As weeds resistant to current herbicide technology continue their march across U.S. fields, farmers can slow or stop their progress by putting together a management program that takes into consideration herbicide site of action and mode of action. While the terms are often used interchangeably, they are not the same processes.
Bryan Young, Purdue University weed scientist explains: Mode of Action (MOA) is the name for the process the herbicide uses to control the weed. Site of Action (SOA) is the location within the plant where the herbicide impacts the weed’s development process.
“So, site of action is ‘where’ and mode of action is ‘how,’” Young says.
Understanding the where and how of herbicide active ingredients can help you address weeds and manage your budget more effectively. That understanding can also help you avoid using the same technology repeatedly in a field, which can contribute to the very resistance problems you’re trying to avoid.
Bob Hartzler, Iowa State University Extension weed scientist offers this example as a for instance: “A triazine herbicide (e.g. atrazine, metribuzin) interferes with photosynthesis by binding to the D1 protein involved in photosynthetic electron transfer. Thus, the site of action for triazines is the D1 protein, whereas the mode of action is disruption of photosynthesis.”
Dig into the details. Manufacturers increasingly package multiple herbicide active ingredients in branded premixes to control tough weeds—especially those weed species with multiple herbicide resistance, such as Palmer amaranth and waterhemp. That means you may have to dig a bit deeper to determine what – and how many – SOAs and MOAs you’re using in the field.
The Weed Science Society of America (WSSA) has a numbering system that classifies individual herbicide active ingredients according to their SOA. Tapping into this information can help you build a weed-control program that features a variety of herbicides and tackles weeds in multiple sites.
The WSSA updated its number system just this past May, so it’s current for herbicides in the marketplace now. You can access the system here: https://wssa.net/wssa/weed/herbicides/
Most herbicide labels specify the SOA with a unified logo. Check with your retailer or company sales representative if product labels don’t provide that information.
Hartzler adds a reminder that while herbicides are still useful tools against weeds, don’t overlook the opportunity to include other tools and practices that can improve your control efforts, including crop rotation, cover crops and, yes, even tillage.
Tackle Tough Weeds in Pastures Like you Would in Corn and Soybeans
Let Nature Target Weeds with Bioherbicides
Be Aware of Purchasing Livestock Feed Containing Weed Seeds
