Weed Control - Timing is Critical
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!