Sites of action: New call to action in weed management


RESEARCH TRIANGLE PARK, NC – As the spread of resistant weeds continues, the term site of action is becoming a factor in managing resistance. It is imperative that growers understand herbicide site of action – and what it means to their weed management plan – because it can spell the difference between losing yield or gaining profit.

Here are four tips to unlock the benefits of herbicide sites of action:

Tip #1: Know the tool
Site of action is a term used by weed scientists to group or classify different herbicide active ingredients.  A critical element in managing or preventing development of herbicide resistant weed populations is employing multiple sites of action during a growing season.  Therefore, classifying herbicides by their site of action provides a tool that enables grower and retailers to design an effective weed management program. 

“Site of action specifically refers to the biochemical site within a plant where a herbicide has its direct impact on weed growth and development,” said Dan Westberg, Ph.D., BASF Technical Market Manager. “In other words, the site of action is where a herbicide controls a weed.”

For example, glyphosate’s site of action is the inhibition of EPSPS synthase; the site of action of Kixor® herbicide technology is the inhibition of the PPO enzyme.  

Tank mixing these two herbicides provides two sites of action for effective broadleaf weed resistance management.

The development of glyphosate resistance is a good example of the need to rotate sites of action. For the past 15 years, growers have relied almost exclusively on glyphosate year after year to control their weed problems. This practice delivered simple and effective results for many years, but overreliance on one herbicide – and in turn one site of action – caused many growers to develop resistant weed populations. Today, growers have to implement a much more comprehensive management approach, which includes at least two different sites of action in one growing season as recommended by experts.1

Tip #2: Recognize the enemy
Weeds – especially resistant weeds – endanger the very livelihood of growers across the country. They threaten crop-yield potential, valuable input investments and land values.

The Weed Science Society of America has confirmed 13 different species of glyphosate-resistant weeds across 28 states.2 Some weeds, such as waterhemp, are developing resistance to multiple herbicide sites of action. Waterhemp, along with its close cousin Palmer pigweed, is especially scary because of its ability to produce at least one million seeds from a single plant. That seed production could result in 6.25 million waterhemp plants in one acre if not adequately controlled with multiple sites of action.3

“I spend a lot of time with growers discussing and recommending programs for weed management,” said Micheal Owen, Ph.D., Iowa State University Agronomy Professor. “I try to get them to recognize the necessity of proactively managing for the ultimate and inevitable evolution of herbicide resistance.”

Growers can stay ahead of resistance and tough-to-control weeds for the next season by closely monitoring the growth of weeds in their fields throughout the year. By identifying weed populations that did not respond to treatment, growers can plan for an effective weed management program the next year. The International Survey of Herbicide Resistant Weeds provides an excellent resource, at, for identifying weeds and tracking the spread of resistance by location, site of action and weed.

Tip #3: Correct the strategy
Growers need a well-planned, strategic approach to make sure they incorporate at least two herbicides with different sites of action in a single growing season.

“Gone are the days of a one-pass glyphosate system for total weed control,” Westberg said. “Ultimately, growers need to go back to a time when planning and implementation of a comprehensive weed management plan based on the needs of their farm were vital.”

First, growers should identify the sites of action for their herbicides. A herbicide’s product label should identify the site of action. If growers are still unsure, they can contact a retailer, herbicide manufacturer, or a crop advisor.

Next, growers should consider application timing. Growers can apply herbicides at multiple times during the season, which enables them to choose from a greater diversity of herbicide sites of action. For example, a burndown and residual herbicide application in the spring enables a grower to select from a number of different herbicides that incorporate different sites of action and helps growers start clean and stay clean all season long. This can be followed by a post-emergence application that uses a different site of action to eliminate any late-season weeds competing for a crop’s resources.

“By selecting herbicides with different sites of action during the course of the growing season, growers can effectively reduce the selection pressure on any single site of action,” said Westberg.

In hard-hit fields where two sites of action may not be enough, a fall burndown provides the opportunity to select another site of action. This tactic helps growers stay ahead of tough-to-control winter annual weeds, such as marestail, and begin the following spring with a clean slate.

Finally, growers can also select tank-mix partners with different sites of action when weather or workload prohibits more than one application during a season to maintain the benefits of a multi-site of action program.

Tip #4: Understand the big picture
Due to resistance, the big picture when it comes to weeds is changing for growers. No single strategy, including herbicide treatment, should be relied upon for weed management.

“BASF is working in the field to provide solutions, technical support and tools to help growers manage weeds based on herbicide best practices,” said Westberg. “One way BASF has done that is by providing the most corn and soybean herbicide sites of action of any crop protection company. But growers should also proactively plan for things like scouting, planning, and cultural and mechanical strategies.”

When scouting during the season, a grower needs to pay particular attention to fields that have escaped weeds. With this information growers can properly select herbicides and prioritize fields for a residual herbicide application the following year.

Growers can take out extra insurance by adhering to cultural and mechanical best practices for weed management. Strategies such as crop rotation, row spacing, planting dates and seed populations provide growers with additional cultural methods for reducing weed pressure. Tillage and cultivation, hand roguing before seeds set, and equipment sanitation between fields are mechanical strategies that also provide protection from yield loss by weeds.

For more information on herbicide sites of action or to get a leg up on planning for 2012, visit with your local, authorized BASF retailer or local extension agent.
1 Sandell, Lowell, Deana Namuth Covert, Greg Kruger, and Mark Bernards. June 29, 2011. (accessed August 4, 2011).

2International Survey of Herbicide Resistant Weeds. (2011). Glycines (g/9) resistant weeds . Retrieved from

3Stachler, J. (2011, May 18). Waterhemp math. Trail County Ag Alert, (2), 1. Retrieved from

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