Glyph-resistant Palmer amaranth likely
spread by flooding in upper Midsouth
By Larry Steckel,
Tennessee Extension Weed Specialist
We have fielded a number of calls since last
Wednesday on glyphosate control failures of Palmer amaranth. These
calls have come from crop consultants, county agents and growers,
primarily in Lake, Lauderdale, Haywood and Dyer counties.
In
a couple fields, we are seeing thick patches of glyphosate resistant
(GR) Palmer that have survived multiple applications of glyphosate.
In other fields, we are seeing Palmer amaranth scattered throughout
the field, having lived through 1 or 2 glyphosate applications.
It appears that the flooding early this spring
has helped spread GR Palmer amaranth seed. Some of the GR Palmer
seed may have also been spread in gin trash. Bob Scott and Ken
Smith, my colleagues in Arkansas, have reported more calls to date
on glyphosate failures than what we have experienced here. It
appears that the flood this spring has moved GR Palmer amaranth seed
in that state, as well.
The level of glyphosate resistance in some of
these Palmer amaranths is very concerning. In previous years, we
have seen glyphosate resistance levels of about 2X to 3X. The Palmer
in those fields would at least show some symptoms 5 to 10 days after
application (DAA) from a 0.75 lb ae/A (22 oz/A Roundup WeatherMax)
application.
But some Palmer amaranth in fields we have
observed this past week are showing no symptoms at all 7 to 10 DAA
even after being sprayed with rates ranging from 1.5 - 3.75 lb ae/A
of glyphosate (44 – 110 oz/A Roundup WeatherMax).
I can not emphasize enough that we need to get
around to all our fields, particularly the ones that were flooded,
and scout them for Palmer control failures. Crop scouts have, for
the most part, identified many of these Palmer control failures.
This service is very valuable.
Early identification before the Palmer is too
big is critical. Palmer can grow 2 to 4 inches a day in warm
conditions with good soil moisture. So, a 4-inch Palmer today can be
a foot tall in four days. Palmer that is much over 8 inches tall is
often too large for PPO inhibitors like Flexstar, Blazer or Resource
to control over the top in soybeans.
In cotton, if we can determine early enough that
the Palmer is showing some glyphosate resistance, we may be able to
apply a post-direct application of Direx plus MSMA early enough to
at least get control in the row middles.
Do not forget to scout corn fields for escaped
Palmer, as well. A couple of the fields where GR Palmer amaranth has
been observed this spring were in corn last year.
Recrop Intervals for Post Direct and Layby
Applications.
Please keep in mind that there are just a few
commonly used layby herbicides that have a recrop interval short
enough where sowing wheat this fall is still on label. The
herbicides are glyphosate (anytime), Aim (anytime), Envoke (3
months), Ignite (70 days), Suprend (3 months), and Valor (30 days).
Listed below are the recrop intervals for some of the more popular
post direct and layby herbicides.
|
Herbicide |
Wheat Recrop |
|
Aim |
anytime |
|
Caparol |
no fall recrop (wheat grown for grain) |
|
Direx |
1 year |
|
Dual Magnum |
4.5 months |
|
Envoke |
3 months |
|
Glyphosate |
anytime |
|
Goal |
10 months |
|
Ignite |
70 days |
|
Layby Pro (Direx + Linex) |
4 months (provided no Direx was used pre) |
|
Staple |
4 months |
|
Suprend |
3 months |
|
Reflex |
4 months |
|
Valor |
30 days |
Label
Change - The label for Valor XLT, a premix of Valor and
Classic, has now been changed in respect to recropping back to
cotton. It was 18 months and now has been reduced to 10 months for
soils less than 7.0 pH.
