Critical Weed Control Period
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 |
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.