Bladex + MSMA6777
Caparol/others + MSMA6566
Cotoran/Meturon + MSMA8576
DSMA or MSMA2011
Karmex/others + MSMA6555
Probe + MSMA8888
1Command by itself is extremely injurious to cotton. Disyston or Thimet insecticides must be used in-furrow at planting to prevent cotton injury.
2A weed control rating of 6 to 7 indicates partial control or suppression.

Incorporated and pre-emergence herbicides

The 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 in the crop.
  • It provides excellent control of many broadleaves and provides fair control of grasses.

Fluometuron also supplies some suppression of velvetleaf, spurred anoda and prickly sida. Like most soil-applied herbicides, the fluometuron rate depends upon soil texture. Cotton has fair tolerance to fluometuron, but injury can occur if rates are excessive or if the soil texture is coarse. Injury can be a problem in mixed soils with sandy areas.

It is important to remember that if pre-emergence broadleaf control is inadequate and weeds grow as tall as or taller than the cotton, post-directed herbicide applications will be ineffective. A small amount of fluometuron injury may be preferable to poor weed control. Most cotton growers apply fluometuron pre-emergence, after planting, in a band over the row. Banding reduces herbicide costs but requires early cultivation to control weeds in the row middles.

A pre-plant-incorporated grass/small-seeded broadleaf herbicide is also essential for the weed pressure in most fields. These products include trifluralin (Treflan, Trilin, Tri-4, Trifluralin) and Prowl. Prowl or Dual (metolachlor) may be used pre-emergence for no-till cotton; however, Dual occasionally injures cotton.

If velvetleaf, spurred anoda or prickly sida is present in the field, the herbicides norflurazon (Zorial) or clomazone (Command) should be used in addition to fluometuron. Zorial is most effective in a split application where half is applied pre-plant incorporated and half is applied pre-emergence (typically banded behind the planter). Command by itself can kill cotton. If Command is to be used, either Disyston or Thimet insecticides must be used in-furrow at planting. These insecticides act as an antidote to "safen" cotton and minimize the risk of Command injury. Command may be applied either PPI or PRE. Low Command rates (4 to 8 ounces of product per acre) may be used for velvetleaf control.

Command should be used with caution since it is extremely active on several vegetable, horticultural and ornamental plants and tends to vaporize and drift from the soil. Because of this problem, incorporation is required in Illinois and the northeastern United States. Consult the Command label for application restrictions. Lower rates and incorporation can be used to reduce the drift problem while still providing excellent velvetleaf control.

With Command or Zorial, it is sometimes possible to forgo a dinitroanaline-grass/small-seeded broadleaf herbicide. However, pigweed often escapes such programs. Test strips should be used on a field-by-field basis to determine if the grass/small-seeded broadleaf herbicide can be eliminated.

The insecticides Disyston and Thimet (which safen cotton to Command) can interact with the herbicides fluometuron (Cotoran, Meturon), diuron (Karmex, Direx) and linuron (Lorox, Linex) to injure cotton. This injury is usually slight with fluometuron, but is usually severe with diuron and linuron.

Post-directed herbicides

With favorable environmental conditions, the soil-applied herbicide program will give cotton a two- to four-week head start on weeds, after which the herbicides break down and weeds begin to emerge. Post-emergence directed herbicide applications are made to control the emerging weeds. "Post-directed" sprayers are often mounted on cultivators so that the grower bands the herbicide on the drill and cultivates the row middles at the same time. If post-directed sprays are not applied in a timely manner, weeds can quickly grow too tall and the cotton-to-weed height differential will be lost. Plan to apply directed sprays approximately two weeks after planting, but scout for weeds before spraying.

An untimely rain can interfere with your spray plans and allow weeds to grow too tall to be controlled!

Cotton typically is treated with post-emergence directed herbicides at a 3- to 8-inch height and at an 8- to 14-inch height. Sometimes a third "lay-by" application is applied when the row middles are lapping and no further cultivation will be possible.

Most directed treatments act as contact herbicides, although many provide residual control. Cotton leaves that are sprayed will be killed. If cotton terminals are sprayed, delayed maturity and yield loss can occur.

The most important factor for post-emergence directed treatments is the adjustment of the sprayer. Figure 1 shows a diagram and photo of typical directed sprayers. The adjustment must balance good weed coverage (for good weed control) with minimal cotton coverage (for minimal crop injury). Off-center spray tips are often used and tips are aimed so that the spray is higher in the row middle but relatively low on the cotton plant. Aiming the spray tip backwards and down (rather than straight down) also reduces the exposure to cotton.

Post-emergence directed sprayersFigure 1
Typical post-emergence directed sprayers with shoes to control height of spray.

Good coverage is important for adequate weed control, but lower pressures (approximately 30 psi) should be used to keep spray mist from blowing into the foliage and causing excessive injury. Taller cotton is more tolerant of directed sprays — the terminal is higher and farther away from the spray and lower leaves are less important. Any variability in cotton height across a field should be considered. Short cotton can be severely 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 cotton stalk. When cotton is 3 to 6 inches tall, it is best to keep the spray off of the foliage. However, if weeds are almost as tall as the cotton, a "Sloppy Directed" application may be made. With "Sloppy Directed" applications, herbicides are applied higher and cover more of the cotton plant. MSMA, DSMA and fluometuron are usually used because they cause less injury to cotton, but crop injury will still be significant. "Sloppy Directed" applications should be reserved for cases where the need for weed control will outweigh herbicide damage to the crop. Again, it is important to keep sprays off of cotton terminals.

Most post-directed applications include the herbicide MSMA or DSMA mixed with a "broadleaf" herbicide. MSMA and DSMA provide good grass control and suppress nutsedge and broadleaves. MSMA and DSMA are particularly good for cocklebur control, although MSMA- and DSMA-resistant cocklebur are occasionally found in the Cotton Belt. MSMA and DSMA can be carefully directed under cotton at least 3 inches tall. Most, but not all, formulations contain surfactants, which improve the activity of MSMA/DSMA and of tank-mix herbicides. Consult herbicide labels to determine if a surfactant or crop oil is needed.

Directed broadleaf herbicides include Bladex, Cobra, Cotoran/Meturon, Goal and Karmex/Direx Lorox/Linex. They also provide some grass control. These herbicides all require a weed-crop height differential and all cause injury if applied to cotton foliage or terminals. Cotoran/Meturon may be directed to cotton 3 inches or taller. Bladex, Cobra and Goal require that cotton be at least 6 inches tall, and Karmex/Direx and Lorox/Linex require a minimum cotton height of 8 inches.

Some growers use fewer directed applications than others. Scouting and a well-planned weed control program can optimize the use of directed sprays.

Over-the-top, broadleaf salvage

The importance of good pre-emergence weed control, followed by post-emergence directed applications, cannot be overemphasized. However, unusual weather conditions (lack of activating rainfall or excessive rainfall) can cause failures in pre-emergence broadleaf weed control and can eliminate the crop-to-weed height differential. Cotton has slight tolerance to foliar applications of fluometuron (Cotoran, meturon) and MSMA/DSMA. These materials are occasionally applied over the top of young cotton as a salvage treatment to create a crop-to-weed height differential for directed sprays.

Fluometuron (at 1/2 pound ai per acre) occasionally provides good salvage weed control and no crop injury when applied over the top of cotyledon-stage cotton. However, this treatment occasionally kills cotyledon- and larger-stage cotton and does not always provide good weed control.

MSMA and DSMA are also used regularly for salvage. Cotton is temporarily injured and discolored, although it often recovers from over-the-top applications. MSMA and DSMA provide little control of velvetleaf.

Although salvage treatments are routinely used by some growers, they have the potential to reduce cotton stands and yields. It is up to the individual grower to decide if the weed control will make the crop injury worthwhile. The future availability of Staple and Buctril-tolerant and Roundup-tolerant cultivars will be a welcome addition for over-the-top, broadleaf weed control.

Post-emergence grass control

If grass weeds escape a weed control program, the post-emergence, selective grass herbicides can be used in cotton just as they are used in soybeans. These include Assure, Fusilade, Poast and Select. These herbicides provide good grass control without injuring cotton. Bermudagrass can be a problem in cotton because it is tolerant of MSMA and DSMA; however, it can be managed with the selective grass herbicides.

Miscellaneous topics

Phenoxy herbicides
Cotton is extremely sensitive to phenoxy (2,4-D, Butyrac) herbicides. Avoid planting cotton near corn, sorghum or rice that will be treated with 2,4-D or near soybeans that will be treated with 2,4-DB. Spray equipment that has applied phenoxy herbicides should be thoroughly flushed, soaked overnight with a 1 percent solution of household ammonia (1 gallon of ammonia in 100 gallons of water) and flushed again before spraying cotton.

It is important to note the following information: In Arkansas, where cotton acreage is significantly higher than corn acreage, it is illegal to apply 2,4-D within one-fourth mile of a cotton field with ground equipment and illegal to make an aerial application within four miles. This law is not in place in Missouri because corn acreage is many times higher than cotton acreage.

Although a few growers successfully use 2,4-D as a burndown in no-tillage cotton, this practice is not labeled because of the risk of crop injury. Even though 2,4-D is a post-emergence herbicide, it has approximately three weeks of residual soil activity.

Roundup may be applied through a wick-bar to control large escaped weeds. It is important that the wick bar drips little and that weeds be at least 6 inches taller than the cotton. If Roundup contacts cotton, there may be few visible symptoms, but fruiting is severely disrupted and inhibited.

Perennial vines
Woody, perennial weeds and vines are often present in parts of cotton fields. Common species include trumpetcreeper and redvine. These weeds grow slowly and generally have little effect on cotton. At this time the best herbicide option is application of Roundup as a part of the defoliation, harvest preparation program. The herbicides 2,4-D and Banvel are not registered for use the winter before cotton is to be planted. Perennials are worse in reduced tillage situations. Spot treatment is recommended in most cases because these weeds usually occur in patches.

Cotton stand failure
Because of the short, cool growing season and poor early planting conditions that frequently occur in the northern Cotton Belt, cotton stands often fail, after which it is too late for cotton to be replanted. If pre-emergence herbicides have been banded, soybeans can often be planted into the untreated zones that previously were the cotton row middles. Soybeans are very tolerant of Command and Treflan, moderately tolerant of Zorial (Zorial is approved for use in soybeans) but are not tolerant of Bladex, Cotoran or Karmex/Direx. Planting soybeans between bands of injurious herbicides leaves little room for error and must be done at the grower's risk.

Reduced tillage
Many growers successfully grow cotton in stale seedbed, reduced tillage or absolute no-tillage situations. Because cotton weed control is difficult, reduced tillage cotton should be grown with caution. If no pre-plant tillage is to be conducted, complete burndown is required. There are virtually no options to clean up escapes from burndown treatments.

Growers who are familiar with the weed pressure on their farm and have carefully developed practices to manage their weeds can obtain excellent burndown with relatively little herbicide. However, the personnel from the University of Tennessee No-Till Station in Milan, Tenn., normally recommend two burndown applications, since there is no shortage of fields with severe weed pressure. Roundup should be applied at 1 quart per acre two weeks before planting, and Gramoxone should be applied at 1 quart per acre (plus surfactant) at planting.

A grower familiar with reduced tillage in other crops can quickly develop a successful no-till cotton program for his or her farm. Growers unfamiliar with cotton or no-till should begin with a small area and should not commit to large acreages until they have developed a successful program for their farm.

Absolute no-till can be more difficult, since cultivation is an integral part of conventional cotton. This makes reduced tillage alternatives such as ridge till, stale seedbed and no-till cultivators popular.

Tips for successful cotton weed control

  • Cotton should not be planted into extremely weedy fields.
  • Cotton grows slowly, weeds grow fast. Weeds cannot be allowed to grow taller than cotton.
  • Use a solid soil-applied herbicide program with fluometuron and a grass/small-seeded broadleaf herbicide (Trifluralin, Prowl or Dual).
  • Apply Command or Zorial if velvetleaf, prickly sida or spurred anoda is present.
  • Plan to apply two post-emergence directed applications; adjust as needed.
  • Scout carefully, cultivate often.
  • Read the herbicide label.
MU does not warrant herbicide performance. Herbicides may perform differently under differing environmental conditions. Trade names may be used to help audiences recognize commonly used materials. The use of a trade name does not constitute recommendation of one product over other (generic) products of similar nature.