Respect the Rotation in Ames, Iowa – Herbicide Resistance By the Numbers

On its final stop in 2011, Respect the Rotation found Iowa growers facing an increase in resistant waterhemp.

Listening to Extension weed scientists address touring growers on the last leg of the Respect the Rotation tour in Ames, Iowa, one got the sense of being admonished by a series of prophets who foresaw a coming storm. The statistics and stories were staggering. The faces of growers in the crowd were a mixture of surprise, cynicism and neutrality. For many, until herbicide resistance appears in their fields, it won't truly be a problem they're prepared to address. But according to Extension researchers, by then it will be too late.

For many growers, until herbicide resistance appears in their fields, it won't truly be a problem they're prepared to address. But according to Extension researchers, by then it will be too late. We assembled some key facts and figures from the two-day event and decided to let the numbers speak for themselves—allowing you to draw your own conclusions.

Resistance By the Numbers: A Look at the Diverse Impact of Weed Resistance

More Expensive Acres. According to one Extension specialist, Arkansas soybean grower Sid Fogg faced a severe infestation of Palmer amaranth and had to spend more than $200 an acre to successfully grow soybeans.

Rapid Resistance. According to Jason Norsworthy, associate professor of weed science at the University of Arkansas in Fayetteville, the first glyphosate-resistant pigweed in Arkansas was discovered in 2005. Today, 80 percent of the state’s farmland has resistant pigweed.

Higher Labor Costs. The pigweed problem is so considerable in Arkansas that many farmers are spending $50 to $60 an acre for hoe crews to hack out the resistant plants.

Iowa State University's Bob Hartzler suggested that 2011 Iowa soybean fields might be the weediest he's seen in more than 10 years, thanks largely to late-emerging waterhemp in wider-spaced rows.


Small Application Window
. Pigweed can grow 1 to 2 inches a day, yet there is no solution that can be effectively sprayed over the top of 3- to 4-inch pigweed.

Three Geographies. Mike Owen, Extension weed specialist and professor of agronomy at Iowa State University, noted three major geographies in which weed resistance is on the rise: the Mississippi Delta, Southeast Atlantic Corridor and the Midwest.

Not Just Herbicide Rotation—Crop Rotation too. John Cantwell, Crop Science technical service representative for northern Iowa, recommended rotating a diversity of traits and modes of action, in addition to crop rotation. He noted that there were probably 5 million corn-on-corn acres in Iowa in 2010.

The Forgotten Canopy Tool. Bob Hartzler, an Extension weed specialist and professor of agronomy at Iowa State, reminded growers that the crop canopy is an under-appreciated weed-management tool. To that end, he noted the importance of keeping row spacing in mind, with 15-inch rows preferable to 30-inch rows from a weed-management standpoint.

The LibertyLink® trait and Liberty® herbicide represent the only nonselective alternative to glyphosate systems. LibertyLink seeds have a built-in tolerance to Liberty.

Population Metrics. Hartzler also noted that there seems to be an inverse relationship between plant populations and weed biomass; the more plants a grower had per acre, the lower the weed biomass.

Smart Cultural Practices and New Technologies

For growers across the country, resistant pests (whether weed, insect or fungi) have become a clear and present danger. Fortunately, new product offerings are giving growers new alternatives to the over-use of glyphosate. LibertyLink® trait and Liberty® herbicide represent the only nonselective alternative to glyphosate-tolerant systems. LibertyLink seeds have built-in tolerance to Liberty.

Of course, as Extension weed specialists are at pains to point out, the repeated use of any single herbicide can cause resistance. The more a weed species faces the same mode of action, the greater the selection pressure is placed on it to evolve a means of resisting the herbicide.

Recognizing this reality of resistance, FiberMax launched FM 9250GL in 2011, a cotton variety that includes both the LibertyLink trait and GlyTol® glyphosate-tolerant trait, giving growers the opportunity to plant a high-performing variety and manage weeds with two nonselective herbicide modes of action.

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

Works Cited

  • Cantwell, John; Hartzler, Bob; Owen, Mike; and Norsworthy, Jason. “Respect the Rotation Field Tour.” Respect the Rotation 2011 Tour, Elmsworth, Iowa. 14 Sept. 2011. Address.
  • The Bayer CropScience Family of Sites. Bayer CropScience. Web. 23 Sept. 2011.

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