Iowa State Herbicide Guide offers strategies to manage resistant weeds

Iowa State Herbicide Guide offers strategies to manage resistant weeds

ISU specialists updated their annual weed control guide for product effectiveness and new products on the market.

Using herbicides that differ in the ways they kill weeds goes a long way to battling herbicide resistance, say Iowa State University agronomists.

Bob Hartzler and Mike Owen, professors of agronomy and ISU Extension and Outreach weed scientists, updated the 2015 Herbicide Guide for Iowa Corn and Soybean Production to reflect the 2014 growing season, product effectiveness and changes in products companies now have on the market in Iowa. The new guide is available at no charge from ISU's Extension Online Store.

WEED KILLERS: ISU Extension weed scientists Bob Hartzler and Mike Owen have updated the 2015 Herbicide Guide for Iowa Corn and Soybean Production. Revised guide reflects lessons learned in 2014, product effectiveness and changes in industry offerings.

"In this era when herbicide resistance is spreading rapidly across the state, it's important to not only develop a weed management program that provides effective control, but also reduces the likelihood of allowing resistance to become established in fields," Hartzler says. "To do this, you need to know the effectiveness of the individual herbicides used and the sites of action of the herbicides (herbicide group). Our updated herbicide guide is a convenient resource to find this information."

Waterhemp continues to be the biggest problem weed in the state and giant ragweed populations are expanding, according to the guide. Marestail/horseweed still is a major problem in southern and southwest Iowa where most of the no-till corn and soybean production is practiced. All three of these weeds have populations resistant to glyphosate and ALS inhibitor herbicides, and many of the populations have multiple resistances.

WEEDS GONE WILD: Waterhemp, shown in this photo in a soybean field, continues to be Iowa's biggest weed problem statewide. Giant ragweed populations are also expanding. Marestail is still a major problem in southern Iowa.

Many farmers have identified herbicide resistant weeds on their farm
Many Iowa farmers have identified weed resistance to herbicide on the land they farm. That's what farmers told The Iowa Farm and Rural Life Poll last year. Conducted by ISU Extension, the poll also showed that most farmers are concerned herbicide-resistant weeds and pesticide-resistant insects will eventually become a problem.  

"Simple and convenient tactics are failing rapidly and farmers must diversify, not just the herbicides they use, but everything," Owen says. "They also need to understand that many herbicide combinations, while advertised as effective on resistant weeds because they include multiple mechanisms of herbicide action, are not a good tactic unless the herbicides are effective on the target weeds."


A greater diversity of tactics is needed to combat herbicide-resistant weeds. Rotation of herbicide mechanisms of action is beneficial. But including multiple effective herbicide mechanisms of action for every herbicide application you make is more effective, say Owen and Hartzler.

Use the numerical system for identifying herbicide sites of action
Herbicides kill plants by disrupting essential physiological processes. This normally is accomplished by the herbicide specifically binding to a single protein, referred to as the "site of action." Herbicides in the same chemical family generally have the same site of action. The mechanism by which a herbicide kills a plant is known as its "mode of action."

The Weed Science Society of America has developed a numerical system for identifying herbicide sites of action. Certain sites of action have multiple numbers since different herbicides may bind at different locations on the target enzyme or different enzymes in the pathway may be targeted.

Most herbicide product manufacturers are including these herbicide groups (designated by "HG" followed by a number) on herbicide labels to aid farmers and crop consultants in developing herbicide resistance management strategies. Prepackage mixes will contain the herbicide group numbers of all active ingredients.

Choose different mechanisms, don't use same one all the time
Varying herbicide mechanisms of action and using multiple effective herbicide mechanisms of action for every herbicide application you make, are necessary steps, say Owen and Hartzler. But they also emphasize farmers should also include non-herbicide tactics in their weed control strategies.

What kind of "non-herbicide tactics" are they talking about? Cultural tactics such as crop rotation, narrow-row spacing and the inclusion of cover crops reduce the selection pressure on weeds placed by herbicides, according to the weed scientists. Mechanical weed control, such as using a cultivator, is an important option for Iowa farmers to use in the management of herbicide-resistant weeds. "Not everyone is going to go back to cultivating," Owen admits. "But the benefits and risks should be evaluated to determine if mechanical tactics have a fit in specific fields."

The ISU herbicide guide also includes Owen and Hartzler's assessment of herbicide pre-mixtures; the latest research on weed management and cover crops; and a review of new genetically engineered crop traits for weed management.

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