MOA or SOA? Knowing the difference is key to effective weed control
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Over the years, the terms mode of action (MOA) and site of action (SOA) have often been used interchangeably. However, these two key components of effective weed control are not the same. Understanding the difference between them is essential when guiding your customers in selecting herbicide programs that both control weeds effectively and reduce the risk of resistance.
"Mode of action is HOW the herbicide works to control weeds or the overall process — uptake, translocation, Interaction at target site, and weed symptomology," explains Jason Gibson, market development specialist with Corteva Agriscience in Nebraska. "Site of action is WHERE the herbicide works within the weed — the specific biochemical pathway at which that herbicide is actively working to provide control.
"Even though Group 14 and Group 22 herbicides both cause rapid tissue necrosis and look like they are controlling weeds in the same way, they target and bind to different sites. They also are taken up by and move differently within the weed tissues.”
SOA drives effective control and resistance management
Herbicide resistance develops due to diversity within weed populations and mutations or modifications to the site where the herbicide binds to that weed. Repeated use of a single herbicide SOA will ultimately select for weeds that are resistant to that SOA. When the herbicide can't bind to the site of action in the weed, it won't work and no longer provides control.
"If you keep using the same site of action over and over, in five to six cycles all the susceptible weeds have been controlled, and the plants that have the mutated or modified genes keep producing seeds. The next thing you have is a population of weeds you can no longer control," Gibson explains.
Matching weed susceptibility to herbicide SOA
To control resistant weeds, it's important to pick an herbicide with a SOA that targets the plant where it is still vulnerable. Understanding which weed species in your area have documented resistance to specific SOAs is the first step. A database at WeedScience.org tracks confirmed resistance cases and is searchable by state, herbicide group and weed species.
Determining an herbicide SOA is as easy as checking the product label.
Designing an effective soybean weed control program
An effective weed control program may mean different things to different people. For Gibson it means stopping problem weeds and the potential for resistance development by using multiple SOAs.
He says Kyber® Pro herbicide, a new preemergence soybean herbicide containing metribuzin, flumioxazin and pyroxasulfone is a good option. The active ingredients in the herbicide have three distinct SOAs: Group 5, 14 and 15.
"If you have problems in your area with hard-to-control ALS- and glyphosate-resistant weeds like Palmer amaranth and waterhemp, a Group 15 herbicide would be a good choice,” Gibson says. “The goal would be to see at least 70% control when using Group 15 by itself. A Group 14 is another good choice and would also provide at least 70% or better control by itself.
"By stacking two or three individual sites of action with each providing at least 70% control independently, you can elevate the level of control to up to 95%, and on a good day, achieve near 100% control. Weeds not controlled by the Group 15, can be controlled by the Group 14 or the Group 5 and vice versa." he explains.
Another new option in soybean weed control is Sonic® Boom herbicide. It has a unique, 2:1 ratio of proven active ingredients — metribuzin (Group 5) to sulfentrazone (Group 14) — for strong, long-lasting residual of up to four to six weeks.
Both Kyber Pro herbicide and Sonic Boom herbicide are convenient premixes that save time during application and provide control by using active ingredients with different SOAs, which can easily be aligned with the type of weed resistance in your customers' fields.
Applying two or three known effective SOAs elevates the level of control and keeps weeds from producing seed.
Controlling weeds until crop canopy
Gibson points out that the residual control provided by many preemergence herbicides unfortunately doesn't last indefinitely. By extending residual control, customers can keep fields cleaner until canopy.
"I believe the bulk of our herbicide resistance issues are due to foliar applications," Gibson says. "The more we can control weeds before they emerge, the more successful we'll be in controlling the proliferation of weeds."
A product such as Enversa™ herbicide will deliver residual weed control for up to four weeks. This soybean and cotton herbicide has a proprietary formulation of encapsulated acetochlor for tank-mix flexibility. It can be used pre- or postemergence. Enversa herbicide is the preferred residual tank-mix partner of choice with Enlist One® herbicide for Enlist E3® soybeans.
"Depending on your preemergence program, adding a product such as Enversa to the postemergence program provides overlapping residual control and the potential to add another site of action to the program, discouraging resistance," Gibson explains.
Gibson points out that the challenges of weed resistance have gone beyond target-site resistance. Today the industry faces non-target site resistance. What this will mean for weed control Is still unfolding, but along with utilizing herbicides with multiple sites of action, more cultural methods will likely be needed. For successful weed control, it will be important to understand The Difference Between Target-Site & Non-Target-Site Herbicide Resistance.
The transgenic soybean event in Enlist E3® soybeans is jointly developed and owned by Corteva Agriscience and M.S. Technologies L.L.C. Following burndown, Enlist Duo® and Enlist One® herbicides with Colex-D® technology are the only herbicides containing 2,4-D that are authorized for preemergence and postemergence use with Enlist® crops. Consult Enlist® herbicide labels for weed species controlled. Enlist Duo and Enlist One herbicides are not registered for use or sale in all states and counties; are not registered in AK, CA, CT, HI, ID, MA, ME, MT, NH, NV, OR, RI, UT, VT, WA and WY; and have additional subcounty restrictions in AL, GA, TN and TX, while existing county restrictions still remain in FL. All users must check “Bulletins Live! Two” no earlier than six months before using Enlist One or Enlist Duo. To obtain “Bulletins,” consult epa.gov/espp/, call 1-844-447-3813, or email ESPP@epa.gov. You must use the “Bulletin” valid for the month and state and county in which Enlist One or Enlist Duo are being applied. Contact your state pesticide regulatory agency if you have questions about the registration status of Enlist® herbicides in your area. ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. IT IS A VIOLATION OF FEDERAL AND STATE LAW TO USE ANY PESTICIDE PRODUCT OTHER THAN IN ACCORDANCE WITH ITS LABELING. ONLY USE FORMULATIONS THAT ARE SPECIFICALLY LABELED FOR SUCH USE IN THE STATE OF APPLICATION. USE OF PESTICIDE PRODUCTS, INCLUDING, WITHOUT LIMITATION, 2,4-D-CONTAINING PRODUCTS NOT AUTHORIZED FOR USE WITH ENLIST CROPS, MAY RESULT IN OFF-TARGET DAMAGE TO SENSITIVE CROPS/AREAS AND/OR SUSCEPTIBLE PLANTS, IN ADDITION TO CIVIL AND/OR CRIMINAL PENALTIES. Additional product-specific stewardship requirements for Enlist crops, including the Enlist Product Use Guide, can be found at www.traitstewardship.com. Enversa™, Kyber® Pro and Sonic® Boom are not registered for sale or use in all states. Enversa is not available for sale, distribution or use in Nassau and Suffolk counties in the state of New York. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Consult the label before purchase or use for full details. Always read and follow label directions.