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August 18, 2003

Herbicide-resistant weed puts down roots in Indiana cropfields

WEST LAFAYETTE, Ind. – Some weeds have a never-say-die attitude about herbicides, and to many farmers' dismay, one weed standing tall against chemical deterrents is marestail.

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For the past three years marestail populations east of the Mississippi River have exhibited resistance to glyphosate, the active ingredient in many popular herbicides, including Roundup. In 2002 the first glyphosate-resistant marestail in Indiana was discovered in Jackson County. It has since spread to Bartholomew, Clark, Jefferson and Jennings counties.

Now that glyphosate-tolerant marestail has entered the Hoosier state, row crop producers – especially those who grow soybeans in a no-till system – should be on the lookout for populations that are unfazed by the herbicide, said Jeff Barnes, Purdue University weed science research associate.

Farmers who suspect their fields harbor glyphosate-resistant marestail can come closer to confirmation by examining the weed control history of their crop acres and noting which weeds are surviving herbicide applications, Barnes said.

"You might look for fields that have a very high population of marestail growing above the soybean canopy and not very many other weeds," Barnes said. "That would be one indication – you've controlled all of your weeds, but you're not controlling marestail. It also would rule out the potential that you just had a bad herbicide application or something happened when you were mixing the herbicide.

"In that case you'd be either in a situation where the horseweed was too large, which could have been a problem, or it does have resistance."

Marestail also is known as horseweed.

A stronger sign that glyphosate resistance has occurred is the presence of healthy marestail in a field full of dead weeds.

"Many times what you'll see in fields that have resistance is a live plant and a dead plant right beside each other," Barnes said. "You can have 10 dead plants and one live plant in the same area, or vice versa. They were probably all about the same size when they were sprayed, but for whatever reason, whether it is resistance or just plant stress, one plant died and one didn't.

"If you've been in continuous soybeans and continuous glyphosate for your burndown and your in-crop weed control, then you do have more potential for resistance than if you've been in a rotation system and have been using other herbicides."

Soybean growers rely heavily on glyphosate. This past spring, the U.S. Department of Agriculture estimated that 83 percent of the nation's soybean acres would be planted to Roundup-tolerant varieties. In Indiana, the percentage of soybean acres expected to be planted to Roundup Ready seed was even higher – 91 percent.

Marestail presents a greater threat to soybeans than corn for other reasons, as well. The weed can become established before soybean plants emerge. The weed grows taller than soybeans, thus competing for sunlight and other plant nutrients. Left undisturbed, marestail can reach a height of 7 feet.

The weed also creates problems at harvest. Marestail plant tissue can get mixed in with soybeans in the combine, reducing the crop's market value.

Marestail is not as significant a problem in corn because corn usually is planted earlier than soybeans and grows taller than the weed.

"In 1995 marestail was considered one of the worst weeds in Indiana and, particularly, in no-till fields," Barnes said. "In no-till when you're trying to plant you want a nice, clean seedbed. Those weeds that have already emerged before you plant your crop are going to be very competitive and will have the potential to decrease your yields."

Glyphosate-resistant marestail populations are expanding, in part, because the weed travels by wind, Barnes said.

"Marestail can spread very quickly because it's a wind-blown seed," he said. "Some research has shown the seed can go up to a half mile away in a fairly low wind, so it has the potential to spread over a wide distance. It also has very rapid growth. As temperatures begin to warm and you start getting ready to plant your crop or already have a crop in the field, the rapid growth of the marestail, in relation to the crop, gives it a competitive advantage."

Farmers can continue using glyphosate-based herbicides to control resistant marestail if they also use FirstRate and Classic brand products, Barnes said. FirstRate and Classic can be applied prior to or after soybean planting. With corn, 2,4-D and dicamba have proved to be effective in knocking out marestail, Barnes said.

Marestail is among more than 240 weeds worldwide that have developed resistance to herbicides. Herbicide-resistant populations of jimsonweed, pigweed, lambsquarter and giant and common ragweed are present in Indiana.

More information about glyphosate-resistant marestail and control options is available in the paper "Identifying Glyphosate-Resistant Marestail/Horseweed in the Field," by Barnes and Purdue weed scientists Bill Johnson and Glenn Nice. The paper can be downloaded online at or by logging onto the Purdue Extension Weed Science Web page.

Writer: Steve Leer, (765) 494-8415,

Source: Jeff Barnes, (765) 494-4645,

Ag Communications: (765) 494-2722; Beth Forbes,;

Related Web site:
Purdue University Department of Botany and Plant Pathology


Glyphosate-resistant marestail (right) and nonresistant marestail stand side by side in an Indiana field. (Photo/Purdue University Department of Botany and Plant Pathology)

A publication-quality photograph is available at

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