Corn: Introduction

Table of Contents

1. Introduction
2. Seed Corn Recommendations
3. Special Notes for Field and Sweet Corn
4. Atrazine and Simazine Soil Residues
5. Herbicide Application Timings
6. Other Herbicide Recommendations for Corn
   • Burndown
   • Soil Applied
   • Postemergence
7. Related Links

Introduction

• Critical Stage: The Critical Stage to control weeds in corn is the 2-8 leaf over stage (3-10 leaf tips).
• Apply all treatments in 150-300 L/ha (60-120 L/ac) water unless otherwise specified.
• Any single method of weed control or the continuous use of the same chemicals can lead to the build-up of weeds resistant or tolerant to that control method. Triazine-resistant lamb's-quarters and pigweed are problematic due to continuous corn and repeated use of triazine herbicides. Rotating to other crops and/or other control methods reduces the chance of new or unique weed infestations.
• To control small annual weed seedlings, blind harrow with a set of light harrows at a shallow depth before the corn has emerged, or use a weeder harrow (with L-shaped flexible tines) when the crop is 5-10 cm high. High speed (10 kph), shallow (2.5-3 cm) cultivation with the rotary hoe when corn is 7-8 cm high also helps control small weed seedlings. These techniques will not reduce herbicide action and may in some years enhance chemical weed control. Inter-row cultivation can be used to complement other weed control measures. Row cultivation is most effective when weeds are small. Shallow cultivation will reduce:
  • germination of new weed seeds
  • moisture loss
  • corn root injury.
• Inter-row cultivation may be required when weeds escape herbicide treatment; consider weeds escapes when they are 5-7 cm high.
• Band treatment of chemical over the row reduces cost by one-half to two-thirds, depending on the row spacing and width of band. Shallow inter-row cultivation will be required to control weeds between the bands.
• Cultivation gives some control of established perennial weeds but may also help to spread them to previously uninfested areas. Machinery sanitation is important when moving from one field to another. Many perennials (i.e. nutsedge) can be spread on tillage equipment. Custom operators should be particularly careful when moving from one farm to another.
• Nitrogen solution can be used as a carrier, instead of water, for preplant and preemergence application of some herbicides. Weed control activity is not increased. Spray before crop emergence. Consult the herbicide label for proper methods of application and use of dispersing agents. Calibrate the sprayer to apply the required amount of nitrogen. Use stainless steel flood jet nozzles of adequate size (e.g. Teejet TK SS 5 to TK SS 10). Nitrogen solution is mildly corrosive, especially to brass; clean the sprayer immediately after use. Unite may be used to improve liquid fertilizer herbicide compatibility and stability when a simultaneous application of a liquid fertilizer and liquid or wettable powder herbicide is desired. Because formulations and rates vary, it is essential to read the label to determine the exact amount and method to be used.
• Do NOT apply nitrogen solution with postemergence herbicides.
• Sweet corn most frequently is grown in areas where the crops in sequence are very sensitive to atrazine residues. Avoid using atrazine rates above 1.1 kg/ha (active) (2.3 L/ha (0.92 L/ac) of 480 g/L) or 1.2 kg/ha (0.48 kg/ac) of (90 WG) product. (See notes on atrazine residues for suggestions).

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Seed Corn Recommendations

Some field corn recommendations are applicable to seed corn, however, there are inbreds, which are susceptible to some chemicals. Check with the contracting company before applying any herbicide. For information on specific weeds see the Table 9-1, Corn (Field and Sweet) Herbicide Weed Control Ratings Table (PDF - 214kb) and then refer to the appropriate section for details about herbicide treatment.

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Special Notes for Corn, Field and Sweet

Precautions: Do NOT use 2,4-D, MCPA, MCPB, 2,4-DB or dicamba later than 2 weeks prior to the first appearance of tassels or ear silk. Use extreme care when applying these herbicides near susceptible crops because of possible herbicide movement. Soybeans, tomatoes and tobacco are extremely sensitive to dicamba and injury symptoms may persist for several weeks. Do NOT use dicamba in the area of susceptible crops when temperatures exceed 25°C on the day of application or if high humidity is expected, due to the possibility of dicamba volatilizing and injuring susceptible crops nearby. Leave several rows of corn unsprayed when adjacent to soybean fields or other susceptible crops.

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Atrazine and Simazine Soil Residues

Atrazine and simazine residues may last for more than one year, particularly if high rates are used more than once and dry weather occurs. If atrazine or simazine is used year after year as in a continuous corn program, triazine residues may be higher. Atrazine when used at rates from 1.2-1.5 kg/ha (active) on corn generally has not caused injury on succeeding crops of oats, barley, mixed grains, or soybeans, except in years where there was very little rain the previous fall which would slow down atrazine degradation in the soil. Postemergence treatments may persist longer than preemergence treatments. Variations from this generality of 1.2-1.5 kg/ha atrazine may occur across the province.

Injury has been reported on tomatoes, white beans, forage seedlings, peas, tobacco, cucumbers, onions, and turnips following applications of atrazine at more than 1.1 kg/ha (active) on corn the previous year.

To reduce the hazard of atrazine residues on succeeding crops:

1. Apply the early postemergence atrazine oil-water treatment with the lower rate of atrazine: 1.1. kg/ha (active). With this lower rate, a lower measure of weed control may result and cultivation may be necessary.
2. Fall plowing will reduce Triazine injury more than will spring plowing.
3. Moldboard plowing will disperse herbicide residue to a greater soil depth than chisel plowing, thus reducing the concentration of herbicide in the upper soil area.
4. Ensure that the sprayer used is adequate and is properly calibrated and adjusted. Spray uniformly without overlaps and do not spray while the sprayer is stopped.

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Herbicide Application Timings

Soil Applied Treatments include:

• Preplant (PP) - Also see Chapter 6, Special Methods: Preplant - Site Preparation for details of products, rates and remarks.
• Preplant Incorporated (PPI) - Two incorporations at right angles operating at a depth of 10 cm using a double disk (7-10 km/hr) or vibrating shank S-tine cultivator (10-13 km/hr) are required unless otherwise stated. Cultivation-type equipment used for herbicide incorporation is known to spread perennial weeds to previously uninfested areas. Pay special attention toward machinery cleanliness, and/or treating fields with perennial weeds last.
• Preemergence (PRE) - Rainfall of 15-20 mm within 10 days after application is necessary to activate preemergence treatments. Shallow cultivation, rotary hoeing or harrowing controls weed escapes and improves herbicide activity in the absence of rainfall.

Postemergence (POST) - Leaf stage of the weeds is critical for good weed control. Smaller weeds are usually more sensitive to herbicide injury. Apply according to recommended leaf stages. Crop stage is important to optimize crop safety. Adjuvants will frequently improve the weed control when used as directed. Weather or other conditions may influence the optimum rate of adjuvant, see the product label for more details. Always use appropriate drift management technology.

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Related Links

• Weeds in Ontario
• Corn in Ontario
• Problem Weed Control in Field Crops
• Integrated Pest Management (IPM)
• Pesticide Application and Safety
• Pesticide Companies and Agents
• OMAFRA Sprayer Calibration Calculator
• Search the PMRA website
• OMAFRA Publication 505: Ontario Weeds
• OMAFRA Publication 75: Guide To Weed Control

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