Corn: Leaf Stages

Table of Contents

1. Introduction
2. Comparative Growth Stages
3. Leaf Over Method of Counting Corn Leaves (Figure 9-1)
4. Corn Herbicide Weed Control Ratings (Table 9-1) (PDF file - 214kb)
   • Annual Grasses
   • Broadleaf Weeds
   • Perennial Weeds
5. Other Corn Weed Control Recommendations
   • Burndown and Residual Control
   • Soil Applied Grass Herbicides
   • Soil Applied Broadleaf Herbicides
   • Soil Applied Grass and Broadleaf Herbicides
   • Soil Applied Tank-Mixes (for Control of Grass and Broadleaf Weeds)
   • Postemergence Grass Herbicides
   • Postemergence Broadleaf Herbicides and Tank-Mixes (for Broadleaf Weed Control)
   • Postemergence Grass and Broadleaf Herbicides
   • Postemergence Tank-Mixes (for Control of Grass and Broadleaf Weeds)
   • Postemergence Grass and Broadleaf Herbicides - for Liberty Link (glufosinate tolerant) hybrids only
   • Postemergence Tank-Mixes - for Liberty Link (glufosinate tolerant) hybrids only
   • Postemergence Grass and Broadleaf Herbicides - for Roundup Ready (glyphosate tolerant) hybrids only
   • Postemergence Tank-Mixes - for Roundup Ready (glyphosate tolerant) hybrids only
   • Directed Postemergence
   • Directed Postemergence for Liberty tolerant hybrids only
6. Related Links

Introduction

Counting leaves on a corn plant may sound like an easy task, but there are complications that can cause miscounting. There are several methods of counting leaves. It is important to know which leaf counting method is being referred to.

One method - the leaf tip method - counts all leaves, including any leaf tip that has emerged from the whorl at the top of the plant. (leaf tip method). The leaf collar method only counts those leaves that are fully emerged and have the leaf collar showing. The leaf collar is a light green band that separates the leaf blade and leaf sheath that wraps around the stem. In some states they refer to corn growth stages as V1, V2, V3, etc., where a V3 stage is a plant with 3 collars showing.

Publication 75 uses the leaf over method, (see Figure 9-1) where counting starts with leaves that have emerged from the whorl and the leaf tip is starting to arch over. This normally occurs when leaves are about 50% emerged. Most product labels also use this method of leaf counting, but check the label or with the product representative to be sure. The comparative growth stages table in the next column gives a comparison among the count methods.

Another complication with leaf counting is where on the plant leaf counting begins. In Publication 75, the first leaf is the bottom leaf of the plant. The first leaf is shorter than other leaves and has a round leaf tip. However, as the plant grows the bottom leaves die and drop to the ground. For example, a 10 leaf corn plant may be incorrectly identified as a 7 leaf corn plant because 3 leaves may be "senesed" or fallen off. These leaves may not be immediately apparent and care must be taken to count them.

Hint: Start counting from the bottom leaf and check the first leaf to look for the rounded leaf tip.

It takes about 75-80 Crop Heat Units to produce each corn leaf. Therefore at temperatures of 30°C day, and 20°C at night, there is one new leaf every 2-3 days; and at 20°C day, and 10°C at night, one new leaf every 5-6 days.

Critical Stage: The Critical Stage to control weeds in corn is the 2-8 leaf over stage (3-10 leaf tips).

Some product labels also use plant height to indicate crop growth stages. In general, plant height is more variable depending on plant genetics and on the weather of the season. The following table gives some comparative heights for each leaf stage but your plants may be slightly more or less than the stage given depending on genetics and weather. 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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Figure 9-1. Leaf Over Method of Counting Corn Leaves

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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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