Glyphosate-resistant Palmer amaranth weed present in Kansas?
Kansas State University scientists say there is evidence to show that some Palmer amaranth weeds in Kansas are resistant to the widely-used herbicide, glyphosate.
“Glyphosate-resistant weeds continue to be an increasing problem in Kansas,” said K-State agronomist Dallas Peterson. “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. Palmer amaranth is a serious weed problem in Kansas but until now, glyphosate-resistant Palmer amaranth had not been confirmed in the state.”
Glyphosate-resistant Palmer amaranth is a serious problem in the Southeast U.S., said Peterson, who is a weed specialist with K-State Research and Extension, adding that it has dramatically impacted weed control programs and even cropping systems.
Hot, dry weather over the past two 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 determine if that is the case, K-State graduate student Josh Putman, working under Peterson’s direction, collected waterhemp and Palmer amaranth seed in the fall of 2011 from various soybean and cotton fields in eastern and south central Kansas. Waterhemp and Palmer amaranth plants from these different populations were grown in a K-State greenhouse and treated with glyphosate at one, two, and four times the typical field use rate of 0.75 pounds acid equivalence per acre to evaluate for resistance. The response of the different populations was compared to susceptible populations of both species collected from the Ashland Bottoms experiment field south of Manhattan. The susceptible check populations of both species were completely controlled by all rates of glyphosate.
“As expected, a number of the waterhemp populations from across eastern Kansas survived glyphosate treatment up to the four-times rate, and appeared to be resistant,” Peterson said. “Two populations of Palmer amaranth from Cowley County in south central Kansas also had a high percentage of plants that survived the one-time and two-times rates of glyphosate, and had some plants that survived the four-times 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.
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