Using herbicides that differ in the ways they kill weeds goes a long way to battling herbicide resistance, according to 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 industry offerings. It is available at no charge at:

“In this era when herbicide resistance is spreading rapidly across the state, it is 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 said. “To do this, you need to know the effectiveness of the individual herbicides used and the sites of action of the herbicides (herbicide group). The 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 the south and southwest where most of the no-tillage 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.

Many Iowa farmers believe they have identified pesticide resistance on the land they farm, the Iowa Farm and Rural Life Poll found last year, and most are concerned that herbicide-resistant weeds and pesticide-resistant insects will become a problem.

“Simple and convenient tactics are failing rapidly and farmers must diversify, not just the herbicides they use, but everything,” Owen said. “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 inclusion of multiple effective herbicide mechanisms of action for every herbicide application is more effective, said Owen and Hartzler.

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 an 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 manufacturers are including these herbicide groups (designated by “HG” followed by a number) on herbicide labels to aid development of herbicide resistance management strategies. Prepackage mixes will contain the herbicide group numbers of all active ingredients.

Varying herbicide mechanisms of action and using multiple effective herbicide mechanisms of action for every herbicide application are necessary steps, say Owen and Hartzler, but producers should include non-herbicidal tactics.

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 is an important option for Iowa farmers to use in the management of herbicide-resistant weeds and the benefits and risks should be evaluated to determine if mechanical tactics have a fit in specific fields.

The guide also includes their 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.