Populations of Palmer Amaranth resistant to glyphosate were first documented in Kansas three years ago. At that time, these populations were limited in range to isolated areas of south central Kansas. Glyphosate resistant Palmer Amaranth has gradually expanded the last couple of years, and now appears to be increasing rapidly, especially through the central part of the state.
Several other weeds have also developed glyphosate resistance in Kansas, including common waterhemp, marestail, kochia, common ragweed, and giant ragweed.
Glyphosate resistance can be confirmed with greenhouse and laboratory tests, but at this point that is probably no longer necessary. If a few Palmer Amaranth plants or patches of plants survived where glyphosate was applied at the recommended rate with the appropriate adjuvants and spray coverage was good, there is a good chance those plants are resistant.
Surviving Palmer Amaranth among dead plants treated with Roundup PowerMax at 32 fl oz/acre when plants were about 6 inches tall. Photo taken at Ashland Bottoms near Manhattan six days after treatment. Plants were confirmed to be resistant by an enzyme assay in the laboratory. Photo by Dallas Peterson, K-State Research and Extension.
Where glyphostate-resistant Palmer Amaranth occurs, producers should plan on taking the necessary control measures. Another application of glyphosate alone probably will not help. It may be possible to use an alternative to glyphosate as a post-emergence treatment in-season this year, although most post-emergence alternatives are effective only on smaller plants.
In soybeans, the only alternatives would be PPO-inhibiting herbicides such as Cobra, FlexStar, Marvel, or Ultra Blazer. If there are just a few scattered plants, removing them by hand or other mechanical methods before they go to seed may help prevent them from spreading rapidly in the field. If those weeds remain growing in the field and set seed, the action of a combine will spread the seed throughout the field, as well as to other fields.
In fallow, it is best to use glyphosate as a tank-mix with 0.25 to 0.5 lb ae of 2, 4-D or Dicamba. Treat fallow as soon as possible because the larger the glyphosate-resistant Palmer Amaranth gets, the harder it is to control.
In the future, growers need to consider using a more integrated weed management approach that includes cultural practices and multiple herbicide modes of action, especially preplant and pre-emergence residual herbicides. Scout fields early after crop emergence and make timely post-emergence applications with the appropriate herbicides for control of escaped weeds.
-Dallas Peterson, Weed Management Specialist
Glyphosate-resistant Palmer Amaranth in Kansas?
Glyphosate-resistant weeds continue to be an increasing problem in Kansas. Glyphosate-resistant marestail, common waterhemp, common ragweed, giant ragweed, and kochia have been previously confirmed in Kansas and have become very problematic in certain areas of the state.
Glyphostate-resistant Palmer Amaranth is a serious problem in the Southeast U.S. and has dramatically impacted weed control programs and even cropping systems. Palmer Amaranth is also a serious weed problem in Kansas, but glyphosate-resistant Palmer Amaranth has not been previously confirmed in the state. The hot, dry weather the last couple of years, has made it difficult to assess herbicide performance and resistance problems, but poor control of Palmer amaranth with glyphosate has raised questions about whether glyphosate-resistant Palmer Amaranth populations are now showing up in Kansas.
To attempt to very this one way or another, waterhemp and Palmer Amaranth seed were collected in the fall of 2011 from various soybean and cotton fields in eastern and south central Kansas. These fields had abundant pigweed escapes after being treated with glyphosate.
Palmer Amaranth escapes in soyeans treated with glyphosate. Photos by Josh Putman, Graduate Research Assistant, Agronomy, K-State Research and Extension.
Waterhemp and Palmer Amaranth plants from these different populations were grown in the greenhouse and treated with glyphosate at 0.75 (1X), 1.5 (2X), and 3 (4X) lb/acre to evaluate for resistance. Response of the different populations was compared to susceptible populations of both species collected from the Ashland Bottoms experiment field south of Manhattan, KS. The susceptible check populations of both species was completely controlled by all rates of glyphosate.
As expected, a number of the waterhemp populations from across eastern Kansas survived glyphosate treatment up to a 4X rate, and appeared to be resistant. Two populations of Palmer Amaranth from Cowley county in south central Kansas also had a high percentage of plants that survived the 1X and 2X rates of glyphosate and had some plants that survived the 4X rate. These Palmer Amaranth populations did not appear to be as resistant to glyphosate as some of the Palmer Amaranth from the Southeast U.S., but weren't being controlled by typical field rates in the greenhouse or with multiple applications of glyphosate in the field. Thus, it appears that glyphosate-resistant Palmer Amaranth may now be present in Kansas.
Palmer Amaranth seed was collected from additional fields this past fall and is now being evaluated in the greenhouse. Preliminary observations suggest that we are seeing similar survival after glyphosate treatments from Palmer Amaranth collected from fields south of Great Bend, along with additional populations collected south of Wichita. Research on inheritance and the mechanism of resistance will need to be conducted to further characterize and confirm glyphosate resistance in these populations. Confirming herbicide resistance is a long and detailed process.
Regardless of whether glyphosate-resistant Palmer Amaranth is present in your area now, there is a good chance it will develop at some point based upon what has happened in the Southeast U.S., especially if growers rely heavily on glyphosate for weed control.
Palmer Amaranth is an extremely competitive weed and the development of glyphosate resistance means it will require an effective integrated weed management program to achieve acceptable control. Continuing to rely only on glyphosate for weed control, will only speed up the development of glyphosate-resistant weeds and diminish its effectiveness. Utilizing residual herbicides with different modes of action throughout the cropping system will help to manage existing glyphosate-resistant weeds and slow the development of new glyphosate-resistant weed populations.
-Dallas Peterson, Weed Management Specialist
-Josh Putnam, Graduate Research Assistant, Agronomy
-Curtis Thompson, Extension Agronomy State Leader and Weed Management Specialist
K-State researchers confirm case of 2,4-D resistance in Palmer amaranth (Released March 8, 2019)
Study is first-ever confirming the noxious weed’s resistance to the common herbicide
A Kansas State University researcher is reporting the first-ever study confirming that Palmer amaranth has developed resistance to the herbicide 2,4-D, findings that may signal an important step in developing future controls for the pesky weed.
Vipan Kumar, a weed scientist at the Agricultural Research Center in Hays, said that since 2015 a few farmers had reported poor control with 2,4-D, but until now, researchers were not able to confirm the resistance levels to 2,4-D in Palmer amaranth.
“Historically, Palmer amaranth was not a problem weed in western to central parts of Kansas, but over the past 10 to 15 years, it has become a major problem and it is present in all crop situations, and even in non-cropland situations,” Kumar said.
Palmer amaranth is extremely aggressive and thus considered the No. 1 weed problem in U.S. agriculture. It is commonly found in Kansas cropping systems and negatively affects soybean, corn, sorghum, sunflower, cotton, wheat, and fallow fields. It is also a serious problem in wheat stubble.
Kumar and his research team have recently tested one strain of Palmer amaranth – known as a biotype – and the results, Kumar says, are sobering.
That biotype has been confirmed with low levels of resistance to 2,4-D, as well as resistance to glyphosate (Roundup PowerMax), chlorsulfuron (Glean), atrazine (Aatrex), and mesotrione (Callisto).
In addition, Kumar said the biotype showed less sensitivity to fomesafen (Flexstar) herbicide, a commonly-used herbicide in soybeans. He added that more research is underway to confirm if this biotype has developed resistance to fomesafen.
“This discovery confirms the first case of 2,4-D-resistant Palmer amaranth biotype that has also developed multiple resistance to four other herbicide modes of action,” Kumar said.
“We have been seeing a lot more populations with multiple resistance, especially with glyphosate, atrazine and HPPD inhibitors. There is increasing concern about Palmer amaranth’s ability to resist multiple modes of action.”
That situation has left farmers with few options to manage the weed.
“I would recommend growers use some of the pre-mixes, or tank mixes, that are still effective to control those biotypes so that they don’t get into seed,” Kumar said, noting that one female Palmer amaranth plant can produce as much as one-half million seeds.
“In addition to using effective herbicide programs, growers should look at crop rotation as a foundational practice of weed control. Grow those crops that are highly competitive with Palmer Amaranth and try to grow some cover crops if you can in the fallow land. Don’t leave fallow – that’s the weakest link in this whole system where these weed species have been gaining resistance.”
Kumar also suggests that farmers consider pre-emergent herbicides, depending on the crop being grown.
“Including PRE herbicide options can help growers manage some of these multiple-resistant weed biotypes, and delay the development of resistance in this weed,” he said.
Kumar’s research group is currently studying about 200 biotypes of Palmer amaranth collected in Kansas fields to determine the extent of multiple resistant Palmer Amaranth throughout the state. He said the group expects to continue their work well into the future.
“My idea is to determine the distribution of these multiple resistant Palmer amaranth biotypes in Kansas, and based on that biological information, what we can do in terms of alternate strategies to manage this problem weed in a cost-effective manner,” he said.
“If we are losing these tools, like 2,4-D or glyphosate, which are the key tools to control these weed species, then this is going to be economically expensive for growers. So we have to bring more tools into our toolbox to tackle these problems.”
Kumar’s findings have been published in the journal, Pest Management Science. The article, titled “Confirmation of 2,4‐D resistance and identification of multiple resistance in a Kansas Palmer amaranth (Amaranthus palmeri) population, is available online.