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Weed Management in Cotton 1

J. A. Ferrell, G. E. MacDonald, and P. Devkota 2

Successful weed control is essential for economical cotton production. Weeds compete with cotton for moisture, nutrients, and light. The greatest competition usually occurs early in the growing season up to 8 weeks after emergence. Late-season weeds, while not as competitive as early-season weeds, may interfere with insecticide applications, cause harvesting difficulties, and add seed to the soil seedbank.

Crop Rotations

Crop rotations are an important part of a good cotton weed control program. Certain weeds, especially nutsedges, may be less difficult to control in a preceding crop such as peanut. Other benefits of crop rotation may include reduction in insect, disease, and nematode problems both in cotton and succeeding crops.

Cultivation

Cultivation can be utilized if effective weed control is not achieved with herbicides. However, if weeds have been controlled with herbicides, there is generally little benefit from cultivation. If cultivation is needed, avoid throwing soil around small cotton plants to minimize disease problems.

Herbicides

Herbicides are the most effective means of controlling weeds in cotton. However, herbicides need to be appropriate for the weed problem in order to be effective. Therefore, before purchasing an herbicide, you should first scout the field, identify the weed, and assess the severity of the weed problem. Once these have been determined, Tables 4, 5, and 6 can help in herbicide selection for your particular situation/production system.

Preplant and/or preemergence (PRE) applications are important to ensure that the cotton has the initial competitive advantage over the weeds. Postemergence (POST) directed applications can then be utilized to extend the weed control throughout the season. The herbicides listed in Table 1 are those that have performed well in trials conducted at the UF/IFAS West Florida Research and Education Center (Jay), the Gainesville area, and the UF/IFAS North Florida Research and Education Center (Quincy and Marianna).

Herbicide Application

Calibrate spray equipment accurately. Rates too high may injure the crop, and rates too low may not provide adequate weed control. This is especially critical with banded applications, which are often utilized in cotton. Follow all label instructions and precautions carefully. Make sure that the application is properly timed in relation to the crop and weed growth stages. Do not allow spray to drift to sensitive crops. Store herbicides behind locked doors in their original containers with intact labels, and keep them separated from seed, fertilizer, and other pesticides.

It is very important to have a clean spray system prior to spraying any crop. After each use, rinse the system thoroughly and use a tank cleaner solution to ensure that no herbicide residue remains. See EDIS document SS-AGR-102, Calibration of Herbicide Applicators (https://edis.ifas.ufl.edu/wg013), for more information on cleaning spray equipment. If products containing dicamba or 2,4-D herbicides are applied, follow the proper sprayer clean-out procedure listed on the product label. Failure to clean spray equipment properly can result in significant damage to the sensitive crops in subsequent spray application.

Hooded Sprayers

Use of nonselective herbicides applied with a hooded sprayer may be desirable for row middles. Care should be taken to avoid contact with the crop. A residual-type herbicide may be used and will extend weed control, which may delay layby herbicide applications.

Herbicide-Tolerant Cotton

Liberty Link

Liberty Link cotton is a technology that allows the application of Liberty herbicide, glufosinate, over the top of green foliage. Glufosinate is completely different from glyphosate. Glufosinate inhibits glutamine synthase enzymes, and toxic levels of ammonia quickly accumulate in plant tissues. This process is radically different from Roundup Ready technology. Applications of Liberty to Roundup Ready cotton will result in death of the crop. Likewise, glyphosate cannot be safely applied to Liberty Link cotton.

Application Timing

The Liberty Link cotton allows an effective broadleaf herbicide to be applied postemergence over the top of cotton. Liberty 280 can be applied to Liberty Link cotton from emergence until early bloom. A spray volume of 15 gallons/A or more with an operating pressure of 40 psi or greater is recommended for use with flat fan nozzles. It may be applied postemergence without visible crop injury, yield reduction, or delay in maturity. No more than 80 oz/A can be applied per growing season. Liberty Link cotton cannot legally be grown south of Tampa, Florida (Route 60).

Herbicide Program

Liberty has no soil residual activity and only controls weeds that have emerged at the time of application. A traditional preplant-incorporated or preemergence herbicide, such as Prowl, Treflan, or Cotoran, may be needed to enhance the level of weed management. The suggested use rate for Liberty 280 is 23–43 oz/A, depending on weed size and species. No more than 43 oz per application, or 87 oz per season, may be applied to cotton. See the label for weed size and rate information. Liberty may be applied from emergence to early bloom. Cultivation should be delayed 5 to 7 days following application. Liberty is rainfast in 4 hours. Therefore, if rain is imminent, applications of Liberty should be delayed. Ammonium sulfate at 3 lb/A has been shown to enhance the activity of Liberty (Table 2).

Liberty provides fair to good grass activity, so applications should target grasses that are 3–6 in high. Postemergence grass control can be supplemented by several postemergence grass herbicides, but these should not be tank-mixed with Liberty 280. For other difficult-to-control weeds, such as pigweeds, Liberty may be tank-mixed with Staple.

*NOTE: Liberty Link cotton use is limited to the area north of Tampa (Florida Route 60). Also, Liberty will kill Roundup Ready cotton, and vice versa.

Rotation Restrictions

Liberty requires a 120-day restriction for all rotational crops, except for wheat, barley, sorghum, and other crops listed on the label.

Roundup Ready Cotton

Roundup Ready cotton has been genetically modified to tolerate glyphosate. There are several products containing glyphosate, but all are not labeled for use on Roundup Ready cotton. Consult your local UF/IFAS Extension agent or the labels; use only those glyphosate products that are labeled for use on Roundup Ready cotton. Glyphosate provides control of broadleaf weeds, grasses, and nutsedges. Proper application timing must be used to avoid crop injury.

Application Timing

Proper glyphosate application timing is essential to avoid injury in Roundup Ready cotton. Glyphosate can be applied over the top of Roundup Ready cotton anytime from cotton emergence until the fourth true-leaf stage. Applications of glyphosate over the top of cotton are prohibited after the cotton exceeds the four-leaf stage, except in salvage situations. These late (after four-leaf) over-top applications will result in bloom abortion and loss of yield. For Roundup Ready Flex cotton, glyphosate can be applied from cotton emergence until 7 days prior to harvest.

Roundup can be applied post-directed until layby. Consult the label for the suggested use rate. Glyphosate can be applied twice over the top of Roundup Ready cotton when the following criteria are met. The two applications must be at least 10 days apart. There must be two nodes of new growth between the applications. Lastly, the second application must be made before cotton exceeds the four-leaf stage. Therefore, if you are going to use two over-the-top applications, you must make the first application prior to the second true leaf, and the second application during the fourth true leaf stage.

Directed glyphosate applications must have minimal contact with the cotton plant when applied after the four-leaf stage. Glyphosate can be applied late-season after 20% of the bolls have cracked. At least 7 days between application and harvest are necessary.

Herbicide Program

Preemergence—An advantage to the Roundup Ready system is the option to eliminate preemergence herbicides. Early-season weed competition should be considered when determining the weed management program. This competition may be avoided by an early glyphosate application (i.e., one-leaf stage). If a preemergence herbicide is used, the glyphosate application can be delayed until the three- or four-leaf stage. Consideration should be given to total cotton acreage, weed spectrum, and acceptable risk level when weather delays postemergence applications. Preemergence herbicides are a type of insurance policy and often maintain a grower's peace of mind. A soil-applied preemergence herbicide should be utilized in the program to provide early-season control of annual grasses and Florida pusley (Table 3).

Tank Mixes

Glyphosate does not provide any soil or residual activity. Therefore, tank mixtures can be utilized for broad-spectrum weed control and/or residual activity. A tank mix of Staple and glyphosate improves control of hemp sesbania, spreading dayflower, and morning glory species (all except tall). A premix of this combination is available under the trade name Staple Plus.

Tables 4, 5, and 6 can be helpful in choosing the herbicide that is best suited for your particular situation.

XtendFlex Cotton

XtendFlex cotton has been genetically modified to tolerate dicamba herbicide. There are several dicamba products, but only newer, low-volatility dicamba-containing products (XtendiMax, Engenia, Fexapan) are labeled for XtendFlex cotton. Other dicamba products (Clarity, Banvel, etc.) are not labeled for use in XtendFlex cotton. Consult your county's UF/IFAS Extension agent and read and follow label instructions carefully to use the appropriate dicamba product in the XtendFlex system. There are several requirements to consider for dicamba application in XtendFlex cotton. These requirements include completion of dicamba training, proper spray record keeping, downwind buffer, application rate, application time, wind speed, nozzles, spray volume, tank-mix partners, spray adjuvants, ground speed, boom height, and spray equipment clean-out. Dicamba herbicide is effective only on broadleaf weed species (less than 4 inches tall) and does not provide control of grass weeds.

Application Timing

Dicamba herbicide products' application window is no later than 60 days after planting or mid-bloom, whichever comes first. Apply dicamba herbicide products at a wind speed between 3 to 10 mph and a minimum of 15 gallons per acre (GPA). Do not exceed a boom height of 24 inches, and do not exceed a ground speed of 15 mph. Do not use ammonium sulfate as a water conditioning adjuvant. Apply one hour after sunrise until two hours before sunset. Do not apply during temperature inversion conditions (e.g., when visible dew and low-lying fog are present in the field). Do not aerially apply Enlist herbicide products.

Enlist Cotton

Enlist cotton has been genetically modified to tolerate 2,4-D herbicide. Only 2,4-D choline herbicide-containing products (Enlist Duo and Enlist One) are labeled for Enlist cotton. Other 2,4-D products (2,4-D Amine, 2,4-D Ester, LV4, etc.) are not labeled for use in Enlist cotton. Consult your county's UF/IFAS Extension agent and read and follow label instructions carefully for appropriate 2,4-D products in Enlist cotton. There are several requirements to consider for 2,4-D application in Enlist cotton. These include application timing/window, tank-mix partners, nozzles, sprayer contamination, wind speed, environmental condition, buffer area, spray volume, spray pressure, herbicide rate, boom height, and spray equipment clean-out. 2,4-D herbicide alone is effective only on broadleaf weed species (less than 4 inches tall) and does not provide control of grass weeds.

Application Timing

Enlist herbicides' application window is after cotton emergence but no later than full flowering or mid-bloom stage. Wind speed between 3 and 10 mph and spray volume of 10–15 GPA are most desirable for Enlist herbicide application. Do not apply at a wind speed greater than 15 mph. Do not apply during conditions of temperature inversion (inversions are more frequent between dusk and dawn). Do not aerially apply Enlist herbicide products.

Tables 4, 5, and 6 can help you choose the most suitable herbicide for your particular situation.

Tables

Table 1. 

Weed management in non-transgenic cotton.

Table 2. 

Herbicide program for Liberty Link cotton.

Table 3. 

Herbicide program for Roundup Ready cotton.

Table 4. 

Estimated effectiveness of recommended herbicides on common weeds in Florida cotton.1

Table 5. 

Estimated effectiveness of recommended herbicides on common weeds in Florida cotton (continued).1

Table 6. 

Estimated effectiveness of recommended herbicides on common weeds in Florida cotton (continued).1

Footnotes

1. This document is SS-AGR-04, one of a series of the Agronomy Department, UF/IFAS Extension. Original publication date January 2000. Revised April 2020. Visit the EDIS website at https://edis.ifas.ufl.edu for the currently supported version of this publication.
2. J. A. Ferrell, professor, Agronomy Department; G. E. MacDonald, professor, Agronomy Department; and P. Devkota, assistant professor, Agronomy Department, UF/IFAS West Florida Research and Education Center; UF/IFAS Extension, Gainesville, FL 32611. Original written by J. A. Tredaway and B. J. Brecke; revised by P. Devkota.

The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication do not signify our approval to the exclusion of other products of suitable composition. All chemicals should be used in accordance with directions on the manufacturer's label. Use herbicides safely. Read and follow directions on the manufacturer's label.