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Controlling weeds requires new solutions

By Staff | Jan 21, 2011

“While resistance will not cover the landscape in the next four to five years, it will start showing up more frequently.” —Dr. Bob Hartzler Professor of agronomy, ISU

STORM LAKE-According to one agronomist, it’s time to start “confusing” weeds by hitting them with a variety of herbicides for season-long control, instead of relying on glyphosate alone to protect Iowa’s corn and soybeans from these yield robbers.

“Cases of herbicide resistance can be found in fields across Iowa, and this poses a bigger threat than ever before,” said Bob Hartzler, a professor of agronomy at Iowa State University, who spoke Tuesday at a Crop Advantage workshop in Storm Lake. “While resistance will not cover the landscape in the next four to five years, it will start showing up more frequently.”

In the past 14 years, glyphosate has been the herbicide of choice for many farmers, Hartzler said, but this has turned into too much of a good thing. In the United States, there are now 12 glyphosate-resistant weed species.

A number of these can be found in Iowa, including marestail, waterhemp and giant ragweed. It’s not just a glyphosate issue, Hartzler added. “Weeds will develop resistance to any herbicide we rely on too heavily.”

Taking action

Bob Hartzler, a professor of agronomy at Iowa State University, answered questions about herbicide resistance following his presentation at a recent Crop Advantage seminar in Storm Lake.

To better manage resistance issues while controlling weeds, Hartzler offered the following strategies:

  • Pay attention to the new herbicide labels. The key to managing resistance is to rely on multiple mechanisms of action. Herbicide labels now include a standardized system to explain each product’s MOA, which eliminates the need for farmers, crop consultants and ag suppliers to learn the MOA of all the active ingredients used in agriculture today.

Herbicide packaging now indicate the product’s herbicide group near the top of the label. The number in the box represents MOA of the active ingredient, based on a system developed by the Weed Science Society of America. Pre-mixes containing more than one mode of action will have multiple numbers listed.

“To use the information properly, you must still know the activity of the individual herbicides on the weeds present in the field to ensure that the target weeds are being affected by multiple MOAs,” Hartzler said.

  • Use a pre-emerge on corn and soybeans. Pre-emerge herbicides should be viewed as foundation products in a weed control program for soybeans, because 80 to 90 percent of the time a grower will need to use a post-emerge product to achieve season-long weed control.

“Unfortunately, pre-emerge products for soybeans are not nearly as good as pre-emerge products for corn,” Hartzler said. Producers should selecting a pre-emerge product that’s active on the weeds in thier fields and should consider any existing resistance problems, crop safety, carryover risk and cost.

“Since this isn’t a one-pass weed control system, you may not need to use a Cadillac pre-emerge product,” Hartzler said.

  • Mix it up. Will tank-mixing with glyphosate work? “A lot of people don’t like using a pre-emerge product in soybeans, because it just doesn’t fit their system,” said Hartzler, who acknowledges that tank-mixing can work in some situations. Paying attention to details is important, however. Select a herbicide based on a specific need, and consider the application parameters, since they may be different for the tank mix and the glyphosate. For example, Flexstar’s label calls for a minimum of 15 gallons of water, 30 to 60 pounds per square inch of pressure and a flat fan nozzle. “How many of you use that much water with glyphosate?” Hartzler asked. “Also, most applicators use air-induction nozzles with glyphosate to minimize drift.”

Select the right herbicides to control specific weeds. Generally, the greater number of MOAs used, the less selection pressure is placed on weeds. However, designing an integrated program is not as simple as randomly adding MOAs, said Hartzler, who recommends hitting weeds with a variety of MOAs over a three- to four-year period. The different MOAs used in the program must have good activity on the important weeds in the field to successfully reduce selection pressure. “You have to know what weeds individual herbicides control,” Hartzler said.

Above all, avoid the worst-case scenario of planting Roundup Ready corn, Roundup Ready soybeans and only using glyphosate to control weeds.

“Now’s the time to get on the bandwagon and start thinking about how you can diversify your weed management program,” Hartzler concluded. “We need to use herbicides more wisely than we have in the past to be sure we have valuable weed control tools available to us in the years to come.”

You can contact Darcy Dougherty Maulsby by e-mail at HYPERLINK “mailto:“mailto:yettergirl@yahoo.com”>yettergirl@yahoo.com“mailto:yettergirl@yahoo.com”>yettergirl@yahoo.com.

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