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Invasive Weed Taking Hold in Kansas
USAgNet - 07/24/2014

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.

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