In the News - March 2008
Soybean weed control program begins at burn-down
LONOKE, Ark. - Starting with a good weed burn-down program in soybeans is
often the most important decision a farmer will make for the whole year,
according to Dr. Bob Scott, extension weed scientist with the University of
Arkansas Division of Agriculture.
"Weeds have evolved, especially in reduced tillage systems that are
complicating many burn-down scenarios," Scott said.
"Glyphosate-resistant horseweed has now been identified in almost every Delta
county in Arkansas," he said. Horseweed and other difficult-to-control weeds
such as cutleaf evening primrose, wild garlic, flowering winter annuals, eastern
black nightshade, pokeweed, common ragweed and giant ragweed are forcing farmers
to make hard decisions early in the growing season.
If horseweed isn't controlled on the front end with a good burn-down program,
he noted, then there are few good later-season options.
Scott said most burn-down programs for horseweed take place in late February
or early March. Research by the University of Arkansas and others has shown that
8 ounces per acre of Clarity (dicamba) herbicide in a tank-mix with glyphosate
is the best choice for control of this weed.
Economically speaking, glyphosate is still in the mix to control other weeds
at burn-down. This tank-mix is also effective on cutleaf evening primrose and
other tough broadleaf weeds.
"My second option is to substitute 1 quart per acre of 2,4-D for the dicamba," Scott said.
"It's typically less than $1 per acre cheaper, but can work if everything is
right."
Gramoxone and Ignite are good choices if the horseweed is small. While
neither product has a rotational issue with soybeans, both need dicamba in the
tank in order to be really effective.
Other products just don't perform as well as dicamba and 2,4-D on emerged
horseweed, he said. Remember, it's important for you need to nail the weed on
the first try. Adding 2,4-D will usually pick up the other winter annual junk
present in the field.
"You can also use combinations of reduced rates of dicamba and 2,4-D to
approach fields with mixed populations of weeds that include horseweed," he
said.
Burn-down treatments with dicamba or 2,4-D need to go out an absolute minimum
of 14 days prior to planting soybean, Scott recommended. These dates assume at
least 1 inch of rainfall.
The way the plant-back interval works is you spray your burn-down, wait until
you get a rain, then start counting. After 14 days with dicamba you can plant
your soybeans, according to University of Arkansas recommendations. These labels
vary by product, so as always read the label.
Another area of concern with these treatments is application. It's essential
to get good coverage for good control.
"Last year, common mistakes made at application were spraying in high winds
and boom height being too high for effective coverage," Scott said. "There is
less concern about herbicide drift this time of year, but it is still important
to keep it in the field.
"If the application is going out by air, be sure to talk to your
aerial applicator about what his best configuration is for this
application. Commercial applicators must understand the importance of this
treatment and do everything they can to make it work."
In areas where horseweed is bad or where pigweed is a problem, many soybean
growers are adding Valor, Synchrony XP, or Canopy EX, or another residual
component to their dicamba plus glyphosate burndown programs. These treatments
looked good in university trials over the past few years, according to Scott.
Another burn-down consideration is Palmer amaranth (pigweed). Using Valor
Pre-plant is an excellent start to a program approach for pigweed, especially
for later planted soybeans.
"Remember, if you miss horseweed early or it comes up and needs treatment
in-season in soybean, the best thing that I have looked at is a full rate (0.3
ounce per acre) of FirstRate," Scott said. Unfortunately, this is really a
payback, suppression type treatment. It will not completely kill the horseweed."
For more information about weed control, contact your county extension agent.
The Cooperative Extension Service is part of the U of A Division of Agriculture.
March 14, 2008
Media Contact: Lamar James
Extension Communications Specialist
U of A Division of Agriculture
Cooperative Extension Service
(501) 671-2187 or (501) 753-0207
ljames@uaex.edu
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