Corn Leaf Counting

Counting the leaves on a corn plant should be an easy task. However, there can be confusion as to when a corn leaf is included in the count. Leaf count becomes important when reading a pesticide label which refers to a specific counting method.

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 - Figure 1).
• 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 stresses 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.

Source: Guide to Weed Control, Publication 75, OMAFRA

Figure 1. Leaf Over Method

| Top of Page |