Farmers Face Added Cost Of Battling Resistant Weed
Bruce Schultz, LSU Ag Center
Tests by the LSU AgCenter have confirmed herbicide-resistant pigweed
at three locations in north Louisiana.
joined the party,” said Daniel Stephenson, LSU AgCenter weed scientist
who conducted the testing.
this party is no celebration – rather, a meeting of the minds
by LSU AgCenter scientists to figure out how to combat the problem.
evaluating alternative weed control programs,” said Jim Griffin,
LSU AgCenter weed scientist. “We’ll assist growers in planning
control programs where weed control issues have occurred.”
of the fields were in Tensas Parish, and the third was in Franklin Parish.
All three populations are resistant to the herbicide glyphosate, which
includes Roundup as well as many other glyphosate products.
Ready soybeans were grown in one of the Tensas Parish fields, while
Roundup Ready cotton was grown in the other two fields, according to
Donnie Miller, LSU AgCenter weed scientist and director of the Northeast
year, Palmer amaranth (pigweed) from Concordia Parish and johnsongrass
in Pointe Coupee Parish were confirmed as resistant to glyphosate.
states from Iowa to Georgia, including Arkansas and Mississippi, have
had severe outbreaks of herbicide-resistant weeds, including pigweed,
specifically Palmer amaranth.
Griffin said it’s not a surprise that the problem finally surfaced
in Louisiana. “It was not a matter of if, but when.”
explained that herbicide resistance is the result of accelerated evolution.
process begins with just a few plants with the genetic capacity to survive
the herbicide treatment. It is believed that these plants, which occur
naturally in the population at a very low level, are not a result of
genetic mutation caused by the herbicide. These inherently resistant
plants, when exposed to the same herbicide over several years, produce
seed. Over time the population slowly shifts such that the resistant
weeds become dominant. Since this process is slow, the producer may
not notice the problem until large scale weed control failures occur,”
pigweed from Concordia Parish survived amazingly high rates of glyphosate,
“To kill half the population would have required 54 times the
normal rate,” Stephenson said.
Concordia Parish weed was found in 2009 from a field where Roundup Ready
cotton had been grown in four consecutive years. In 2009, the farmer
noticed pigweed that had not been killed by an aerial application of
glyphosate, Stephenson said. Samples of those plants were used to obtain
seeds that were grown in a greenhouse and tested with varying rates
same protocol was used on johnsongrass that could not be killed with
glyphosate alone. It was found in a field in Pointe Coupee Parish where
Roundup Ready soybeans had been grown for 10 years.
in Concordia Parish, a cousin of Palmer amaranth, tall water hemp, is
suspected of having herbicide resistance, and it has been confirmed
in Mississippi by Mississippi State University, he said. In other states,
including Arkansas and Mississippi, herbicide-resistant weeds have caused
headaches for farmers, with reduced yields and harvest problems caused
by the large weeds.
amaranth can grow up to an inch a day, expanding to a 4- to 6-inch diameter
trunk that can damage harvest equipment. It thrives in heat that normally
would suppress other weeds, Stephenson said. Its pollen can move up
to 600 meters, making neighboring fields vulnerable. One plant can produce
up to 2 million seeds. Their small size makes it impossible to clean
all seeds from farm equipment.
weed experts say the problem in Louisiana is manageable.
“Louisiana is at a point where we can mitigate this,” Stephenson
said. “We’ve got the opportunity to not be like our neighbors.”
said growers in Concordia Parish are working together to fight the weed.
They are using herbicides with residual action.
said alternative herbicides, such as Valor, Dual, Reflex or Magnum,
which have different modes of action to kill pigweed, can be used separately
or with glyphosate.
Miller said chemical companies are offering rebates and other incentives
for growers to use alternative herbicides.
warned that using reduced rates of herbicides to save money is false
During field scouting for insects and diseases, he said, farmers can
destroy weeds to prevent seed production.
Liberty link soybeans where Ignite is used for weed control would also
be a means to manage glyphosate-resistant weed problems,” Griffin
The Liberty Link gene is also available for other crops, including corn
Griffin said crop rotation is another strategy, provided the rotational
crop would be something other than one that relies on glyphosate for
primary weed control. For example, he said, rice would be a good rotational
said addressing the problem will cost farmers more money, but it is
less expensive than what farmers are facing in Arkansas. “It comes
down to pay me now, or pay me later.”
He said the Palmer amaranth problem may be overshadowed by the herbicide-resistant
Palmer, I have bullets to control it, but with johnsongrass, I only
have one bullet,” he said. That is because in some areas of the
state johnsongrass has already been shown to be resistant to herbicides
that could be used as alternatives to glyphosate. That puts us in a
He said chemical companies are working on new compounds to fight the
weeds, but a new product is at least 10-12 years away. Weed scientists
agree that glyphosate was the discovery of a lifetime, and nothing like
it is on the horizon.
is no silver bullet coming,” Stephenson said.
was discovered by a team of researchers led by John E. Franz, a Monsanto
Co. chemist, in 1970.
herbicide kills plants by interfering with an enzyme required for growth.
It is absorbed through its leaves and moves throughout the plant, and
it is quickly broken down in the soil by bacteria.
herbicide-resistant crops has enabled farmers to lessen their environmental
impact by reducing their reliance on plowing to kill weeds. Using minimal
or no-till practices leaves soil more intact and reduces runoff from
fields into waterways. It also means farmers don’t have to burn
as much fuel to grow a crop, keeping food prices lower and helping farmers
cut their expenses.
Roundup Ready technology has been accepted in cotton, soybeans and corn
in the northern United States where sugar beets are grown, litigation
involving the Roundup Ready version of that commodity has growers in
a quandary. A federal judge has ruled that Roundup Ready beets cannot
be planted this year until a full environmental impact study is conducted
for the genetically altered variety, even though the genetically modified
variety has been grown for the past six years on 95 percent of the sugar
beet acreage in 10 states.
U.S. Department of Agriculture decided this month it will allow Roundup
Ready alfalfa to be commercially grown. The USDA reversed an earlier
decision that would have imposed restrictions on where it could be grown.
A 230-page environmental impact study was completed, but opponents are
contending it was not an in-depth examination.