FEATURE STORY
Weed Control - Timing is Critical
By Greg Stewart and Hugh Martin
Producers
who rely heavily on post-emergent herbicides need to be keenly aware of three
things:
1) the critical period of weed control for corn.
2) the leaf stage of your corn crop.
3) the herbicide label rating for safe application to corn.
Critical period of weed control
Research at the University of Guelph by Dr. Clarence Swanton has determined the critical weed-free period for corn
to be from the third- to the eighth-leaf stage. 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 third-leaf stage. However if
these weeds are not brought under control early, they will affect final yields. This weed-free period in corn must
extend to at least the eighth-leaf stage. Weeds that emerge after the corn passes this eighth-leaf stage will generally
have little effect on corn yields given near normal conditions.
Leaf stage of your corn crop
In OMAFRA Publication 75 (Guide to Weed Control), and on most herbicide labels,
corn leaves are counted on the “leaf over” 
basis.
In this system, a leaf is counted once it has emerged far enough out of the whorl
to no longer stand vertical. It begins to arch and the end of the leaf blade leans
over. When corn plants have three leaves that are long enough to arch over, then
the corn is at the three leaf stage and the critical weed-free period has commenced.
These plants will have at least one other leaf emerging from the whorl, but as
long as it remains straight it is not counted as a leaf stage. Effective post-emergent
programs require producers to be aware of their corn’s stage of development.
Herbicide label
Producers also need to know the growth stage interval during which they can safely apply specific herbicides to
the corn crop. This information then can be used to guide them in selecting the most suitable herbicides. Some
herbicides have a wide application window (for example, 1 to 8-leaf stage) when the herbicide will not injure the
corn plant, but if you wait to the end of this period you will lose yield through weed competition. Figure 1 illustrates
corn yield loss from delayed applications of Roundup on Roundup Ready corn in Peter Sikkema’s research at Ridgetown
College, University of Guelph. In this study there was more than 10 bu/ac yield reduction from spraying at the
eighth-leaf stage compared to spraying at either the third or fifth-leaf stage. Bottom line is you can delay post-emergent
applications in some situations well into the critical period and often get excellent weed control, with the field
appearing very clean at harvest. Unfortunately, yield will have been surrendered because of weed competition experienced
by the corn crop in the early growth stages!
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