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Corn+Soybean Digest

Controlling Super Weeds

There's no mystery ammunition when it comes to controlling “super weeds” like marestail, common lambsquarters and giant foxtail, according to Bill Johnson, Purdue University weed specialist.

Producers are facing bigger challenges in combating tough winter annuals and simple perennials, he says. Johnson has been surveying farmers in Indiana over the years and says winter weeds — especially marestail, also known as horseweed — are becoming more prevalent and persistent.

“Many of the fields may look clean from the road. However, once you start scouting these soybean fields you'll find that many of them have been infested with marestail,” he says. “This weed is definitely becoming a bigger problem.”

Johnson estimates that glyphosate-resistant marestail has infested at least 20-25% of the counties in Indiana. In southeast Indiana, infestation rates of up to 90% can be found in the hardest hit counties.

In 2000, marestail was the first glyphosate-resistant weed to appear in soybean fields in the U.S., according to a white paper by the University of Wisconsin — Extension (UW).

Initially reported in Delaware, there are now nine states with glyphosate-resistant marestail (Delaware in 2000; Tennessee in 2001; Indiana, Maryland, New Jersey and Ohio in 2002; and Arkansas, Mississippi and North Carolina in 2003). Six weeds have developed resistance to glyphosate in the past eight years, according to the UW white paper.

Johnson says glyphosate-resistant marestail problems have also been reported in parts of Illinois, Kentucky and Ohio.

Marestail and other winter annual weeds have become more of a problem due to a growing resistance to glyphosate-based herbicides, he says.

“There are no magic bullets because herbicide discovery efforts are low right now; there's not much new material in the commercial pipeline,” says Johnson.

There are many reasons why herbicide resistance is a growing concern. “Among them include the widespread adoption of Roundup Ready soybeans, increased use of no-till practices, and the lack of cheap herbicide alternatives to glyphosate (aka Roundup),” he says.

Aaron Hager, crop scientist at the University of Illinois (U of I) agrees that herbicide resistance has increased.

For example, by 1993 U of I researchers had documented herbicide resistance in only three weed species biotypes in Illinois, and this resistance was limited to only one herbicide family. However, by 2003 the list of herbicide-resistant weed biotypes in Illinois had grown to include 10 species, and resistance had spread to include four herbicide families.

The most recent addition to the list of herbicide-resistant biotypes is a waterhemp population with resistance to three herbicide families — namely, triazine, ALS (acetolactate synthase) inhibitors and PPO (protoporphyrinogen oxidase) inhibitors.

While the majority of herbicide-resistant biotypes in Illinois are broadleaf-species, Hager says concern is growing over resistance to ALS-inhibiting herbicides among grass species like giant foxtail and shattercane.

“This increasing occurrence continues to reduce the effectiveness of many herbicide options,” he says.

Based on experiences in Indiana, Johnson estimates that the added costs to control marestail can easily range anywhere from about $1 to $7/acre or more. “That cost largely comes from the need for a second herbicide tankmixed with glyphosate or applied in a separate trip across the field,” Hager says.

Competition for moisture and poor pod set can also reduce yields dramatically, and marestail can even be a host to the plant disease aster yellows.

Other problematic weeds include: common lambsquarter, common ragweed, giant ragweed and Eastern black nightshade. In corn, giant ragweed and bur cucumber have caused problems in Indiana, Johnson says.

He adds that perhaps the best way to combat so-called “super weeds” is to get them before they get super tall.

“What we're telling producers is to control them when they are small (less than 6 in. tall) and not to go after marestail with just glyphosate,” he says. “They need to use another mode of action on it; or they need to use tillage. We're finding in some of our work from 2003 that about 20% of the populations that are resistant to glyphosate are also resistant to Classic and First Rate herbicides. So they're cross-resistant to arguably the second best herbicides or treatment for it.”

Johnson says using more than one herbicide can help control and minimize the spread of glyphosate-resistant weeds.

In the meantime, he recommends scouting fields in early spring for weed development.

“Look for emerging weeds in March, and then plan to spray in April,” Johnson suggests. “If weeds get too big or re-emerge after planting soybeans, then it's difficult to control them.”

Not all weeds that survive the first application of herbicide should be considered resistant, he says. “It could be that the application rate was too low, the spray coverage was spotty or the environmental conditions were not optimal for herbicide activity.”

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