Weed Control

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 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 or before the crop emerges, which is especially useful for the Stale Seedbed Technique (see the Weed Management section for more information). These herbicides are also used to kill the cover crop in a minimum- or no-till system.

• glyphosate (Roundup Power Max): PHI 7d, REI 4h, Group 9.
• pelargonic acid (Scythe): PHI 1d, REI 12h, Group 0.
• paraquat (Gramoxone SL 2.0*): REI 48h, Group 22. *Restricted use: Applicators must complete an EPA-approved paraquat training every 3 years: https://www.epa.gov/pesticide-worker-safety/paraquat-dichloride-training-certified-applicators.

Preemergent Herbicides (before weeds germinate)

HRAC Group 3:

• pendimethalin (Prowl H2O): 2 to 4 pt/A. REI 24h. Apply after planting corn seed at least 1.5" deep, but before weeds and crop emerges, or after corn emerges until it is 20-24” tall or at the V8 growth stage. Use drop nozzles to direct spray to the soil surface if applying after crop emergence. Do not apply in minimum- or no-till systems.

HRAC Group 5:

• simazine (Princep 4L): Up to 2 qt/A. PHI 45d for sweet corn, 60d for popcorn, REI 12h. 1 application per year. Apply preplant incorporated or to the soil surface immediately after planting before corn emerges. Does not control annual grasses. Do not apply on highly erodible soils unless there is at least 30% plant residue cover.

HRAC Group 14:

• saflufenacil (Sharpen): 2 fl oz/A. REI 12h. Sweet corn for processing only and popcorn. Do not exceed 6 fl oz/A per year. Apply preplant incorporated up to 14 days before planting, or immediately after planting before corn emerges. 

HRAC Group 15:

• acetochlor (Surpass EC): 1.5 to 3.75 pt/A. REI 12h. Do not exceed 3.57 pt/A per year. Do not apply to sandy soils or within 50’ of any well where the depth to groundwater is 30’ or less. Apply as close as possible to planting or after planting and incorporate 0.25” into the soil with water or cultivation. If applied more than 14 days before planting, incorporate 2” into the soil. Do not apply after sweet corn has emerged. Delay cultivation as long as possible after applying.
• dimethenamid (Outlook): 8 to 24 fl oz/A. PHI 50d, REI 12h. Do not exceed 24 fl oz/A per year. Can be split-applied. Apply up to 45 days before planting and until corn is 12” tall or at layby for popcorn.
• pyroxasulfone (Zidua): 1 to 4 fl oz/A. PHI 37d, REI 12h. Do not exceed 2.75 to 5 fl oz/A per year. Apply after planting until the V4 stage, to corn planted at least 1" into the soil.
• s-metolachlor (Dual Magnum): 1.33 to 2.6 pt/A. PHI 30d, REI 24h. Do not exceed 3.9 pt/A per year. Make beds before applying if planting into beds. Apply before planting until corn is 40” tall. When corn is more than 5” tall, direct spray towards the base of plants.

Pre- and Postemergent Herbicides

HRAC Group 2:

• halosulfuron-methyl (Sandea): 0.67 to 1 oz/A. PHI 30d, REI 12h. Do not exceed 2 oz/A per year. Apply 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 applied only with directed drop nozzles to avoid application into the corn plant whorl. Do not use on “Jubilee” sweet corn.

HRAC Group 5:

• atrazine (Aatrex 4L): 2.4 to 4 pt/A. REI 12h. It is recommended to only use 2 pt/A in Massachusetts. Do not exceed 5 pt/A per year. Apply before planting until corn is 12” high, as the primary broadleaf herbicide treatment instead of an application at planting or as an emergency treatment if the soil-applied broadleaf herbicide treatment fails. On highly erodible soils with less than 30% residue cover at planting, use a maximum of 3.2 pt/A. 

HRAC Group 27:

• mesotrione (Callisto): 3 to 7.7 fl oz/A. PHI 45d, REI 12h. Do not exceed 7.7 fl oz/A per application. Can make 1 application before corn emergence and 1 after emergence, or 2 applications after corn emergence when corn is less than 30” tall or at the 8-leaf stage. Applying after crop emergence may cause crop bleaching in some sweet corn varieties, but should not affect yield. Adjuvant restrictions apply to sweet corn applications. Do not apply to white popcorn varieties. Severe corn injury resulting in yield loss may occur if any organophosphate or carbamate insecticide is applied foliar postemergence within 7 days before or after Callisto application.

Postemergent Herbicides (after weeds germinate)

HRAC Group 0:

• pelargonic acid (Scythe): 3 to 10% solution. PHI 1d, REI 12h. Apply preplant, as a directed and shielded spray during crop growth, or before emergence. Contact with crop will cause damage.

HRAC Group 1:

• sethoxydim (Poast): 1.5 pt/A. PHI 30d, REI 12h. Use only with Poast Protected corn hybrids. Do not exceed 3 pt/A per year. Apply to actively growing grasses, until the onset of pollen shed. Do not apply after pollination. BASF has stopped production and will discontinue the registration at the end of 2027.

HRAC Group 2:

• nicosulfuron (Accent Q): 0.45 to 1.8 oz/A. PHI 30d, REI 4h. Not for use in NH. 1 application per year. Apply broadcast before corn is 12” tall to actively growing grasses. If corn is 12-18" tall, apply with drop nozzles. Do not apply if corn is greater than 18". Do not apply to white popcorn varieties. Check label for specific precautions related to previous insecticide applications. Do not use if the crop exhibits herbicide injury from previous applications. 

HRAC Group 4:

• 2,4-D (Amine 4): 1 to 2 pt/A. PHI 45d (sweet corn), 7d (popcorn), REI 48h. Do not exceed 3.15 pt/A per year. Apply once 7 to 14 days before planting or after planting, before corn emergence and once after corn emergence when corn is less than 8” tall or with drop nozzles if greater than 8” tall through the dough/denting stage. Do not use on sandy soil. Can apply to popcorn in the dough stage. Do not cultivate for at least 7-10 days after application to corn that has emerged or stem breakage may occur. Use only the amine formulation of 2,4-D and avoid spray drift to desirable broadleaf crops.
• clopyralid (Stinger): 0.33 to 0.67 pt/A. PHI 30d, REI 12h. Do not exceed 0.67 pt/A per year. Apply to actively growing weeds. Apply from corn emergence until it is 18” (sweet corn) or 24” (popcorn) tall. To target Canada thistle, apply when the majority of thistle plants have emerged (rosette to 8") and do not cultivate for 14-20 days before or after application.
• clopyralid (Clean Slate): 0.33 to 0.67 pt/A. PHI 30d, REI 12h. Do not exceed 0.67 pt/A per year. Apply to actively growing weeds. Apply from corn emergence until it is 18” (sweet corn) or 24” (popcorn) tall. To target Canada thistle, apply when the majority of thistle plants have emerged (rosette to 8") and do not cultivate for 14-20 days before or after application.

HRAC Group 5:

• ametryn (Evik DF): 2 lb/A. REI 12h. Popcorn only. 1 application per year. Apply as a directed spray to the weeds after the shortest corn is at least 12” tall.

HRAC Group 6:

• bentazon (Basagran): 0.8 to 1.6 pt/A. REI 48h. Do not exceed 3.2 pt/A per year. Apply when corn has 1-5 leaves. An effective treatment in an emergency situation to control certain broadleaf weeds and fairly effective against 4- to 6”-tall yellow nutsedge. Spray drift injury risk is less than that of 2,4-D.

HRAC Group 9:

• glyphosate (Roundup Power Max): 15 fl oz to 3.1 qts/A. PHI 7d, REI 4h. Do not exceed 5 qt/A per year. Apply 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. Spray contact can cause plant death in non-Roundup Ready varieties. Spray additive recommendation varies with formulation–see label. To target quackgrass, apply after corn harvest to actively growing quackgrass 6-8" tall. Wait at least 5 days after application before plowing. Most effective on bindweed and milkweed at or after bloom.

HRAC Group 14:

• carfentrazone (Aim EC): 0.5 to 1 fl oz/A. PHI 14 leaf collars, REI 12h. Do not exceed 2 fl oz/A per year. Apply as a pre-plant burndown up to 1 day after seeding until corn is at the 14 leaf-collar stage. Use drop nozzles with higher rates. Foliar speckling often occurs after application.
• fluthiacet-methyl (Cadet): 0.6 to 0.9 fl oz/A. PHI 40d, REI 12h. Sweet corn for processing only and popcorn. Do not exceed 1.25 fl oz/A per season. Apply as a preplant burndown or after corn emerges until corn is 48” tall or tasseling occurs. Do not cultivate within 2 days before or after applying. Do not apply to severely stressed or wet crop.

HRAC Group 22:

• paraquat (Gramozone SL 3.0*): 0.7 to 1.3 pt/A. REI 48h. Do not exceed 8 pt/A per year. Apply in a banded over-row or broadcast application before corn emergence, or with a hooded or shielded sprayer after emergence. Can apply without a directed sprayer when corn is at least 10" tall and nozzles spray no higher than 1/3 of the corn stalk. Spray contact can cause crop death. May be fatal if swallowed or inhaled. *Restricted use: Applicators must complete an EPA-approved paraquat training every 3 years: https://www.epa.gov/pesticide-worker-safety/paraquat-dichloride-training-certified-applicators.

HRAC Group 27:

• tembotrione (Laudis): 3 fl oz/A. REI 12h. 1 application per year for sweet corn (3 fl oz/A per year), up to 2 applications per year for popcorn (6 fl oz/A per year). Apply after corn emergence until the V7 (sweet corn) or V8 (popcorn) stage, or as a postharvest burndown. Do not use on “Merit” or “Shogun” sweet corn.
• tolpyralate (ShieldEx 400 SC): 1 to 1.35 fl oz/A. PHI 35d for sweet corn, 45d for popcorn, REI 12h. Do not exceed 2.7 fl oz/A per year. Apply when corn is emerged up to the V6 stage or 20” tall.
• topramezone (Impact): 0.75-2 fl oz/A. PHI 45d, REI 12h. Do not exceed 2 fl oz/A per year. Apply after corn has emerged up until the V8 stage. Crops under stress may show transient bleaching.

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 to expand the spectrum of control (e.g., 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 2 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.