Corn: Weed Control

Excerpt from Agronomy Guide for Field Crops (Chapter 3)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Introduction
2. Critical Period for Weed Control
3. Corn Leaf Stages
   • Comparative Growth Stages - Table 3-25
4. Crop Losses Due to Weeds
   • Percent Corn Yield Loss Due to Weeds at Known Populations - Table 3-26
5. Mechanical Weed Control
6. Herbicide Resistance
7. Integrated Weed Management
8. Updates on Corn: Weed Control
9. Related links...

Introduction

The critical period to control weeds in corn is the 2- to 8-leaf over stage (3 to 10 leaf tips or V1 to V6).

Critical Period for Weed Control

Season-long weed control is not necessary to protect yield. Maintaining good weed control during the period from the 2- to 8-leaf stage has been shown in research to keep the yield loss due to weeds to less than 5%. Weeds germinating later have minimum impact on crop yields and result in relatively low production of weed seeds. The herbicide label indicates the growth stage of the weeds when optimum control can be obtained. The timing of weed control should optimize the critical period for the crop and the optimum growth stage of the weed. Weed control should occur within the labelled timing for the crop but should be timed primarily on the weed leaf stage and density. The critical period for weed control will vary within the 2- to 8-leaf stage depending on factors such as soil type, soil moisture, precipitation, weed species and density, growing conditions, etc. The timing is earlier within the window for conditions that result in greater stress potential to the crop. For example, weed control in corn grown on coarse-textured soils with high weed densities and low soil moisture will require weed control to occur at an earlier stage within the critical weed-free period.

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Corn Leaf Stages

Counting the leaves on a corn plant sounds like an easy task, but there are a few complications that can cause one to miscount. It is important when reading pesticide labels or other information to know which leaf counting method is being referred to.

There are several methods of counting leaves:

• One method counts all leaves including any leaf tip that has emerged from the whorl at the top of the plant (leaf tip method).
• The second method only counts those leaves that are fully emerged and are arched over with the next leaf visible in the whorl but standing straight up (leaf over method).
• The leaf collar method used extensively in the U.S. refers to the leaf collar being visible, where the leaf collar is the light green to whitish band that separates the leaf blade from the leaf sheath, which wraps around the stem. The corn stages for corn are referred to as V1, V2, V3, etc., where the V3 stage is a plant with three collars showing.

In OMAFRA Publication 75, the Guide to Weed Control, leaves are counted that have emerged from the whorl and are starting to arch over (leaf over method). This normally occurs when leaves are about 50% emerged. Most herbicide labels refer to this method of leaf counting, but check the label or contact the product representative to confirm.

Another complication with leaf counting is that it starts with the bottom leaf known as the first leaf. The first leaf is shorter than other leaves and has a round leaf tip. As the plant grows, the bottom leaves start to die and drop to the ground. A 10-leaf corn plant may have one to three leaves that have died, and these dead leaves may or may not be visible. Care must be taken to account for them as well in leaf counts.

Hint: Start counting from the bottom leaf, and check that the first leaf visible is the one with the rounded leaf tip.

It takes approximately 75 to 80 Crop Heat Units to produce each corn leaf. Therefore at temperatures of 30°C (86°F) during the day, and 20°C (68°F) at night, there is one new leaf every 2 to 3 days; and at 20°C (68°F) during the day, and 10°C at night, one new leaf appears every 5 to 6 days.

Some herbicide labels may use plant height to indicate crop or weed growth stage. Actual plant height may vary due to genetics and the weather or other stesses of the season. Table 3-25. Comparative Growth Stages, gives some comparative heights for each leaf stage, but plants may vary from the stage in the table. The standing height is measured from the ground surface to the top of the plant as it stands. "Leaf extended" refers to the height of the plant with the leaves pulled up to their full height.

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Source: Guide to Weed Control, Publication 75, OMAFRA

Crop Losses Due to Weeds

Certain weeds are more competitive than others. Table 3-26. Percent Corn Yield Loss Due to Weeds at Known Populations, shows some comparative yield losses due to weeds, assuming that the weeds emerge with the crop. Crop scouting is essential to determine the weed species and weed populations present in the field. Consider the competitiveness of weed species when deciding whether to treat escapes. Effects on crop quality and harvest procedures also need to be considered. These estimates are based on normal weather conditions and adequate soil moisture. Yield losses may increase under drier soil conditions and be variable under conditions of plant stress.

Source: Herbicide Application Decision Support System 2001

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Mechanical Weed Control

Small annual weed seedlings can be partially controlled by blind harrowing, before the corn has emerged, with a set of light harrows operating at a shallow depth or by using a weeder harrow (with L-shaped flexible tines) when the crop is 5-10 cm tall. High speed (10 km/h or 6.2 mph), shallow (2.5-3 cm) cultivation with the rotary hoe when corn is 7-8 cm (2 3/4-3 3/16 in.) high will help 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. Cultivation should be shallow to reduce germination of new weed seeds, soil moisture loss and corn root injury. Inter-row cultivation may be required when weeds escape a herbicide treatment. Weeds should be considered as escapes when they are 5-7 cm (2-2 3/4 in.) high.

Band treatment of chemical over the row will reduce cost by half to two-thirds, depending on the row spacing and the width of the band. Shallow inter-row cultivation will be required to control weeds between the bands. Consider the combination of the two operations when evaluating the economics of treating weeds in this manner.

Herbicide Resistance

Herbicide resistance has been found in Ontario. Resistance to triazine herbicides was first found in 1974 and is now present in several species of broadleaf weeds and grasses. In 1997, resistance was found when using ALS (Group 2) herbicides to control pigweeds and in 2000, ALS-resistant Eastern black nightshade and common ragweed were found. Herbicide resistance usually develops from repeated use of the same type of herbicide. Using principles of integrated weed management helps to reduce the onset of resistance and to manage resistance when it is found.

Integrated Weed Management

Weed control in the corn crop is influenced by:

• crop rotation
• seeding rate
• row width
• hybrid characteristics
• tillage practices
• use of custom crop harvesting or planting equipment

In general, the agronomic practices that produce a healthy, fast-growing crop will provide the best competition with weeds.

Crop rotations can be effective in reducing certain perennial weeds that may grow best in a particular crop. Rotating to a different crop can allow different herbicides or tillage practices to be used, which may be particularly effective on specific weed species.

Narrow rows and higher populations tend to increase the crop canopy density and reduce the growth of weeds, but care should be taken to balance the benefits of weed control with the other requirements of the crop.

Fertilizers tend to stimulate the germination of some plant species. Using banded phosphorous and potassium tends to concentrate the nutrients most where the crop has access to them. Side-dress nitrogen applications disturb the soil, which may stimulate the germination of weeds but also places nitrogen in a narrow band and below the depth from which most weeds germinate and grow.

Equipment can carry weed seeds from field to field. Combines, tillage equipment, wind and soil erosion can all transport weeds. Weeds can also infest a field from manure application or other soil amendments. Proper sanitation and cleaning of equipment along with maintenance of field border areas all benefit long-term weed management in the field. Birds may also carry seeds of weeds from field to field.

Please refer to OMAFRA Publication 75, Guide to Weed Control, for more information, including herbicide products and rates or to order Publication 75 - Guide to Weed Control.

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Updates on Corn: Weed Control

No updates available at this time.

Related links...

• Corn
• Weeds
• Publication 75, Guide to Weed Control
• Field Crop Protection Guide - Publication 812

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