Weed Control

NOTES: For the herbicides listed below, one product trade name and formulation is provided for each active ingredient along with preharvest interval (PHI), restricted entry interval (REI), resistance management group number, and example of rates and special instructions. In many cases there are other products available with the same active ingredient. However, not all products with the same active ingredient are registered for use in a crop. Always check the product label to be sure that the crop is listed before using. 

Delayed Herbicide Applications

Growers using atrazine (Aatrex) and or mesotrione (Callisto) for broadleaf weed control and either metolachlor (Dual) or pyroxasulfone (Zidua) for grass control in corn should consider delayed applications in their earliest plantings. Reasons for delaying herbicide applications in the earliest sweet corn plantings include:

1. Corn is most tolerant to both Dual and Zidua after emergence. Supersweet varieties and some "se" sweet corns are easily injured by these herbicides when the soil is below 60°F. Both Dual and Zidua as well as atrazine and Callisto can be applied at any time as long as the corn is less than 5" high. Bicep Lite II, a very commonly used prepack of atrazine and Dual, can also be used this way as can Lumax, a prepack of metolachlor, atrazine, and mesotrione.
2. If the first few plantings of sweet corn are sprayed on the same day, the amount of cleaning and rinsing of the spray tank is reduced since it does not need to be cleaned between applications when the same herbicide is used.
3. Delaying herbicide applications on the earliest plantings will also extend the activity of the herbicide later into the season. This is important in maintaining good weed control throughout the season to minimize weed seed production. Growers often rely on sweet corn rotations to reduce weed seed populations in the soil.

Growers should consider waiting until their first planting of sweet corn is 3"-5" high to apply herbicide. At this time all fields of sweet corn planted to-date can be treated. Each field sprayed will be at a different stage of growth. After that, each field should be sprayed soon after it is planted, since soils will be warmer and grasses are more likely to emerge soon after seeding the crop.

A possible problem with delayed applications involves the potential for poor control of grasses. Dual, Zidua, Bicep (atrazine + Dual), or Lumax (atrazine + Dual + Callisto) must be applied before grasses emerge. If grasses are not controlled in corn, yield reductions will likely occur. Also, as the soil warms up, grasses are likely to emerge soon after seeding.

Reduced Herbicide Rates for Corn Weed Management 

Be sure to follow the herbicide rates recommended in this guide. Only 1 lb active ingredient of atrazine is recommended for sweet corn in New England. This is well below the rate on the label and constitutes best management practices for groundwater protection. This rate could be reduced further, although the grower should be prepared to make a second application of atrazine if any weeds escape. Rates for Dual and Lasso should be selected based on soil type. Follow the label to determine the correct rate. Reducing the rate of Dual or Zidua is risky since it is very difficult to control grasses in sweet corn once they emerge.

Special Atrazine Precautions

The Environmental Protection Agency and Syngenta have revised the label uses of all atrazine products to reflect the potential for surface and groundwater contamination. Be sure to read and follow all directions and restrictions listed on the label. The preceding section on Reduced Herbicide Rates for Corn Weed Management reflects these concerns. All of the rates listed in this guide for atrazine are within the guidelines of the label. These include uses for single applications (applied before crop emergence or early postemergence) and sequential applications (applied both at planting and postemergence). Please read all labels carefully.

Stale Seedbed

The following herbicides are nonselective and are used to control weeds which are present in a field prior to planting the sweet corn or before the sweet corn emerges (see Stale Seedbed Technique in the Weed Management section). If a grower is using "no-till" or "minimum tillage," these herbicides are also used to kill the cover crop that may be present in the field.

• glyphosate (Roundup Power Max): REI 12h, Group 9. Apply to emerged annual or perennial weeds prior to crop emergence. Do not feed crop residue to livestock for 8 weeks following treatment. Consult the manufacturer's label for specific weeds and rates. May be tank mixed with atrazine, simazine or alachlor.
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• paraquat (Gramoxone SL 2.0*): restricted use. REI 12h, Group 22. Use 2 – 4 pts/A.  Apply in 20 to 60 gallons spray mix to emerged weeds. Field should be prepared several days ahead of treatment to allow maximum weed emergence. Use a nonionic surfactant at a rate of 16 to 32 oz per 100 gal spray mix. May be tank mixed with atrazine or simazine preemergence. Can also be applied as a preemergence. Check label for directions. May be fatal if swallowed or inhaled. Applicators must complete an EPA-approved paraquat training listed on the following website https://www.epa.gov/pesticide-worker-safety/paraquat-dichloride-training-certified-applicators(link is external). The training must be completed a minimum of every three years.
• pelargonic acid (Scythe): PHI 1d, REI 12h, Group 17. Use a 3 -10% solution (3 to 10 gallons per 100 gallons).

Preemergent Herbicides (before weeds germinate)

• acetochlor (Surpass): REI 12h, Group 15. Apply 1.5-3.17 pt/A per application. Do not exceed 1.5-3.17 pt/A per year. Rates based on soil texture and tillage system used (conventional, reduced, or no-till). Do not apply to sandy soils or within 50’ of any well where the depth to groundwater is 30’ or less. Apply in a minimum of 10 gal/A of spray. Can be applied to the soil surface up to 30 days before planting, but for best results, apply as close as possible to planting and prior to weed emergence. Do not apply after sweet corn has emerged. Cultivation should be delayed as long as possible after applying or the herbicide may be diluted. Can also be applied to the soil surface and incorporated into the top 2” of soil anytime up to 14 days before planting. Poor soil tilth or excessive crop residue may reduce control. Do not move soil after incorporation. Can also incorporate into the soil with 0.25” of water within 7 days of application before or after planting corn, as long as corn has not emerged.
• dimethenamid (Outlook): PHI 50d, REI 12h, Group 15. Apply 8-24 oz/A per application, up to 2 appliations allowed per year, a minimum of 14 days apart. Dot not exceed 24 oz/A per year. If applying a split application, use 8-16 oz/A for the first application and the remaining 8-16 oz/A for the second application. Rate based on soil texture and organic matter. Can be applied before up to 45 days before planting, after planting but before emergence, or after crop emergence until corn is 12” tall. If applying 15-45 days before planting, use a higher rate and apply as part of a split-application program where the second application is made after planting. If applying 2 weeks before planting or after planting but before crop emergence, incorporate 1-2 “ into the soil with shallow cultivation or water through irrigation or rainfall.
• pendimethalin (Prowl H2O): REI 24h, Group 3. Apply 2-4 pt/A per application, only 1 application per year. Rate based on soil texture. Can be applied after planting but before weeds and crop emerges, or after corn emerges until it is 20-24” tall or at the V8 growth stage. Use crop nozzles to direct spray to the soil surface if applying after crop emergence. When applying after planting but before emergence, there is potential for crop injury if herbicide is moved into the seed zone. Minimize the potential for crop injury by preparing a firm seedbed and planting corn seed at least 1.5" deep and covered with soil. May cause crop injury if heavy rains follow an application to dry soil.
• pyroxasulfone (Zidua): PHI 37d, REI 12h, Group 15. Apply 1-4 oz/A per application, multiple applications allowed per year. Do not exceed 2.75-5 oz/A per year. Rate based on soil texture. Can be applied before planting, after planting but before corn emerges, or early after corn has emerged. Plant corn seed at least 1" into the soil. If applying from 14 days before planting up until the V4 stage, apply broadcast and do not incorporate. Do not apply earlier than 14 days before planting for sweet corn. For popcorn, can be applied in the fall or winter to manage winter annual weeds, up until 14 days before planting, and in that case, incorporate less than 2” into the soil with shallow tillage.
• s-metolachlor (Dual Magnum): PHI 30d, REI 24h, Group 15. Apply 1.33-2.6 pts/A per application, multiple applications allowed per year, a minimum of 14 days apart. Do not exceed 3.9 pts/A per year. Rate based on application timing, soil texture, and organic matter content. Can be applied before planting, after planting but before crop emergence, or at lay-by. To target glyphosate resistant Italian ryegrass, apply from Sept. 1- Dec. 1 after harvest to the soil surface. Do not apply to frozen ground. The combined applications in the fall plus spring must not exceed the maximum allowed annual rate. Only 1 fall application per crop cycle. Can apply in the early spring between 14-30 days before planting, or after planting but before crop emerges. Do not till deeper than 2-3” after application. If applying within 14 days of planting, only incorporate 2” into the soil. Incorporate if dry weather is expected. Reduce soil movement after application for improved weed control, apply and incorporate after bed formation if planting into beds. Can also be applied after corn has emerged until it is 40” tall. For best results, direct towards the base of corn plants, when they are more than 5” tall.
• simazine (Princep): PHI 45d, REI 12h, Group 5. Apply up to 2 qt/A preplant incorporated or to the soil surface immediately after planting before corn emerges. Use alone only if annual grasses are not a problem. Do not apply on highly erodible soils unless there is at least 30% plant residue cover.

Pre- and Postemergent Herbicides

• 2,4-D (Amine 4): PHI 45d, REI 48h, Group 4. Apply 1-2 pt/A per application, multiple applications allowed per year, a minimum of 21 days apart. Do not exceed 3.15 pts/A per year. Can make 1 application before planting or before corn emergence. Can make 1 application after corn emergence. Do not till for at least 7-10 days after application. Do not use on sand soil or unacceptable crop injury may occur. Plant corn a minimum of 7 days after application at 1 pt/A and a minimum of 14 days after application at 2 pts/A. In reduced tillage systems, apply after corn is planted but before it has emerged. The seed furrow must be completely closed at application. Can also apply when corn is less than 8” tall, as long as leaves have not just unfolded. Use drop nozzles to direct spray toward base of corn if it is over 8" tall. Do not apply from 7-10 days before tasseling through dough stage. Do not apply to open whorls. Reduce rate of 2,4-D if temperature is hot and soil is wet. Use only the amine formulation of 2,4-D. Do not use any ester or low-volatile ester formulations. Use all precautions to avoid spray drift to desirable broadleaf crops.
• atrazine (Aatrex 4L): REI 12h, Group 5. Apply 2.4-4 pts/A per application, up to 2 applications allowed per year, 1 before corn emergence and 1 after. Do not exceed 5 pt/A per year. Can be applied before planting and incorporated into the soil less than 2 weeks before planting, or applied to the soil surface immediately after planting, or after corn emergence until it is 12” heigh. Do not move treated soil out of the row or move untreated soil to the soil surface or weed control will be diminished. If applying after corn and weeds have emerged, add either 1 qt/A of crop oil concentrate or 1 gal/A of spray oil. Apply when weeds are 1-1.5" high. This treatment can be used as the primary broadleaf herbicide treatment instead of an application at planting or it can be used as an emergency treatment if the soil-applied broadleaf herbicide treatment failed. If applied correctly, this application will control the emerged weeds and will continue to provide residual control for later-emerging broadleaf weeds. On highly erodible soils with less than 30% residue cover at planting, use a maximum of 3.2 pts/A. It is recommended to only use 2 pts/A in all of Massachusetts.
• Halosulfuron-methyl (Sandea): PHI 30d, REI 12h, Group 2. Apply 0.67-1 oz/A per application, up to 2 applications allowed per year. Do not exceed 2 oz/A per year. Can be applied over the top of corn, or with drop nozzles from the spike through layby stage of the corn. If making sequential applications, the second application should be at 0.67 oz/A, applied only with drop nozzles semi-directed or directed to avoid application into the corn plant whorl. Do not use on “Jubilee” sweet corn. Has not been tested on all varieties, crop tolerance should be verified before treating the entire field. The use of non-ionic surfactant is recommended for use on emerged weeds (1 to 2 qt/100 gal spray). Works on germinating broadleaf weed seeds, as well as emerged broadleaf weeds.
• saflufenacil (Sharpen): REI 12h, Group 14. Apply 2 oz/A per application, multiple applications allowed per year, a minimum of 14 days apart. Do not exceed 6 oz/A per year. Can be applied as a burndown herbicide, up to 14 days before planting, or after planting before crop has emerged. Do not apply after corn emergence or severe crop injury will occur. Can incorporate 1-2’’ into the soil if applying before planting. Apply at 1 oz/A before corn has emerged. No residual weed control will occur. If applying as a burndown, apply 1 oz/A with an appropriate adjuvant. Not for sweet corn grown for fresh market - processing sweet corn varieties only. Has not been tested on all varieties, crop tolerance should be verified before treating the entire field. Do not apply to course-texture soils with less than 3% organic matter. Plant corn at least 0.5” deep and ensure the seed row is closed to reduce risk of crop injury. May result in delayed crop emergence and stunting in cool wet conditions after application.
• mesotrione (Callisto): PHI 45d, REI 12h,  Group 27. Apply 3-7.7 oz/A per application, up to 2 applications allowed per year, a minimum of 14 days apart. Do not exceed 7.7 oz/A per application. Can make 1 application before corn emergence and 1 after emergence, or 2 applications after corn emergence. If applying before corn has emerged, apply 6-7.7 oz/A to the soil surface after seeding, with 10-30 gal/A of spray. If applying after crop emergence, apply when corn is less than 30” tall or at the 8-leaf stage. Apply up to 3 oz/A per application (if making 2 post-emergence applications, the maximum rate is 6 oz/A per year). Applying after crop emergence may cause crop bleaching in some sweet corn varieties, but typically has no effect on final yield. Adjuvant restrictions apply to sweet corn applications. Severe corn injury resulting in yield loss may occur if any organophosphate or carbamate insecticide is applied foliar postemergence within 7 days before or 7 days after Callisto application.

Postemergent Herbicides (after weeds germinate)

• bentazon (Basagran): REI 48h, Group 6. Apply 0.8-1.6 pts/A per application, multiple applications allowed per year. Do not exceed 3.2 pts/A per year. Rate based on weed composition. Apply to actively growing weeds when corn has 1-5 leaves and weeds are small. Do not apply to crops subjected to stress conditions such as insufficient moisture or hot or cold temperature, or crop injury may result. Do not graze corn for at least 12 days after treating. An effective treatment in an emergency situation to control certain broadleaf weeds and fairly effective against yellow nutsedge when 4-6" tall. There is less chance of adjacent crop injury from spray drift than with 2,4-D.
• carfentrazone (Aim EC): PHI 14d, 3d for harvest aid, REI 12h, Group 14. Apply 0.5-1 oz/A per application, multiple applications allowed per year. Do not exceed 2 oz/A per year. Can be applied as a preplant burndown 1 day after planting by seed up until corn is at the 14 leaf-collar stage. For best results, make application to actively growing weeds up to 4” tall and rosettes less than 3” across. Use drop nozzles to avoid spraying into the whorl and on the corn foliage. Expect to see speckling of the crop foliage after application. Initially, the injury appears to be substantial, but it is not systemic and the corn outgrows the injury rapidly. Has not been tested on all varieties, crop tolerance should be verified before treating the entire field. Weather conditions may also affect the degree of injury observed. Injury may be more severe during periods of warm, cloudy weather with high humidity and plentiful soil moisture when corn growth is rapid and soft.
• clopyralid (Stinger): PHI 30d, REI 12h, Group 4. Apply 5-11 oz/A per application, multiple applications allowed per year, a minimum of 21 days apart. Do not exceed 11 oz/A per year. Apply anytime between corn emergence until it is 18” tall in 10-20 gal/A of spray. Rate based on weed composition. Do not cultivate before or after application, if cultivation is necessary, wait 14-20 days after application. Spray additives are not recommended.
• clopyralid (Clean Slate): PHI 30d, REI 12h, Group 4. Apply 0.33-0.67 pts/A per application, only 1 application per year. Apply 0.25-0.5 pts/A of this product from weed emergence up to the 5-leaf stage of weed growth. To target Canada thistle control, apply up to 0.67 pts/A of this product as a broadcast treatment to the entire infested area when the majority of thistle plants have emerged (rosette to 8") and thistles are at least 6-8" in diameter or height up to bud stage. Cultivation can disrupt translocation to the roots of Canada thistle, so for best control, do not cultivate before or after application for 14-20 days.
• fluthiacet-methyl (Cadet): PHI 40d, REI 12h, Group 14. Apply 0.6-0.9 oz/A per application. Do not exceed 1.25 oz/A per season. Can be applied as a preplant burndown or after corn emerges until corn is 48” tall or until tasseling occurs. Do not cultivate within 2 days before or after applying. Do not irrigate or apply within 4 hours before a rainfall as it will wash off the Cadet. Does not control grasses, but can be tank mixed with many other herbicides. See label for details. Do not apply to severely stressed or wet crop.
• glyphosate (Roundup Power Max): PHI 7d, REI 12h, Group 9. Apply 15 oz to 3.1 qts/A per application, multiple applications allowed per year. Do not exceed 5 qts/A per year. Rate based on target weed species. Can be applied after crop emergence as a broadcast spray in roundup-ready varieties or with a completely enclosed hooded sprayer after the corn is 12” tall in non-roundup-ready varieties. If applying in non-roundup-ready corn varieties, avoid any contact of herbicide with crop or crop death could result. Can be applied as a spot treatment before silking of corn. Do not treat more than 10% of the total field area to be harvested. Any crop plants receiving spray in the treated area will be killed.  Can also be applied after corn harvest to actively growing quackgrass 6-8" tall. Wait at least 5 days after application before plowing. Can use as a preharvest aid (up to 60 oz/A) once kernel-fill is complete and grain moisture is 35% or less. Can be used after harvesting corn but the application must be made a minimum of 30 days prior to planting any crop not listed on the label. Glyphosate is the only postemergent herbicide that can provide excellent control of perennial weed species commonly found in corn, including quackgrass, bindweed and milkweed. Most effective on bindweed and milkweed at or after bloom.
• nicosulfuron (Accent Q): PHI 70d, REI 4h, Group 2. Not for use in New Hampshire. Apply 0.45-1.8 oz/A, only 1 application per year on sweet corn. Do not exceed 1.8 oz/A per year. Best results are obtained when broadcast applied before corn is 12” tall to young, actively growing grasses. If corn is 12"-18" tall, applications should be made with drop nozzles. Do not apply if corn is greater than 18". Has not been tested on all varieties, crop tolerance should be verified before treating the entire field. If organophosphate insecticide has been applied to corn, crop injury may occur. Do not use if the crop exhibits herbicide injury from previous applications. Applications must include either a non-ionic surfactant, crop oil concentrate, or an ammonium nitrate nitrogen fertilizer.
• paraquat (Gramozone SL 3.0): REI 12h or 24h (application type), Group 22. Apply 0.7-2.7 pts/A per application, up to 3 application before planting or corn emergence and 3 applications after corn emergence are allowed, a minimum of 7 days apart. Do not exceed 8 pts/A (preplant/preemergence), 4 pts/A (after corn emergence), or 1.3 pts/A (harvest aid). Apply in a banded over-row or broadcast application in a minimum of 10 gal/A of spray before corn emergence. Seed with a minimum amount of soil disturbance. After corn emergence, apply with a directed spray (hooded or shielded) or without a directed sprayer when corn is at least 10" tall and nozzles spray no higher than 3" of corn stalk or 1/3rd of the corn stalk if corn is over 20" tall. Rate is dependent on weed composition and application use. May be fatal if swallowed or inhaled. Applicators must complete an EPA-approved paraquat training listed on the following website https://www.epa.gov/pesticide-worker-safety/paraquat-dichloride-training.... The training must be completed a minimum of every three years
• pelargonic acid (Scythe): PHI 1d, REI 12h, Group 0. Apply a 3-10% solution (3-10 gal per 100 gal). Rate based on target weed species. Apply in 75-200 gal/A of spray or with a spot sprayer until foliage is wet but not to the point of runoff. Repeat applications as necessary. Can apply for vegetative burndown during site preparation, prior to crop emergence from soil, as a directed and shielded spray during crop growth taking care to avoid contact with foliage and green tissue of crops, or after harvest.
• sethoxydim (Poast): PHI 45d for grain and fodder, 30d for fresh, REI 12h, Group 1. Apply up to 1.5 pt/A per application, multiple applications allowed per year, a minimum 10 days apart. Do not exceed 3 pt/A per year. Use only with Poast protected sweet corn hybrids. Apply until the onset of pollen shed. Do not apply after pollination. Use with crop oil concentrate (2.0 pt/A) or methylated seed oil (1.5 pt/A).  Note that crop oil can cause injury under hot and humid conditions. Can also be used as a spot-spray by mixing 1-1.5% (1.3-1.9 oz/gal) Poast and 1% v:v crop oil concentrate (1.3 oz/gal). Spray to wet, but do not allow runoff of spray solution.
• tembotrione (Laudis): REI 12h, Group 27. Apply 3 oz/A per application, only 1 application per year for sweet corn, up to 2 applications allowed per year for popcorn, a minimum of 7 days apart. Do not exceed 3 oz/A per year for sweet corn or 6 oz/A per year for popcorn. Can be applied after corn emergence until the V7 stage for sweet corn and V8 stage for popcorn. Can also be used as a burndown application after corn harvest, but maximum use rates still apply. Add oil methylated seed oil (MSO) or concentrate (COC) to be 1% of the spray solution (1 gal/100 gals of spray solution). In addition, the label requires the addition of nitrogen liquid fertilizer (1.5 qt/A) or AMS (1.5 lb/A). Do not apply tank-mixes of Laudis and atrazine to corn greater than 12" tall. Do not use if Callisto, Lumax or Lexar was used preemergence due to herbicide resistance concerns. Most broadleaf weeds should be treated before they are 6" tall and grass weeds before they are 2" tall. Has not been tested on all varieties, crop tolerance should be verified before treating the entire field. Do not use on Merit or Shogun corn varieties.
• tolpyralate (ShieldEx 400 SC): PHI 35d, REI 12h, Group 27. Apply up to 1.35 oz/A per application, up to 2 application allowed per year, a minimum of 14 days apart. Do not exceed 2.7 fl oz/A per year. Apply when corn is emerged up to the V6 stage or up to 20” tall, whichever is more restrictive. Use an adjuvant for optimum activity. Rainfast within 1 hour after application.
• topramezone (Impact): PHI 45d, REI 12h, Group 27. Apply 0.75-2 oz/A per application, multiple applications allowed per year. Do not exceed 2 oz/A per year. Apply when weeds are actively growing, after corn has emerged up until the V8 stage. Crops under stress may show transient bleaching. These symptoms are temporary and occur infrequently. Do not use postemergence if Callisto, Lumax, or Lexar was used preemergence due to herbicide resistance concerns. Most broadleaf weeds should be treated before they are 6" tall and grass weeds before they are 2" tall. May not control species when larger than the recommended size for treatment. Rainfast within 1 hour after application.

Mixing Herbicides for Weed Control in Sweet Corn

Many corn herbicides can be tank mixed with one or more other herbicides to extend the spectrum of weed control.  Labels will have detailed instruction about compatible tank mixing partners and instructions on selecting rates, etc. when tank mixing.

Formulated Mixes (Prepackaged Tank Mixes)

In recent years, many products have come on the market as prepackaged combinations of herbicides which were often recommended in the past as tank-mix combinations of products purchased separately. They are designed for to expand the spectrum of control (i.e. grass herbicide mixed with broadleaf herbicide). 

There are 3 questions that a grower or pesticide applicator should ask when deciding whether to use a formulated mix or the individual herbicide.

1. Which is less expensive on a per acre basis?
2. Which formulation is easier to apply? Most pesticide applicators prefer to avoid wettable powders, for example.
3. Is the ratio of the two herbicides in the formulated mix appropriate for the given field situation? For example, Bicep Lite contains 2/3 as much atrazine as Bicep and should allow growers a better opportunity to obtain good grass control without using more atrazine than is needed.

Following are some examples of formulated mixes registered for use on sweet corn. There are many others in addition to those listed.