Weed Resistance: Recent History

Some eye-opening facts about the rapid evolution of herbicide reistance.

In 1996, 19 different herbicides were employed on more than 5 percent of fields. By 2005, only one was used: glyphosate.

The Respect the Rotation tour made its final stop of 2011 in Ames, Iowa, on September 13 and 14. Growers saw the firsthand impact that resistant waterhemp had on corn and soybean crops. At the banquet following the event, Jason Norsworthy, Associate Professor of Weed Science at the University of Arkansas in Fayetteville, made a compelling presentation titled, "A Mid-South Perspective on Glyphosate Resistance." His address included a series of charts that dramatically illustrated the historical impact of glyphosate on American farm practices and agribusiness in general.

A Decline in Diversity.

Norsworthy opened his presentation with a look at the number of herbicides used on more than 5 percent of soybean acres in the United States. In 1996—the year Roundup Ready soybeans were introduced by Monsanto and two years before Roundup Ready corn was launched—19 different herbicides were employed on more than 5 percent of fields. By 2005, only one was used: glyphosate.

The days when growers could tell themselves, I can get one more year out of glyphosate,” are rapidly drawing to a close, as glyphosate-resistant broadleaves like pigweed, waterhemp and giant ragweed have become commonplace on farms across the country.

A Rise in Resistance.

A second data set illustrated the connection between the increased use of glyphosate and the increased incidence of weed resistance. In 1996, fewer than five weeds globally had evolved resistance to glyphosate. By 2011, 26 different species worldwide had become glyphosate resistant, including six species in Arkansas alone. In other words, Arkansas today has more glyphosate-resistant weeds than the entire world did 15 years ago.

A R&D Shortfall

Today Arkansas has more glyphosate-resistant weeds than the entire world did 15 years ago. Growers are turning to LibertyLink soybeans and Liberty herbicide to prevent further resistance.

Partly in response to the remarkable efficacy of glyphosate, the herbicide pipeline has greatly diminished. According to Norsworthy, between 1986 and 1995, 96 different herbicide chemistries were brought to market. Over the next decade, between 1996 and 2006, he noted that just 20 new herbicide chemistries were launched.

Clear Alternatives.

Norsworthy’s research punctuates the need for greater diversity across the board—from crop rotation to herbicides and herbicide-tolerant traits to modes of action—in an effort to combat weed resistance and stem the rapid decline of glyphosate as a viable technology. These three forms of diversity represent central themes of the Crop Science Respect the Rotation™ initiative, launched to promote herbicide diversity and integrated weed management.

As growers become aware of the threat of resistance, new options are emerging, thanks to researchers like Norsworthy and programs like Respect the Rotation. Norsworthy noted one Arkansas grower who faced a devastating outbreak of Palmer amaranth. He was able to save his farm by turning to the LibertyLink® trait and Liberty® herbicide, the only nonselective alternative to glyphosate systems.

Although LibertyLink® and Liberty represent the most reliable weed management solution and a welcome advance in herbicide diversity, the repeated use of any single herbicide will exert added selection pressure on weeds in a field, dramatically increasingly the likelihood of resistant strains. As the principles of Respect the Rotation outline, rotating herbicides across different crops is a method of reducing the likelihood of herbicide resistance and preserving the efficacy of all herbicide technologies. The Crop Science product portfolio features more than 20 herbicides for use on multiple crops, making herbicide rotation easy.

To learn more about safeguarding your fields against resistant biotypes, contact your local Bayer sales representative or your regional agronomist.

Works Cited

  • Norsworthy, Jason. “A Mid-South Perspective on Glyphosate Resistance.” Respect the Rotation 2011 Tour, Prairie Moon Winery and Vineyards, Ames, Iowa. 14 Sept. 2011. Address.
  • The Bayer CropScience Family of Sites. Bayer CropScience. Web. 23 Sept. 2011.

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