Managing Glyphosate-Resistant Palmer Amaranth In Grain Sorghum
JACKSON, TENN.
It appears that Tennessee
may have more grain
sorghum planted in 2012.
Several growers have figured
that they can be more profitable
with grain sorghum
than soybean on some of their
farms. Some of these fields
that are slated to go to sorghum have been more
or less continuous soybean due to the productivity
of the soil type and/or are prone to flooding
and a June planting date. Factor in that
many of these fields also have significant
glyphosate-resistant (GR) Palmer pressure and
one can see where sorghum is a viable option.
The question is how to manage the Palmer pressure
which is quite high in some of these fields
in a crop of grain sorghum.
Grain sorghum does have more herbicide options
for Palmer control than soybeans. However,
there is not near the arsenal of Palmer
amaranth effective herbicides registered for
grain sorghum compared with corn. Grain
sorghum is not as robust as corn when it comes
to emergence and stand establishment. As a result,
herbicides like simazine that is labeled in
corn is NOT labeled in sorghum, as it will wipe
out an establishing stand. Another corn herbicide,
2,4-D is not labeled pre emergence or preplant
for the same reason. Even dicamba can
only be applied at an 8 oz rate and only up to 15
days before sorghum planting due to stand establishment
concerns.
Atrazine can be used pre and post in sorghum
and will be a key herbicide in Palmer infested
fields. Consider split applying the atrazine with
some of it pre and the rest early post for best results.
Atrazine is a good first step but it should
not end there for good pigweed control. There
are some herbicides along with atrazine in grain
sorghum that will provide residual control of
Palmer amaranth. They are Dual Magnum,
Guardsman and Degree and the atrazine premixes
of these (Bicep II Magnum, Guardsman
Max and Degree Xtra). There has not been anything
new from a herbicide standpoint in grain
sorghum for many years but that changed when
Lexar received a preplant/preemergence grain
sorghum label. Lexar is a three way premix of
atrazine, Dual Magnum and Callisto. All three
of these herbicides will provide Palmer amaranth
control provided they are activated by a
rain. Again some care is needed when utilizing
this herbicide. Lexar should not be used on
course soil types. It is also much safer to an establishing
sorghum stand if it is applied preplant
7 to 21 days before planting than pre
emergence.
From a post standpoint the labeled products
that can provide Palmer control are 2,4-D,
dicamba, atrazine, Dual Magnum, Degree, Degree
Xtra, Guardsman and Guardsman Max.
Atrazine and the atrazine premixes (Degree Xtra
and Guardsman Max) can be applied up to 12”
tall sorghum. The Degree Xtra and Guardsman
Max herbicides would be the “go to” options if
similar products were not used before planting.
An option here would be to start clean with a
paraquat and atrazine preplant application followed
with either Degree Xtra or Guardsman
Max post emergence. Remember not to go over
2.5 lbs of atrazine for the season.
Grain sorghum rotation can be a good way to
reduce Palmer amaranth pressure in a field.
However, for fields with heavy GR Palmer pressure,
it takes a preplant followed by early postemergence
herbicide application approach to
best manage this
weed. Δ
DR. LARRY STECKEL: Extension Weed Specialist,
University of Tennessee