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Reasons not to tank-mix herbicides and soybean-rust fungicides
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From Wisconsin Crop Manager newsletter
by Chris Boerboom, Ext. Weed Scientist
and Craig Grau, Ext. Plant Pathologist
University of Wisconsin
1/312005 -- Soybean rust has been confirmed in the southern United States. While it is unknown when the rust will be introduced
into Wisconsin, fungicides will be applied to control the
rust when it appears.
At that time, there may be interest
in tank-mixing fungicides with herbicides to reduce
application costs. However, there are three good
reasons why a herbicide-fungicide tank mixture may not
be the best idea: improper timing, drift and labeling
precautions.
Trying to make a tank mixture work for
weed control and soybean rust control may result in
poor performance of both pesticides. Instead, we
recommend that herbicides and fungicides are applied
separately for the best control of weeds and rust.
Improper timing
Considering that most soybeans are
Roundup Ready, the first application of glyphosate
should be applied to these soybeans when they are in
the V2 or V3 growth stage. This early timing is
required to control weeds before they compete with the
soybeans, which would cause yield loss. It is also the
best timing to get the greatest herbicide activity because
the weeds are still small.
However, this timing is likely to be too early for fungicide treatments to control soybean rust. Based on the experience in other
countries, the earliest fungicide treatments are often
made around the R1 soybean growth stage. Because of
Wisconsin’s northern latitude, the rust may not arrive
until even later in the season. If a fungicide-herbicide
tank mix was sprayed at the correct timing for the rust,
weed control could be reduced because of the large size
of the weeds.
In addition, you should not delay your
herbicide application in anticipation of a herbicide-fungicide
application, because the rust may not even
develop in Wisconsin each year. Herbicide-fungicide
applications might be timed right for one pest, but not
the other, or the application might be timed wrong for
both.
One situation when a herbicide-fungicide tank mixture
might be appropriately timed for both pests is when a
second herbicide application is being made to control
late-emerging weeds. If this is the case, drift and label
precautions still need to be considered.
Drift
The goals for spray coverage with herbicides and
fungicides differ. For translocating herbicides like
glyphosate, uniform spray distribution is required, but
complete coverage of each leaf surface is not. Contact
herbicides require more thorough leaf coverage than
translocating herbicides, and their application may be
more similar to the needs of fungicides.
Fungicides require smaller droplets and higher spray volumes to
achieve maximum coverage of the soybean canopy than
herbicides. This may require either different spray
nozzles or higher spray pressure to generate the smaller
droplets.
Some recommendations suggest that fine- to
medium-sized droplets (150 - 350 VMD) are
appropriate for fungicides. (VMD = volume mean
diameter and is measured in microns.) However, we
want larger droplets during herbicide applications to
reduce the risk of drift. Medium or coarse-sized
droplets (250 - 450 VMD),which are less prone to drift, are recommended for
herbicides.
If the application of a herbicide-fungicide tank mixture is
optimized with larger droplets to reduce herbicide drift,
the performance of the fungicide may suffer. If smaller
droplets are used to optimize the fungicide
performance, herbicide drift is more likely.
For the best performance, fungicide applications by
ground rigs should be made in a minimum of 15 gallons per acrea of
water, and 20 gpa may be preferred. If a herbicide-fungicide
application were to be applied at the lower spray
volumes (e.g. 10 - 15 gpa) typically used for many
herbicides, spray coverage with the fungicide may be
compromised as well.
Label precautions
Most herbicide and fungicide labels
are vague regarding the recommendations of herbicide-fungicide
tank mixtures. Many soybean herbicide
labels make no mention of fungicide tank mixtures,
while others make generic statements that tank mixtures
with other pesticides may reduce efficacy.
The product Headline allows tank-mixing with herbicides, but tank mixtures
with herbicides are not addressed on most other
fungicide labels. Chemical companies may make more
specific recommendations in regard to the risk of
incompatibility or reduced performance with tank
mixtures as more experience is gained with the soybean
rust.
Until that time, it may be wise to avoid tank
mixtures. Likely, the greatest risk with a tank mixture
is somewhat-reduced performance of glyphosate if it is
applied in the higher spray volumes that are
recommended for fungicides. (This reduced
performance might also be associated with the fact that
weeds are likely to be larger at the time of this tank mix
application.)
From a legal standpoint, if both labels do not prohibit a
tank mixture, it is legal for the applicator to mix and
apply the tank mixture. If desired, you could try the
tank mixture in a small field test, but the responsibility
for performance would basically be your own.
On the Web: Wisconsin Crop Manager
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