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By Ryan McGeeneyThe Cooperative Extension ServiceU of A System Division of Agriculture
LONOKE, Ark. — Weed experts with the University of Arkansas System Division of Agriculture
are warning soybean growers to prepare for a challenging season ahead as one of the
area’s most popular herbicides meets with increasing resistance from a pervasive weed.
Bob Scott, extension weed specialist with the Division of Agriculture, said PPO-resistant
Palmer Amaranth — commonly known as pigweed — has been confirmed in soybean fields
in four counties in northeast Arkansas: Woodruff, Lawrence, Clay and Independence.
Scott said “suspect” samples from Phillips and Crittenden counties have also been
sent to the University of Illinois for testing.
“It’s an evolving thing right now,” Scott said. “There are more samples that have
been sent in for testing, so I expect that number to continue to increase.”
Protoporphyrinogen oxidase inhibitors, commonly known as PPO, are the chemical basis
for some of the most popular and effective pre- and post-emergence herbicides in use
throughout the region. Scott said he and other agricultural experts have long believed
it was only a matter of time before pigweed, which affects a variety of crops, would
develop a resistance to the herbicide family.
“We’ve been relying very heavily on this class of chemistry for pigweed control for
several years now,” Scott said. “Unfortunately, as history has shown, when a class
of chemistry works well — in this case, it seems to be a whole lot of Valor herbicide
applied pre-emergence, followed by Flexstar applied post-emergence —we have a tendency
to over-rely on it, to use it over and over again.”
Tom Barber, extension weed scientist for the Division of Agriculture, said PPO herbicides
have been used in farming for about four decades.
“We’ve used them forever,” Barber said. “They’ve been a mainstay over the years, even
before Roundup-Ready crops. Since the early 1980’s, we’ve used PPO herbicides for
broadleaf weed control in soybeans and rice. “
Scott said the first samples of PPO-resistant pigweed were collected from Lawrence
County in 2011 by a University of Arkansas graduate student, although the samples
weren’t tested until 2013.
“It kind of got lost in a PhD student’s work in Fayetteville,” Scott said. “It was
an isolated incident back then. Now it seems like the stuff’s popping up everywhere.”
Researchers in both Arkansas and Tennessee detected new PPO-resistance in July after
sending samples to the University of Illinois, where Pat Tranel, Professor of Molecular
Weed Science, developed a test to identify the point-source gene mutation that allows
for PPO resistance. Scott said the test has significantly reduced turn-around time
in detecting resistance.
“In the past, we used to have to grow these things out, collect seed, pant it in a
greenhouse, then spray herbicide on it when it came up,” he said.
Scott said researchers and farmers throughout the state don’t yet have a clear picture
of how prevalent the PPO-resistant pigweed is, but said growers would be well-served
to begin weighing their options now.
“A lot of the growers are asking, ‘well, what are the alternatives to this chemistry?’”
Scott said. “Unfortunately, because of the previous development of resistance to other
modes of action in Arkansas, we’re really down to only a couple left.”
Although two new herbicides, Dow Agroscience’s Enlist Duo (glyphosate plus 2,4-D and
choline) and Monsanto’s M1691 (dicamba), were provisionally approved for use by the
Arkansas State Plant Board in December 2014, neither are on the market yet. Barber
said switching to one of these new technologies may be a smart move in the coming
“Enlist Duo herbicide currently has a federal label and is registered for use on Enlist
soybean technology,” Barber said. “Limited quantities of seed should be available
for growers to try in 2016. Xtend soybean technology which provides crop tolerance
to dicamba herbicide has been deregulated and some seed supply should be available,
but as of today, even if growers could switch to Xtend technology, no formulations
of dicamba have been approved for use, or have a federal label for application on
“Probably 75-80 percent of all soybeans in Arkansas are roundup-ready technology,”
Barber said. The remaining acreage is split between Liberty-Link beans and conventional
soybeans, he said.
“I think these growers need to start looking a little closer at Liberty beans,” Barber
said. “I think Liberty offers another alternative as a post-emergence control option
for pigweed, and we ought to rotate into that system on more acres.
“Regardless of the technology used in future growing seasons, the fact that PPO resistance
is here drives home the point that multiple herbicide modes of action are needed in
every weed management plan, regardless of the crop grown,” he said.
Scott said that without overstating the problem, growers should take the appearance
of the PPO-resistant pigweed seriously.
“I think it’s an alarm bell,” Scott said. “Pigweed can spread via pollen and by seed
movement on equipment. Just muddy tires can move this seed from one field to another.
“We’re going to go at this thing like we have a severe problem,” Scott said. “We’re
going to make recommendations for growers to take action. Even if they don’t think
they have a problem, now would still be a good time to alternate your chemistry, before
this problem develops.
“It gets worse, worse and worse if we just keep doing the same thing,” he said.
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action institution. If you require a reasonable accommodation to participate or need
materials in another format, please contact your County Extension office (or other
appropriate office) as soon as possible. Dial 711 for Arkansas Relay.
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Media Contact: Mary HightowerDir. of Communication ServicesU of A Division of AgricultureCooperative Extension Service(501) firstname.lastname@example.org