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War on weeds

Gil Gullickson Updated: 05/09/2012 @ 9:21am Crops Technology Editor for Successful Farming magazine/Agriculture.com

If you keep controlling weeds the same way year after year, don't sweat it. You'll always have a job, along with jobs for your friends.

“Glyphosate-resistant Palmer amaranth is the species that has brought new employment opportunities to mid-South cotton fields,” jests Aaron Hager, University of Illinois Extension weeds specialist. “It has brought back chopping crews.”

In the South, chopping Palmer amaranth is the only way to rid fields of it. Resistance has rendered herbicides useless in many cases.

Learn more about the history of herbicides and herbicide-resistant weeds

These may be a predictor of things to come in the Midwest.

“When I look at fields in the Midwest and see waterhemp and giant ragweed problems, it looks like Arkansas in 2005 and 2006,” says Bob Scott, University of Arkansas Extension weeds specialist “We knew we had a problem, but it wasn't bad enough to take action. Using glyphosate-tolerant crops and applying glyphosate after glyphosate have led us to the problems we have.”

Download the War on Weeds game!

Weed resistance to herbicides isn't new, says Mike Owen, Iowa State University (ISU) Extension weed specialist. Resistance to atrazine, a popular corn herbicide, first surfaced in the U.S. in 1970. ALS inhibitors, a widely used herbicide mode of action, suffered rampant weed resistance in the early 1990s following repeated applications of the then-popular herbicide Pursuit.

“Today in Iowa, you can't swing a dead cat by the tail without hitting an ALS-resistant weed,” jests Owen. “With glyphosate, the same thing is happening again.”

Waterhemp way

Waterhemp is the weed that changed the rosy Roundup Ready weed-control scenario of the 1990s.

When Pat Tranel took an ISU under-graduate weed science class over 20 years ago, waterhemp wasn't even on the list of weeds to learn. “Twenty years ago, it was also unknown in Illinois,” says the University of Illinois weed scientist. “Now it's the worst weed in Illinois.”

Historically, this pigweed family member grew in wet habitats like pond margins. Waterhemp became common in crops in the late 1980s and early 1990s. Herbicide-use patterns helped prompt its proliferation. ALS-inhibitor herbicide use approached 90% in soybeans and 65% in corn. This heavy usage helped prompt ALS-resistant waterhemp that first was identified in 1991. In 1998, a waterhemp biotype was identified that resisted triazines (atrazine) and ALS inhibitors.

“It is now hard to find waterhemp (in Illinois) that is sensitive to ALS-inhibitor herbicides,” says Tranel.

More woes surfaced in 2000, when weed scientists identified a waterhemp biotype that resisted both PPO inhibitors and ALS inhibitors. Then came 2001, when a weed-resistance triple stack of PPO inhibitors, triazines, and ALS inhibitors was found in a waterhemp biotype.

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The Researched Potential Issues With The "Other He 04/12/2012 @ 3:01pm Nor is Glyphosate the only problem INVESTIGATION OF RESISTANCE MECHANISMS TO MESOTRIONE IN A WATERHEMP (AMARANTHUS TUBERCULATUS) POPULATION FROM ILLINOIS. R. Ma1, D. McGinness1, N. E. Hausman1, P. J. Tranel1, A. Hager1, S. S. Kaundun2, T. Hawkes2, G. D. Vail3, D. E. Riechers*1; 1University of Illinois, Urbana, IL, 2Syngenta, Bracknell, England, 3Syngenta, Greensboro, NC (412) ABSTRACT Waterhemp (Amaranthus tuberculatus) is a difficult-to-control weed in Illinois soybean and corn production. This is in part due to the evolution of multiple herbicide resistances in waterhemp, which is facilitated by its dioecious biology and genetic diversity. A population of waterhemp (designated MCR) from a seed corn field in McLean County, Illinois displays resistance to mesotrione and other 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors, as well as to atrazine and certain ALS-inhibiting herbicides. Growth chamber and laboratory experiments were conducted to determine if differential herbicide uptake or metabolism are the basis for mesotrione resistance in the MCR population. Mesotrione-resistant (MCR) and mesotrione-sensitive (ACR and WCS) biotypes of waterhemp were treated with radiolabeled mesotrione for analyses of uptake and metabolism during time course experiments. Absorption studies in intact plants revealed that mesotrione uptake was not significantly different among biotypes. However, initial rates of mesotrione metabolism were faster in MCR than in ACR and WCS (using whole plants) and half-lives of mesotrione in MCR and corn were shorter than in ACR and WCS (using an excised leaf assay), which correlate with phenotypic responses to mesotrione applied postemergence in these waterhemp biotypes and in corn. The cytochrome P450 inhibitors malathion and tetcyclacis significantly inhibited mesotrione metabolism in excised leaves of MCR and corn at 6 and 24 HAT, but had no effect in ACR leaves. Our results indicate that enhanced metabolism in MCR may contribute significantly to mesotrione resistance, but further research is needed to determine if additional non-target site mechanisms may also contribute to mesotrione resistance in the MCR population. Mesotrione and Glufosinate in Glufosinate-Resistant Corn(Currently Being Used As Added Traits in “Stacked” Corn Traited Seeds) Gregory R. Armel, Robert J. Richardson, Henry P. Wilson, and Thomas E. Hines* *Former Graduate Research Assistant, Graduate Research Assistant, Professor, and Lab and Research Practitioner IV, Eastern Shore Agriculture Research and Extension Center, Virginia Polytechnic Institute and State University, Painter, VA 23420. Current address of first author: 2431 Joe Johnson Drive, 252 Ellington Plant Services Bldg., Knoxville, TN 37996; current address of second author: Box 7620, Raleigh, NC 27695. Corresponding author's E-mail: hwilson@vt.edu Abstract Field experiments were conducted in 1999 and 2000 to evaluate early POST (EPOST) and late POST (LPOST) control of common ragweed and giant foxtail with mesotrione at 70, 105, and 140 g ai/ha alone and in mixtures with glufosinate at 300 g ai/ha in glufosinate-resistant corn. Glufosinate-resistant corn injury was frequently higher with mixtures of mesotrione plus glufosinate than with mesotrione applied alone. Mixtures of mesotrione with glufosinate applied EPOST injured corn 6 to 21% in 1999, but in 2000, injury from mixtures was 23 to 30% from LPOST applications. Common ragweed control was above 77% with all treatments, which included 105 g/ha mesotrione. Giant foxtail control was higher at 76 to 78% by mixtures of mesotrione with glufosinate applied LPOST than by mesotrione alone. Corn yields were highest when glufosinate was included in treatments at either application timing. In the greenhouse, mixtures of mesotrione with glufosinate-injured glufosinate-resistant corn 11% or less, but corn biomass was reduced by 25% for the mixture of mesotrione at 105 g/ha plus glufosinate at 350 g/ha. Mixtures of mesotrione with glufosinate can be more effective than mesotrione alone but control of common ragweed and giant foxtail might not be commercially acceptable.

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