University of Wisconsin-Extension
Dry
weather after planting causes many concerns, including the
impact of weeds on annual crops. Many herbicides lose effectiveness
during dry periods; growers who use herbicides on corn and
soybean crops are likely to be affected. Fortunately, an awareness
of herbicide effectiveness and the aggressive use of mechanical
weed control measures can make a difference.
- Preplant
incorporated herbicides. These are applied before planting
and mixed into the soil. They work best when:
- the
product is mixed uniformly with soil to the depth recommended
by the manufacturer;
- soils
have reasonable moisture levels after incorporation has
been completed.
- If
the soil is only slightly dry, incorporated herbicides
generally perform adequately. Seldom is it so dry early
in the season that incorporated treatments fail. In a
true drought, however, they may not give acceptable weed
control. Therefore, be prepared to cultivate if weeds
appear.
- Pre-emergence
herbicides. These depend totally upon rainfall after
applications to "activate" the product. Rainfall
positions the chemical in the upper soil surface where the
weed seeds germinate; there is no chemical change as perhaps
the term "activate" suggests. To obtain adequate
herbicide activity, however, rain must fall within 10 to
14 days after the seedbed was prepared. Without such precipitation,
pre-emergence herbicides generally fail to give acceptable
weed control even if a true drought does not develop. Therefore,
mechanical weed control may become critical within weeks
of planting. Two examples:
- If
a field is prepared to plant on April 30, corn is planted
on May 1 and a pre-emergence herbicide is applied on May
2, rainfall of at least one-quarter to one-half inches
is needed within 10 to 14 days to assure adequate performance.
If rainfall does not occur by May 12, the grower should
begin rotary hoeing.
- If
a field is prepared on April 30 and corn is planted on
May 10, followed by pre-emergence herbicide on May 11,
plan to rotary hoe on May 12 unless rainfall is very likely
in the immediate future.
As illustrated
above, when planting and spraying are close to the field preparation
time (example a), there is more time to get the needed rainfall
to make a surface-applied herbicide perform adequately. As
time between field preparation and spraying increases (example
b), there are fewer days after an application to get a timely
rain. Thus, rotary hoeing becomes necessary sooner.
Rotary
hoeing kills weeds that have germinated but have not yet emerged.
These weeds are in the "white root" stage of development.
After weeds emerge, rotary hoeing is less effective. Rotary
hoeing also helps place the herbicide in the upper soil surface
so that when rains do fall, the herbicide is in a better position
to be quickly taken up by weed seedlings and hopefully kill
them. If it has not rained within seven days of the first
rotary hoeing, make a second ass with the rotary hoe to kill
the next generation of weeds.
Post-emergence
herbicides also may fail in dry weather. These treatments
work best when weeds are actively growing. When weeds are
stressed by lack of adequate soil moisture, chemical control
declines. If you decide to apply post-emergence herbicides
under very dry conditions, be aware that crop injury may occur
and weed control will be poor.
In all
situations, be prepared to cultivate once or twice following
rotary hoeing. Some growers mistakenly believe that soil loses
more moisture when cultivated. But remember that weeds transpire
water into the atmosphere every day they are in the field;
the longer weeds live, the more soil moisture is lost and
unavailable for the crop, and the harder they are to eradicate.
So it is always a wise decision to cultivate weeds early on.
- Cultivate
when the weeds are relatively small and the crop is large
enough (at least 4 inches tall) to allow you to roll some
soil into the row without covering the crop.
- The
crop should be at least three times as tall as the weeds
when the first cultivation is done (for example, the crop
is 6 inches tall and the weeds are 2 inches or less tall).
This way, the weeds in the row can be covered with minimal
effect on the crop.
- The
cultivator need not be set any deeper than a couple of inches
to dislodge the weeds; little if any moist soil will be
brought to the surface.
- A
second cultivation can be done when the crop is 14 to 18
inches tall. This requires timely mechanical practices,
but keep in mind that in drought years, a few uncontrolled
weeds cost more in reduced yield than in years with ample
moisture.
Additional resources:
Your county agricultural agent
Related publications:
UW-Extension publication "Reduced Herbicide Rates: Aspects to Consider," (A3563).
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NASD Review: 04/2002
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