Critical Weed Control Period
by Greg Stewart, OMAFRA Corn Specialist

At the South West Ag Conference this year in Ridgetown, Peter Sikkema of Ridgetown College, University of Guelph, outlined the rapidly expanding use of post-emergent weed control on Ontario corn. Survey results indicate that close to 90 per cent of the corn acreage in Ontario was treated with a weed control program that employed post-emergent applications. That’s a dramatic increase even since 1993. In addition, we sit on the verge of perhaps even greater reliance on post-emergent weed control as corn hybrids with genetic resistance to over-the-top, broad spectrum herbicides increase in acceptance and availability.

With this trend in mind, a review of the importance of timing in herbicide applications becomes appropriate. The critical period of weed control is an important concept to understand when employing post-emergent herbicides. Prof. Clarence Swanton, Department of Plant Agriculture, University of Guelph has done extensive research to illuminate this concept.
Table 1 Critical Period for Weed Control  
Crop Critical Period for Weed Control  
Corn excellent weed control must be achieved from the 4th leaf to the 10th leaf stage (tip of the 10th leaf emerging above the whorl)  
Soybeans critical period extends from the 1st to the 2nd trifoliate stage of soybean growth (V2 to V3); the V3 growth stage occurs approximately 20 days after crop emergence  
Whitebeans from the second trifoliate to the first flower stage (V2-R1)  
 
C.J. Swanton, University of Guelph
  
Swanton defines this critical period 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 postemergent herbicide applications, reduce the reliance on season long residual herbicides and eliminate the unnecessary late application of herbicides.

Table 1 illustrates the critical period for weed control in three crops. Practical implications for corn producers are that weeds which emerge with the crop have little effect on corn yields until the corn gets to the 4th leaf stage. However, these weeds start to seriously affect yield if not brought under control by the 4th leaf stage. This weed-free period in corn must extend to the 10th leaf stage. Weeds that emerge after the corn passes the 10th leaf stage will generally have little effect on corn yields given near-normal conditions.

Some of these concepts are quite familiar to growers. However, corn producers making extensive use of post-emergent weed control programs should review their spraying practices to harmonize with these critical-timing concepts. In reviewing the post-emergent herbicides that are, or will be available for use, keep in mind that a large application window (i.e., 1st to 8th leaf) for applying the herbicide without damaging the corn plant should not lead you to believe you necessarily have that same window before weed pressure reduces yields.

 

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