July 13, 2009

Herbicide diversity needed to keep Roundup effective

WEST LAFAYETTE, Ind. - Using a diverse herbicide application strategy may increase production costs, but a five-year Purdue University study shows the practice will drastically reduce weeds and seeds that are resistant to a popular herbicide.

Excess usage of glyphosate-resistant crops has led to weeds, such as marestail, that also are resistant to glyphosate, the herbicide used in Roundup. Bill Johnson, a Purdue associate professor of weed science, said changing management practices can almost eliminate resistant marestail and its viable seeds in the soil.

"Another herbicide application is expensive, and it means more trips across the field," Johnson said. "But we can reduce the population and density of resistant weeds, which increases the crop yield potential."

The results of Johnson's five-year study were published in the journal Weed Science.

Marestail, also known as horseweed, was the first weed to develop resistance to glyphosate. Other weeds also are adapting, Johnson said, reducing the effectiveness of products such as Roundup, the most widely used herbicide on the market.

It is Roundup's popularity that is contributing to its diminished effect. Johnson said farmers have come to rely on Roundup Ready crops that resist glyphosate as an easy way to control weeds. But overuse of any herbicide allows weeds to adapt and develop resistance.

Johnson's study found that farmers should diversify the herbicides they use. Using a variety of herbicides in addition to Roundup before planting and alternating between Roundup and other herbicides in corn can significantly reduce marestail.

Fields that had three resistant weeds for every susceptible weed while using only Roundup and Roundup Ready crops saw weed populations drop to one resistant weed for every six susceptible weeds while rotating herbicides as Johnson suggests. That rotation also may lead to a 95 percent decrease in the number of viable marestail seeds in the soil.

Continuing with only Roundup and Roundup Ready crops can intensify the problem, Johnson said.

"Glyphosate-resistant marestail develops very quickly in a field. Populations reach staggering levels of infestation in about two years after it is first detected," Johnson said. "For us, marestail being the first weed that developed resistance showed that a weed-management system that is solely reliant on glyphosate is starting to break down. However, a system that incorporates other herbicides with glyphosate can be sustainable for quite some time."

The Indiana Soybean Alliance, BASF, Dow AgroSystems, Monsanto and Syngenta funded Johnson's study. His next step is looking at management strategies that reduce the prevalence of other weeds that have built up resistance to glyphosate.

Writer: Brian Wallheimer, (765) 496-2050, bwallhei@purdue.edu

Source: Bill Johnson, (765) 494-4656, wgjohnso@purdue.edu

Ag Communications: (765) 494-8415;
Steve Leer, sleer@purdue.edu
Agriculture News Page


Influence of Weed Management Practices and Crop Rotation on Glyphosate-Resistant Horseweed Population Dynamics and Crop Yield-Years III and IV

 Vince M. Davis, Kevin D. Gibson, Thomas T. Bauman, Stephen C. Weller, William G. Johnson

Horseweed, Conyza canadensis, is an increasingly common and problematic weed in no-till soybean production in the eastern cornbelt due to the frequent occurrence of biotypes resistant to glyphosate.  The objective of this study was to determine the influence of crop rotation, winter wheat cover crops (WWCC), residual non-glyphosate herbicides, and preplant application timing on the population dynamics of glyphosate-resistant (GR) horseweed and crop yield.  A field study was conducted from 2003 to 2007 in a no-till field located at a site that contained a moderate infestation of GR horseweed (approximately 1 plant m-2).  The experiment was a split-plot design with crop rotation (soybean-corn or soybean-soybean) as main plots and management systems as subplots.  Management systems were evaluated by quantifying in-field horseweed plant density, seedbank density, and crop yield.  Horseweed densities were collected at the time of postemergence applications, one month after postemergence applications (MAP), and at the time of crop harvest or 4 MAP.  Viable seedbank densities were also evaluated from soil samples collected in the fall following seed rain.  Soybean-corn crop rotation reduced in-field and seedbank horseweed densities versus continuous soybean in the third and fourth years of this experiment.  Preplant herbicides applied in the spring were more effective at reducing horseweed plant densities than when applied in the previous fall.  Spring-applied, residual herbicide systems were the most effective at reducing season-long in-field horseweed densities and protecting crop yields since the growth cycle of horseweed in this region is primarily as a summer annual.  Management systems also influenced the glyphosate-resistant and glyphosate–susceptible (GS) biotype population structure after four years of management.  The most dramatic shift was from the initial GR:GS ratio of 3:1 to a ratio of 1:6 after four years of residual preplant herbicide use followed by non-glyphosate postemergence herbicides.


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