Giant ragweed has officially been named the seventh glyphosate-resistant weed in the United States. Three different populations in Ohio and one in Indiana are now confirmed resistant to the herbicide. (See Glyphosate-resistant giant ragweed confirmed in Indiana, Ohio).

Glyphosate-resistant giant ragweed is a significant discovery because it already heavily impacts growers. “Giant ragweed is the most competitive broadleaf weed in Indiana and Ohio, and it’s the one growers, crop advisors and weed scientists are most fearful of becoming resistant,” said Bill Johnson, Purdue University weed scientist who worked with the populations. Yield loss from a dense giant ragweed population (3 to 4 plants per square yard) can decrease yields as much as 70 percent. Compared to horseweed (marestail), also a problem for many of the same growers, yield loss may average 10 to 20 percent.

“Giant ragweed is pretty dominant. If you get enough of it, it’ll force out most other weed species in the population,” said Mark Loux, weed scientist at the Ohio State University, also a collaborator in the resistance work. “In the populations we looked at, it was primarily a monoculture of giant ragweed.”

The field in which the confirmed Indiana resistant population was found has been in continuous glyphosate-tolerant soybeans for six of the last seven years. The past two years, normal rates of glyphosate have failed to control giant ragweed, and in 2004 and 2005, three applications of glyphosate provided unacceptable control.

The Ohio populations have a similar field history, with glyphosate the sole herbicide applied. “Several fields have ragweeds that are being treated at 15 to 25 inches in height,” Loux commented. “Growers are using two applications of glyphosate and they’re not seeing control. Glyphosate is being managed differently today compared to when glyphosate-tolerant soybeans were first available. Growers have taken advantage of the simplicity of the system and some are applying it too late and on weeds that are too large.”

Johnson agreed and added, “There’s still a perception in the field that we’re crying wolf about glyphosate resistance. That’s definitely not the case.”

As we move into 2007, Loux anticipates the problem will increase. “The resistance curve can be brutal. You start out with a few fields demonstrating resistance and then you have a lot that show resistance at the same time. I think that’s where we’re heading – next year we’ll see an immense number of fields with a giant ragweed problem. I wouldn’t be surprised if it’s 10-fold worse next year.”

To compound the control problem with giant ragweed, ALS-resistance is also widespread. Though none of the current populations are confirmed resistant to both modes of action, the lack of potential control options presents a tricky and costly challenge. “There is a window for other types of post-emergence herbicides to be effective, but growers need to start with a clean field, and have timely post applications,” Johnson said.

For effective control of giant ragweed, Johnson and Loux recommend the following steps:

• Start clean by using tillage or a pre-plant burndown herbicide.

• Apply pre-emergence herbicide(s) to reduce early season giant ragweed density and the rate of weed growth, creating more flexibility in the post-emergence application window and improving post-emergence glyphosate efficacy. 

• Apply initial post-emergence treatments when giant ragweed is 6 to 10 inches tall. 

  • Use one of the following in the first post application: glyphosate - 1.5 lbs ae/A; Flexstar® - 1.3 pts/A; Cobra®/Phoenix™ - 12.5 oz/A; or FirstRate® - 0.3 oz. Do not use FirstRate if the giant ragweed population is known or suspected to be ALS-resistant.

• Make a second post application of glyphosate 3 to 4 weeks after the first, using a rate of 0.75 lb ae/A. Do not wait until giant ragweed plants have regrown above soybean canopy to make the second application or plants may not be controlled.

Syngenta recommends the use of pre-emergence, residual herbicides on glyphosate-tolerant crop acres and following 2-1-2 as a resistance management program for corn and soybean growers – no more than two applications of glyphosate on one field over a two-year period. In soybeans, Prefix™ herbicide provides a pre-emergence, non-glyphosate and non-ALS herbicide option for effective early season control of giant ragweed. For more information on resistance management options and specific product recommendations, visit resistancefighter.com and view the Resistance Fighter™ solutions module.

Syngenta is a world-leading agribusiness committed to sustainable agriculture through innovative research and technology. The company is a leader in crop protection, and ranks third in the high-value commercial seeds market. Sales in 2005 were approximately $8.1 billion. Syngenta employs some 19,000 people in over 90 countries. Syngenta is listed on the Swiss stock exchange (SYNN) and in New York (SYT). Further information is available at www.syngenta.com.