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Watch for Fomesafen Carryover Injury in Corn

If your
soybean herbicide last season contained the active ingredient fomesafen, you
may want to take a stroll through your cornfield. In the past few days, Kevin
Bradley, University of Missouri (MU) Extension weed specialist, has received
numerous calls in Missouri regarding carryover injury in corn that received fomesafen
applications last season.  

Here’s
what Bradley writes about his findings in this week’s MU Integrated Pest and
Crop Management newsletter: 

Fomesafen
is the active ingredient in the herbicides Dawn, Flexstar, Flexstar GT, Prefix,
Rhythm, Marvel, and various generic products. In recent years, it’s become one
of the most common active ingredients applied postemergence to control glyphosate-resistant
waterhemp in soybeans. Of all soybean postemergence herbicides, fomesafen is one
of the most persistent with one of the longest periods of soil residual
activity.

Due to
this soil persistence as well as the sensitivity of corn to fomesafen residues,
the label of most fomesafen products like Flexstar and Prefix requires a
10-month rotational interval between fomesafen applications and corn planting.
 

In some
of the calls Bradley has received, this 10-month rotational interval between
application and planting was not followed. However, in some of these instances,
more than 10 months elapsed between application and planting.

Two
factors influence the likelihood of fomesafen carryover injury to corn:


1. 


Dry conditions following application.

2. 


The rate and timing of the herbicide application. 

Last winter’s colder-than-normal
temperatures may also play a role in fomesafen carryover, believes Bradley.  “I can’t find any research data to support this, but it makes sense to me that
microbial degradation may have been reduced in response to the extended winter
we just experienced,” he says.

Precipitation
between application and planting is the main factor that influences the
likelihood of fomesafen carryover. Soil moisture is critically important
for herbicide degradation, says Bradley.  If adequate rainfall is not received
after application, then the chemical and microbial processes that degrade
herbicides are reduced significantly. Herbicide molecules are more likely to
become bound (adsorbed) to soil particles, says Bradley.

All this
means less herbicide degradation and increases the likelihood of herbicide
carryover.  Injury may also be more noticeable on sandy soils, as these
areas are usually better drained and hold moisture for shorter periods of time.
In some locations that have reported fomesafen carryover this week,
precipitation totals have been off by as much as 17 inches from the 15-year
average for the time period following application to now, says Bradley.

 


Rate and timing

These are
two other factors that influence the likelihood of fomesafen carryover injury
to corn.  Simply put, the higher the rate of fomesafen applied and the
later the herbicide application was made, the greater the chance that some
fomesafen may remain to cause carryover injury to corn. Since the labels
of most fomesafen-containing products require a 10-month rotational interval
between application and corn planting, late-season applications of these
products in soybeans and early planting of corn the following spring can often
make satisfying these intervals difficult. 


Symptoms

The most
common corn injury symptom caused by fomesafen carryover is a whitening of the
leaf veins, commonly referred to as veinal chlorosis. Affected areas of corn
leaves often take on a striped appearance, can become necrotic, and tissue near
the leaf midrib may totally collapse in that region.  The root
system of affected plants usually remains normal, and plants can take on
somewhat of a droopy appearance as well.


Will it kill corn?


Bradley
has rarely seen a fomesafen
carryover issue in corn that is bad enough to justify killing the corn and
starting over. He also doesn’t believe this is warranted for any of the
fields or photos he has seen this week. Usually, the most effective strategy is
to wait for good growing conditions and heat units so that roots will grow out
of that fomesafen “zone.” He advises farmers to closely monitor new
growth in the next five to seven days. As long as the new leaves have a healthy green
color, they will be fine for the rest of the season. 

 

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