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Palmer amaranth continues to wreak havoc in farm fields across U.S.
August 3, 2014
SPURRED by late-spring and early-summer rainfall, row crops across many states are thriving, but so are the weeds, including Palmer amaranth, an aggressive and invasive weed that used to be controlled by the herbicide glyphosate.
Increasingly, however, Palmer amaranth is resisting glyphosate.
"We have had numerous calls about poor control of Palmer amaranth with glyphosate this year," Dallas Peterson, a Kansas State University agronomist, said. "Glyphosate-resistant Palmer amaranth was first confirmed in Kansas three years ago but seems to be exploding across central Kansas this year. Poor control doesn't mean you have resistance, but if the herbicide treatment provides good control of some plants and not others, that is an indication you may have resistance."
The weed, also known as Palmer pigweed, is a warm-season annual weed that generally starts to emerge in May as soil temperatures warm and continues to germinate into summer, especially following rainfall events, Peterson said. It grows rapidly with hot conditions, maybe as much as 1-2 in. per day. It is competitive with crops and is a prolific seed producer, up to several-hundred-thousand seeds per plant.
The hardy weed has been a serious problem in several states for many years, and some populations had previously developed resistance to atrazine and the acetolactate synthase-inhibiting herbicides. With the introduction of Roundup Ready crops in the late 1990s, glyphosate helped solve some of those problems initially, but glyphosate resistance has now become a problem.
Glyphosate-resistant Palmer amaranth first showed up in the southeastern U.S. and has had a dramatic impact on farmers' production systems and weed control costs there.
"Producers need to use an integrated approach to weed control that utilizes a variety of cultural practices and herbicide modes of action to help control weeds and minimize herbicide resistance," Peterson said. "The use of effective pre-emergence residual herbicides is probably going to be very important to help manage Palmer amaranth in the future.
"If a producer notices just a few scattered Palmer amaranth that have escaped a glyphosate treatment, it may even be worth hand-removing those from fields to prevent seed production. If not, the resistant biotypes will increase and get spread across the field and to other fields by the combine," he said.
"If poor control was achieved with glyphosate, it is probably best to assume that it is resistant and plan accordingly both this year and in the future," Peterson said.
In addition to using a variety of herbicides to fight pigweed, University of Missouri weed scientist Kevin Bradley also recommends deep soil tillage for extreme cases.
"I'm not advocating a return of moldboard plowing, (but) burying weed seeds with deep tillage does provide one more tool in dealing with resistant pigweeds," he explained.
Bradley recently spoke to producers about the extreme control methods they will need to use for Palmer amaranth and waterhemp — both herbicide-resistant weeds.
In extreme cases, resistant pigweed can overshadow a soybean field, sharply reducing yields.
"A return to tillage can help kill resistant weeds. Pigweed seeds like waterhemp, and Palmer amaranth germinates only in upper soil layers, so they thrive in no-till fields," Bradley said. "Without soil turnover, weed seeds remain at shallow levels."
When control is gone and resistant weeds take over, then bury the seeds 6 in. or more, Bradley explained.
The second part of the recommended control strategy is to never allow resistant weeds to set seeds again; kill them before they set seed.
"Palmer amaranth and waterhemp control will take management of the seed bank in the soil," Bradley said. Deep tillage, which he called extreme tillage, can be an option once it infests a field. Then, growers can return to no-till with more careful management after that.
In recent years, herbicide control won out over tillage by moldboard plowing. With these tough species, it's something to consider again, Bradley said.
"Palmer amaranth is our number-one weed to watch in ... the Midwest right now," Bradley said. "It's probably too late to lock the barn door; this weed is out there more than we know. Most farmers don't even know they have it."
Cotton growers in Texas who are also battling Palmer amaranth recently petitioned the Environmental Protection Agency to use an emergency application of propazine for 3 million acres of cotton, but EPA rejected the request.
Shawn Wade, director of policy analysis and research for Plains Cotton Growers Inc., said EPA conducted a risk assessment but would not add another use for the class of chemical.
Approval would have provided one more "tool in the toolbox" to fight the weed, and Wade said only about 10% of those acres would have needed the application. The cotton growers plan to revise the petition to reflect more conservative numbers and resubmit it in 2015.
The economic impact of the weed this year remains unknown, but Wade said some farmers had to abandon some fields that were unsalvageable, and harvest will be challenging in fields with dense pigweed populations.
Additionally, he said some farmers have to hire workers to hand-pull the weed, while others have to use additional herbicide applications for variety. There is also a potential for yield and harvest losses, according to Wade.
"It's something that producers are just going to have to deal with and continue to fight this growing season," Wade said, adding that educational efforts to teach producers about how to control the weeds also need to continue.
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