Control Weeds Early in Corn

Corn planting in Manitoba is pretty well complete for 2012 and the next order of business for Manitoba corn producers is weed control.  It is important for producers to remember that early weed control and timing is an important step in maximizing yield potential.  This is particularly true when many post emergent weed control options have little to no residual activity. As with any crop type, the critical period of weed control in corn is an important concept to understand when considering the weed control program. 

The critical period of weed control is defined as an interval in the life cycle of the crop when it must be kept weed-free to prevent yield loss. Understanding this period helps determine the most effective time for post-emergent herbicide applications and eliminate the unnecessary late application of herbicides. 

In Ontario, the critical weed free period for corn is from the 3 to 8 leaf stage when using the leaf over method.  However, there are many ways to stage corn and the following table compares the most commonly used systems. 

If Using The... The Critical Weed-free period is from ...
Leaf Tip Method 4 to 10 leaf stage
Leaf Over Method 3 to 8 leaf stage
Collars Method (ie V2 stage) 2 to 6 leaf stage

It is important to note that this period can be dynamic and in any given year or field situation the critical period can vary.  The time when it starts and its duration are dependent on many factors. Climatic conditions, time of weed emergence, weed density, weed species, soil type and crop canopy are all contributing factors. Scouting and good record keeping allows weed control timing to match the critical period in any field situation.

One important thing to keep in mind is that a large window of application for a product (i.e. 1st to 8th leaf) for application does not equal the same window before weed pressure reduces yields. In a corn weed control program, early removal of weeds is important to maximize yield.  Waiting for those last few weeds to emerge can be costly in terms of lost yield since they are usually "not competitive".


Staging Corn for Herbicide Applications

When applying post-emergent herbicides, proper corn growth staging is extremely important.  Herbicide labels often refer to plant height, crop growth stage (leaves or collars), or both, when discussing corn growth stage limits for the application of postemergence herbicides.  Below is a review of some common methods for determining growth stage.  For each method, the stage of the corn plant in Figure 1 will be determined.
Figure 1. Corn Plant

Corn Height Method
To determine corn plant height, measure from the soil surface to the highest point of the arch of the uppermost leaf whose tip is pointing down.  Do not measure to the "highest point" on the plant, which is often the tip of the next emerging leaf above.  Refer to Figure 1 on how to correctly determine the height of a corn plant. 
Both environmental and management conditions can have a great impact on the height of a corn plant.  In cool, wet springs, corn often grows more slowly from a height standpoint but it is still advancing physiologically.  A delayed seeding date, differences in tillage, and differences in soil type can also have a pronounced effect on plant height but relatively little effect on the stage of vegetative development.  Hybrid can also have an effect on plant height as shorter-season hybrids tend to produce shorter plants. Since corn height varies a great deal due to growing and crop management conditions, it is not a very accurate way to stage corn plants.  However, it is a quick and universally understood method. 
Leaf Tip Method
The leaf tip method counts all the leaves including any leaf tip that has emerged from the whorl.  In Figure 1, the corn plant would be at the 6 leaf stage. The leaf tip method is not commonly used.
Leaf Over Method
The leaf over method is a more common way of measuring leaf number than compared to the leaf tip method.  The leaf over method counts the number of leaves, starting from the lowest one (the coleoptile leaf which has a rounded tip) up to the last leaf that is arched over (tip is pointing down). Do not count leaves younger (inside) than this one, even though they are present in the whorl.  In Figure 1, the corn plant would be at the 4 leaf stage.
Leaf Collar Method (V-stage)
The leaf collar method is generally the easiest to use. It also relates better to the physiological stage of the plant and thus to the effects of herbicides. Staging by the leaf collar method is done by counting the number of leaves with visible collars. Collars are not visible until the leaves are developed enough to emerge from the whorl.  In Figure 1, the corn plant would be at the 3 leaf stage (V3).
When comparing the leaf counting methods, the stage of the corn plant in Figure 1 differs by one to three leaves.  However, all methods are valid in determining crop growth stage.  What is important to know is which method the manufacturer is using to indicate correct herbicide application timing. Use the Guide to Field Crop Protection which outlines manufacturer’s recommendations for post-emergent herbicide application timings based on corn growth stages. For more detailed information on various corn herbicides, please refer to the product label for application details and precautions.
For products whose labels indicate crop height and growth stage, it is important to follow the more restrictive of the two.  Also when tank mixing be aware the window of application may change depending on the tank mix partner’s staging restrictions.
  • “Staging Corn Plants and Implications Associated with Herbicide Applications” by A. Hager, E. Nafziger, and D. Nordby.  The Bulletin: Pest Management and Crop Development Information for Illinois.  University of Illinois, No. 7 Article 9/May 12, 2006.
  • Guide to Crop Protection 2012, Manitoba Agriculture Food and Rural Initiatives.
  • “Critical Weed Control Period” by Greg Stewart. OMAFRA Corn Specialist.