Mississippi County
Agriculture
Glyphosate Resistant Horseweed Confirmed in County
Glyphosate resistant horseweed has been documented in Mississippi County. In
the spring of 2003 many producers were reporting fields where glyphosate failed
to control horseweed or marestail. Dr. Ron Talbert, Professor and Weed Scientist
with the University of Arkansas, Fayetteville, traveled to Osceola to sample
populations of horsweed with suspected resistance to glyphosate. Greenhouse
studies revealed that the plants were resistant to applications of Roundup
WeatherMax at four to twelve times the normal use rate. That translates into
applications of more than two and a half gallons of Roundup WeatherMax per acre.
Horseweed is not controlled 100% of the time by glyphosate applications. The
2004 Recommended Chemicals for Weed and Brush Control manual from the University
of Arkansas lists glyphosate as providing only 70% control of horseweed. The
typical horseweed plant produces 50 to 200,000 seeds. With 30% of the plants
surviving and potentially going to seed, a huge seed bank of horseweed that can
tolerate the normal use rates of glyphosate is created. The adoption of no-till
or reduced tillage systems has compounded the problem by eliminating tillage
passes that previously controlled horseweed. As a result, farmers have
unintentionally selected a naturally occurring resistant population of
horseweed.
Confirming the presence of resistant horseweed in Mississippi County is
important. Determining how to control glyphosate resistant horseweed is a top
priority. The best options for resistant horseweed control will include 2,4-D or
Clarity in the burndown program. Be sure to follow the plant back restrictions
on the product label. Currently 2,4-D must be applied 30 days prior to planting.
The plant back interval for Clarity is 21 days per 8 ounces used plus one inch
of rainfall.
Horseweed emerged in the late spring and was hard to control in 2003. Few
herbicides will control resistant horseweed after cotton emergence. In a
Fayetteville field study, Gramoxone, Gramoxone + Direx, Buctril, 2,4-D, Liberty,
Clarity, MSMA, Roundup WeatherMax, MSMA + Direx, and Stinger provided greater
than 90% control of a glyphosate susceptible horseweed population six weeks
after treatment. Envoke, Firstrate and Gramoxone + Canopy provided 87 to 89%
control by six weeks after treatment. Caparol, Cotoran, & Direx alone did not
control horseweed more than 20%. The diphenylether herbicides like Goal, Cobra,
Reflex, and Blazer did not control horseweed adequately. Command, Staple, Aim,
and Valor also provided less than 20% horseweed control.
One mistake a producer can make is ignoring the issue of herbicide
resistance. If you were able to control horseweed last year with glyphosate
products you might not be so fortunate this year. The presence of glyphosate
resistant horseweed in Mississippi County means that every farmer should adopt a
weed control program to manage horseweed as if it is resistant. The easiest way
to have a weed free crop to harvest is to control weeds before or as soon as
they emerge.
Information about resistant horseweed was presented at the Southern Weed
Science Society’s annual meeting in Memphis, TN. View slide show in
PowerPoint or
PDF.
The horseweed plants in the pots labeled “R” for resistant were taken from a
field in Osceola. They were able to tolerate an application rate up to 4 times
the normal use rate. The plants in the pots labeled “S” for susceptible were
taken from a field near Fayetteville, AR with no history of glyphosate
application.
These plants were from Pritchett’s Corner in Poinsett County. These resistant
horseweed plants were able to withstand an application of glyphosate at 16 times
the normal use rate.
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