There have been several phone calls over the past few weeks about Palmer amaranth (Palmer pigweed). Several producers and local agronomists are noticing that it is not being controlled effectively in places with Glyphosate. I was e-mailed a news release this week that will give some information about what is being observed in the state, especially in Central Kansas at this time. As always, if you have any questions or concerns, you can get a hold of me by phone, email or stopping in the Extension office.
Judging by the number of phone calls he receives, Dallas Peterson is very popular this time of year.
The typical topic is weeds – something Peterson, a Kansas State University agronomist, knows well.
Spurred by late spring and early summer rainfall, farmers’ row crops across much of the state are thriving. And so are the weeds they’re trying to control, including Palmer amaranth, an aggressive and invasive weed that used to be controlled by the popular herbicide glyphosate. Increasingly, however, Palmer amaranth is resisting glyphosate.
“We have had numerous calls about poor control of Palmer amaranth with glyphosate this year,” said Peterson, who is a weed specialist with K-State Research and Extension. “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 summer 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 to 2 inches per day. It is competitive with crops and is a prolific seed producer, up to several hundred thousand per plant.
The hardy weed has been a serious problem in Kansas for many years and had previously developed populations resistant to atrazine and the ALS (acetolactate synthase) inhibiting herbicides. With the introduction of Roundup Ready crops in the late 1990s, glyphosate helped to solve some of those problems initially, but glyphosate resistance has now become a problem because of the heavy reliance on it for weed control.  
Glyphosate resistant Palmer amaranth first showed up in the southeast United States 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.”
“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,” he said.  
Alicia Boor is the Agriculture and Natural Resources agent for Barton County K-State Research and Extension. You can contact her by e-mail at aboor@ksu.edu or calling 620-793-1910