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: both Woodruff and Independence counties in the Three Rivers Edition coverage area, as well as Lawrence and Clay. 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 1980s, 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 Ph.D. 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 turnaround time in detecting resistance.
“In the past, we used to have to grow these things out, collect seed, plant 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 is on the market yet. Barber said switching to one of these new technologies may be a smart move in the coming season.
“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 Xtend crops.”
“Probably 75 to 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.”
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.