Prevention is Best Strategy For Resistant Weed Management
Posted by Unknown in Weed Management on Sunday, 12 May 2013
South West Farm Press
 |
| Some of these
weeds died as a result of spray application. The live plants also
received the same spray application and are examples of resistant weeds. (Photo byScott Russell, Texas AgriLife Extension IPM agent for Terry and Yoakum counties.) |
Dealing with
Palmer amaranth—pigweed—is nothing new for the Texas High Plains. It’s
always claimed the number one spot on the list of weed problems for
cotton farmers.
So developing a
program to deal with glyphosate resistant pigweed should not become the
nightmare that has bedeviled cotton and soybean farmers in the
Mid-South and the Southeast, says Wayne Keeling, Texas AgriLife
agronomist and weed scientist who works out of the Lubbock Research and
Extension Center.
Keeling
discussed resistant-weed management recently at the South Plains Ag
Conference and Trade Show in Brownfield. Morningglory and Russian
thistle also claim spots near the top of the most troublesome weed list
for High Plains producers.
“Johnsongrass is not as much of a problem in the High Plains anymore,” he said.
Keeling said
most farmers (91 percent) in the region relied on preplant incorporated
herbicides before the Roundup Ready era. Also, 20 percent used a
pre-emergence herbicide and 40 percent spot-sprayed and almost everyone
cultivated to control weeds.
“Yellow herbicides and cultivation were the keys,” Keeling said.
Adoption of
Roundup Ready technology offered significant benefits to High Plains
farmers, he said. “We greatly reduced silverleaf nightshade, for
instance. Roundup Ready accomplished a lot of good, but we may have
become too reliant on Roundup.” That reliance selected for resistant
pigweed survival. “And pigweed produces a lot of seed. But the good news
is we can control pigweed. We have a lot of good cotton herbicides.”
Identification
of glyphosate resistance in the Texas High Plains is recent—2011. “They
identified glyphosate-resistant Palmer amaranth in the Mid-South five or
six years ago,” he said. “Farmers in the High Plains had been doing a
lot of things right during that time, but we did see resistance begin to
show up in 2011. We collected seed from some escapes, grew them out in
the greenhouse and treated them with Roundup. At two times the
recommended rate, we observed almost no control.
“We also asked
for farmers to collect samples of suspected resistant pigweed. We got 12
samples from four or five counties and determined that eight of those
samples were resistant. In 2012, we received more and more reports of
weeds that farmers couldn’t kill.”
Prevention is key
Prevention, Keeling said, is the best strategy. Keys to prevention include:
- Using herbicides with multiple modes of action.
- Using soil residual herbicides.
- Applying post-emergence tank-mix herbicide treatments.
- Using sequential weed control techniques.
- Going back to some tillage and cultivation.
- Adopting new technology.
He said
residual herbicides such as trifluralin or Prowl are critical to
preventing herbicide-resistant weed infestations. Incorporation, either
through tillage or irrigation, is necessary, he said.
Those pre-plant
materials are important to control small broadleaf weeds and annual
grasses. “Rate will be relative, depending on soil types,” Keeling said.
“Heavier soils may require higher rates. Higher rates also may be
needed if the material is incorporated with irrigation.
“The value of
yellow herbicide application is evident in the field,” he said.
“Tumbleweeds, for instance, are controlled with yellow herbicides.
Pre-emergence
residual herbicides, materials such as Caparol, Direx, Cotoran, Dual and
Staple, “can help with weeds the yellows don’t control. On sandy soils,
Caparol is the only one I’m comfortable with,” Keeling said. “North of
Lubbock, any of the others will do.”
He said Dual
Magnum and Staple may offer a broader spectrum of control than the
dinitroaniline herbicides. “But we may have some residual issues with
Staple.”
Postemergence treatments options include Staple, Dual Magnum, Prowl H2O and a few others. Some can be tank-mixed with Roundup.
In some cases,
cotton weed control “has come full circle, back to post-direct spray
applications at lay-by.” Product options include Caparol, Direx,
Cotoran, and Layby Pro, applied alone or with Roundup. These treatments
offer residual pigweed and morningglory control.”
New technology
may be another key to delaying herbicide resistant weed infestations,
Keeling said. GlyTol Liberty Link technology, for instance, is tolerant
to Liberty and Roundup herbicides.” Tank-mixing those herbicides,
however, is not effective because of an antagonism effect.”
Roundup is not
as effective on morningglory but does a better job on pigweed than
Liberty. “If a farmer has both weeds a sequential approach is best,”
Keeling said. “If morningglory is the worst problem, apply Liberty first
then follow with Roundup.”
He said dicamba
with glyphosate offers improved control of problem annual and perennial
weeds as a burndown or as an in-season treatment. “It’s especially
helpful with glyphosate-resistant pigweed.”
New technology
with tolerance to dicamba and glyphosate may pose some drift issues,
Keeling said. Nozzle selection may help. He recommends coarse to very
coarse spray and ground-only application.
He said a new
technology, Enlist weed control system, which enables plants to have
tolerance to 2, 4-D, may be approved for corn and soybean production in
2014. Approval in cotton may be 2016. Keeling said trials have shown “no
significant injury to cotton.”
But he expressed concern with drift with either the dicamba or 2, 4-D products.
Tillage
practices also may play a role in herbicide resistance management,
Keeling said. “Conservation tillage protects the soil (and seedlings)
from blowing, but it complicates weed control. Weed control, residue
management and stand establishment are all considerations with
conservation tillage. It is important to kill small weeds.”
Keeling said
identifying glyphosate resistant weeds in the Texas High Plains was not a
welcome event but not necessarily a surprising one either. A history of
battling Palmer amaranth and following sound weed control programs
delayed the onset of resistance to some degree and cotton farmers’
experience with controlling pigweed at least gives them a head start on
managing the problem
And new production techniques and technology will aid their efforts, Keeling said.
