By Brad Haire
University of Georgia
Georgia has the world’s first population of Palmer amaranth
weeds
resistant to glyphosate, a herbicide commonly sold under the
brand name Roundup. This will cause problems for cotton farmers,
says a University of Georgia weed specialist.
Right now, Roundup-resistant Palmer amaranth is known to infest
about 500 acres of cotton in central Georgia. Stanley Culpepper,
a UGA Cooperative Extension weed scientist studying the
outbreak, said seeds from at least 100 fields in the area have
been harvested to determine any further distribution.
A wicked weed
“This could be a real threat to future cotton production in our
region,” he said. “It’s the one weed cotton farmers didn’t want
resistant to Roundup.”
Palmer amaranth, also called pigweed, is found throughout the
state. The troublesome weed can quickly grow more than 8 feet
tall with a thick stalk and suck valuable nutrients from nearby
plants. It can clog a cotton picker, too, making it hard to
harvest the crop.
In 1997, farmers started planting cotton that was developed to
stay healthy when sprayed with Roundup. They could spray the
herbicide over the top of this cotton, killing weeds, but not
the cotton. This saved farmers time and money because they
didn’t have to repeatedly plow between rows to kill weeds.
“Roundup Ready” varieties cost more than conventional cottons.
But farmers gladly embraced the new technology, Culpepper said.
About 94 percent of Georgia’s 1.21 million acres of cotton this
year is Roundup Ready.
“Roundup has been our most effective tool to manage this weed in
Roundup Ready crops,” he said. “Most alternative control options
are much less effective than Roundup in controlling a normal
population of Palmer amaranth.”
True resistance
Each year, some Georgia farmers have to deal with some Palmer
amaranth plants that continue to grow after a spray with
Roundup. This usually happens due to weather conditions or
improper spraying.
Specialists with the UGA College of Agricultural and
Environmental Sciences last fall suspected some Palmer amaranth
weeds in central Georgia had resistance to Roundup. Many field
and greenhouse trials and heritability studies now show that the
Palmer amaranth population in central Georgia has true
resistance, he said.
Scientists with Monsanto, registrant of Roundup Ready, are
providing technical expertise and other help to address the
problem, Culpepper said.
Farmers need to watch their fields carefully this year and
remove any Palmer amaranth not hurt after a spray with Roundup,
he said. This could help keep resistant plants from spreading.
It’s too early to say what long-term effect this will have on
cotton production in Georgia. But if farmers are no longer able
to control this weed with Roundup, things will have to change.
Farmers may once again have to plow fields to manage pigweed,
Culpepper said. This will cost them time and money. The
resistant weed could keep farmers from using conservation
tillage, too, an environmentally friendly process that builds
soils.
Too much
Farmers have relied too heavily on Roundup to control weeds in
cotton, Culpepper said. This has given nature the upper hand.
Herbicides don’t cause a plant like Palmer amaranth to change
genetically or become a resistant mutant, he said.
All it takes is one weed plant in a field to be genetically
different — in this case, resistant to glyphosate. All the
other weeds are killed when sprayed, but not the resistant one.
It makes seeds. The next year, a few more resistant plants grow
from those seeds. If the process is allowed to continue, the
offspring of that one resistant weed could eventually cover the
field.
This is what has happened in central Georgia. But it could
happen anywhere, Culpepper said.
