Improving Weed Control
Poor timing of post-emergent herbicides does nothing to reduce weed control costs and can mean significant yield losses in corn in some situations. Focusing on some pre-emergence options may help spread the risk and lower overall control costs.
Emphasis
on Timing
Stress on the corn plant reduces corn yield. Weeds competing with the corn crop
for light, water and nutrients can be a huge source of stress. Hence, weed control,
and perhaps more importantly, timely weed control is critical for optimizing
corn yields. Research within Ontario has developed the concept of the ‘Critical
Weed-Free Period’. This period for corn is from the 3rd leaf stage until
the 8th leaf stage. This implies that weeds that emerge with the corn must be
brought under control by the 3rd leaf stage to prevent yield loss. Similarly,
weeds that emerge in the crop up until the 8th leaf stage need to be controlled
to secure yield potential. Weeds that emerge after the corn has passed the 8th
leaf stage generally will not cause enough yield reduction to warrant control.
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Table
1. Weed control achieved with a range of pre-emergent herbicides when
applied at both the full label rate and one-half the full label rate.
Elora, Woodstock, Cambridge and Markdale, 2001.
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| Herbicide Treatment |
Weed
Control (%) at Full Rate
|
Weed
Control (%) at One-Half Rate
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||
|
Annual
Broadleaves |
Annual
Grasses |
Annual
Broadleaves |
Annual
|
|
| Grasses |
|
|
||
|
|
|
|||
| Control (weed free) |
100
|
100
|
||
| Axiom |
55
|
94
|
35
|
82
|
| Converge |
99
|
90
|
87
|
73
|
| Dual II / Marksman |
98
|
93
|
91
|
90
|
| Frontier / Marksman |
98
|
93
|
90
|
86
|
| Prowl / Atrazine |
100
|
95
|
97
|
87
|
| C. Swanton and K. Chandler, University of Guelph. | ||||
The 3 to 8 leaf
stage critical period should be used by corn producers as a guide to fine-tune
their weed control strategies, but other factors need to be considered. Take,
for example, a field where weed pressure is very light or where weeds are slow
in emerging. Aiming for complete control of the few weeds present when the corn
hits the 3rd leaf stage may be a waste of money. As more weeds emerge after
the 3rd leaf stage, an additional herbicide application may well be required,
particularly if there is no residual activity in your post-emerge spray. On
the other hand, if your post-emergent herbicide has a wide window of crop safety,
you may be tempted to delay spraying until closer to the 8th leaf stage in order
to kill more weeds. This approach may give you clean fields at harvest, but
if there has been significant weed pressure competing with the crop since the
3rd leaf stage, you may have suffered substantial yield loss. Scouting your
fields, knowing your weeds and weed pressure, and getting the timing right are
all part of cost-effective weed control.
Dr.
Susan Weaver (AAFC) and Ian McDonald (OMAFRA) pounded this point home at their
presentation at the 2002 Southwest Agricultural Conference in Ridgetown. They
stressed that, under certain situations, delayed applications of post-emergent
herbicides cost yield and save nothing. Their presentation included data from
research conducted by Dr. Al Hamill (AAFC) (see Figure 1) and shows corn yield
is affected by the timing of post-emergent applications. The red line, which
represents a sandy soil where weed pressure was high, illustrates that delayed
applications – even though within the critical period and within the crop
safety window – resulted in significant net losses to the grower.
Spread
Risk, Reduce Costs
In some cases, poor post-emergent timing is a result of having too many acres
dependent on post-emergent herbicides, and running the risk of not getting enough
good weather to get all of the spraying done on time. Reducing this risk can
be accomplished by using a combination of weed control tactics with application
timings ranging from early pre-emerge to later in the post-emerge window. Recent
research conducted by Dr. Clarence Swanton at the University of Guelph highlighted
some pre-emerge options applied at both the full label rate and half-rate. These
sites were monitored for weed pressure where annual broadleaf weeds consisted
mostly of Wild Mustard, Lamb’s Quarter, Wild Buckwheat and Pig Weed, and
where annual grass pressure was mostly Green Foxtail. Table 1 indicates that
a range of herbicide programs (with varying product costs), and a range of application
rates (full and one-half label rate) produced some reasonably effective weed
control.
Growers should consider pre-emerge applications as a way of removing some of
the burden from post-emerge programs which may suffer from the inability to
apply herbicides on time because of large acreage or availability of custom
applicators. In some cases, these pre-emerge opportunities may include lower
cost herbicides (most will include atrazine) or options that include using label
rates that are lower than the maximum recommended ones. Care should be taken
to select fields that have weed species that fit the strengths of your selected
herbicide. Also note that 10 years ago, looking for lower cost pre-emerge herbicide
options for corn was a much riskier venture. Back then, the post-emerge products
you could use to clean up corn fields paled in comparison to today’s arsenal,
so you ran the risk of having weed escapes and very poor options for getting
things under control.
The message today is to look for options in the pre-emerge window where you
can cut costs, at least on a portion of your acreage, and then scout the fields
to assess where you may need to come back in with some post-emerge applications.
The result can be more cost-effective weed control, a little more attention
to scouting, and improved timing in the post-emerge window.