Phyllis Bongard, Extension content development and communications specialist, Tom Peters, Extension sugarbeet agronomist, Debalin Sarangi, Extension weed specialist, and Eric Yu, Extension educator-crops
As herbicide resistance continues to expand and challenge producers, using an integrated approach will help maintain herbicide effectiveness. Dr. Tom Peters, Extension sugarbeet agronomist, Dr. Debalin Sarangi, Extension weed specialist, and Dr. Eric Yu, Extension educator-crops, joined moderator Ryan Miller, Extension educator-crops, to discuss emerging and familiar weed management strategies in the February 5, 2025, session of Strategic Farming: Let’s talk crops.
View recordings of this and all Strategic Farming sessions at https://z.umn.edu/StrategicFarmingRecordings
When this information is combined for individual populations, a picture of multiple resistance emerges. Ninety-one percent of the populations were resistant to at least two herbicides. Twenty-five populations were resistant to Raptor and glyphosate and another 18 were resistant to these two herbicides plus atrazine. Overall, nearly 60% of the populations were resistant to three or more herbicide sites of action. Most concerning were the two populations that were resistant to all of the herbicides screened except for Liberty.
Using preemergence (PRE) herbicides to start clean and stay clean paves the way for successful postemergence applications.Postemergence weed control in the pictured field where no PRE was applied would be difficult because the weeds are so dense (Figure 1). Control may only reach 50-70% in situations like this. In contrast, corn growth is healthier when the PRE sets the field up for a successful postemergence treatment.
To illustrate this further, Sarangi explained that soybeans treated with a PRE had an 11-bushel yield advantage compared to soybean plots with no PRE in a three-year study at Rosemount, MN.
While PREs are an important tool for maintaining postemergence herbicides, weather can complicate PRE applications. If conditions are too dry, there may not be enough moisture to activate the herbicide and weed control may be delayed. On the flip side, when there is excess rainfall, the risk for surface runoff or leaching increases. For example, the Minnesota Department of Agriculture has reported acetochlor (Group 15) in surface water in southern Minnesota. One strategy to minimize this risk would be to rotate PREs and save the acetochlor for the layered residual treatment with the postemergence application when runoff is less likely to occur. Other Group 15s (e.g., Dual II Magnum, Zidua), Group 14 (Valor) or Group 27 (Callisto) herbicides are examples that could be rotated with acetochlor.
In his recent graduate program, Dr. Yu studied the potential for cereal rye (CR) cover crop to suppress weeds in soybeans. The cereal rye had been seeded the previous fall (late Sep to early Oct) following corn silage. The study included two soybean planting dates (May 15 and June 1) and four termination treatments:
CR biomass increased as the termination was delayed. The greatest amount of biomass, about 6000 lb/a, was produced in the late-planted soybeans when the CR was terminated 7 days after planting (June 8). In contrast, roughly half that amount (2600-3000 lb/a) was produced in the early planted soybeans at a similar 7 DAP termination time (May 22).
Weed suppression increased with CR biomass, but complete weed reduction didn’t occur until biomass production reached 4500 lb/a. This level of biomass production most closely corresponded to the June 8 termination time in the late-planted soybean treatment. However, complete weed suppression came at a price as soybean yields took a hit in these late-planted treatments.
In the early planted soybean, waterhemp suppression reached 80% when CR was terminated at planting, but suppression increased to 90% when a PRE herbicide was added. Similar results were achieved for yellow foxtail. Soybean yields were greatest in the early planted soybeans when CR was terminated at or after planting.
If new to a cereal rye cover crop, Yu recommends planting soybeans early and terminating the cover crop at planting with or without a preemergence herbicide or 7 days after planting.
Cover crop planting dates matter, too. In an earlier study out of Lamberton, CR biomass production was greater with September seeding dates vs. later October seedings. For more information see https://blog-crop-news.extension.umn.edu/2020/09/planting-date-matters-for-cover-crops.html
How does it work? The electricity that the Weed Zapper produces is transferred through the boom. When weeds touch the boom, the heat destroys plant cell integrity and the plant immediately wilts. Within hours, cross-sections of the apical and basal stems of a broadleaf weed turn brown and necrotic.
Dr. Kevin Bradley, University of Missouri, studied electrocution control of several weeds, including waterhemp, at growth stages from 12-inches through seed set. He found that electrocution tended to be more effective at later growth stages (pollination and seed set) when weeds are taller. Visual control of waterhemp 42 days after treatment reached 82% with two electrocution passes. Some waterhemp survival may have been due to electricity reaching only part of the plant. Since waterhemp can easily reestablish growing points, some plants may branch and regrow from the lower part of the plant. Despite this potential, a significant advantage of electrocution is that it also impacts seed viability. In the case of late-season waterhemp, seed viability was reduced 59% in this study.
Weed electrocution may fit well into an integrated program to eliminate escapes. Keep in mind that the bar must remain above the crop canopy or damage will occur. Control is inconsistent and less effective on grasses and kochia, because, unlike waterhemp, they don’t have a good conduit for the electricity to pass through.
How many passes are recommended? Peters suggests two passes but separate them by 5-7 days rather than coming back on the same pass immediately. This would be particularly beneficial where weed pressure is high. If kochia is an issue, it may take several times to set the weed back.
However, we have been relying heavily on this group and this selection pressure could result in resistance. Illinois has already reported the presence of waterhemp that is resistant to Group 15 herbicides.
The key is to rotate both crops and herbicides. For residual waterhemp control, rotate the Group 15 herbicides with those from Groups 5, 14, and 27. Using herbicides from these other groups will provide diversity in the of sites of action and slow resistance development. To demonstrate how frequently Group 15 herbicides are used, Peters urges growers to map out pigweed control programs across the rotation. How many times is a Group 15 herbicide being used? Are Group 14, 5, and 27 herbicides represented? Mix it up and take advantage of the chemical diversity that’s available to maintain Group 15 effectiveness.
Integrated weed management depends on individual farm operations and cropping systems. There is no "one-size-fits-all" approach to weed management. If you're interested in a new tactic, try it on a small portion of the farm. If it's successful, it can be expanded.
Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program.
Products are mentioned for illustrative purposes only. Their inclusion does not mean endorsement and their absence does not imply disapproval.
As herbicide resistance continues to expand and challenge producers, using an integrated approach will help maintain herbicide effectiveness. Dr. Tom Peters, Extension sugarbeet agronomist, Dr. Debalin Sarangi, Extension weed specialist, and Dr. Eric Yu, Extension educator-crops, joined moderator Ryan Miller, Extension educator-crops, to discuss emerging and familiar weed management strategies in the February 5, 2025, session of Strategic Farming: Let’s talk crops.
View recordings of this and all Strategic Farming sessions at https://z.umn.edu/StrategicFarmingRecordings
Setting the stage
Status of waterhemp herbicide resistance
Managing waterhemp is increasingly difficult as populations develop resistance to an expanded number of postemergence herbicides. How extensive is the issue? To address this question, Dr. Sarangi’s lab surveyed 90 waterhemp populations from 47 counties in Minnesota between 2020 and 2022. They screened those populations against 8 postemergence herbicides:- Raptor (imazamox, Group 2)
- Enlist One (2,4-D choline, Group 4)
- Xtendimax (dicamba, Group 4)
- Atrazine (atrazine, Group 5)
- Roundup PowerMax (glyphosate, Group 9)
- Liberty (glufosinate, Group 10)
- Flexstar (fomesafen, Group 14)
- Callisto (mesotrione, Group 27)
When this information is combined for individual populations, a picture of multiple resistance emerges. Ninety-one percent of the populations were resistant to at least two herbicides. Twenty-five populations were resistant to Raptor and glyphosate and another 18 were resistant to these two herbicides plus atrazine. Overall, nearly 60% of the populations were resistant to three or more herbicide sites of action. Most concerning were the two populations that were resistant to all of the herbicides screened except for Liberty.
Integrated weed management tools
Mortenson, et al. in a 2012 Bioscience article stated that if herbicide resistance was addressed with only herbicides, then evolution would most likely win. Producers need a variety of and strategies to preserve herbicide effectiveness.Start clean and stay clean
 |
| Fig. 1. Difference in weed control between corn field without a preemergence herbicide application (left) and one with a PRE application (right) |
To illustrate this further, Sarangi explained that soybeans treated with a PRE had an 11-bushel yield advantage compared to soybean plots with no PRE in a three-year study at Rosemount, MN.
While PREs are an important tool for maintaining postemergence herbicides, weather can complicate PRE applications. If conditions are too dry, there may not be enough moisture to activate the herbicide and weed control may be delayed. On the flip side, when there is excess rainfall, the risk for surface runoff or leaching increases. For example, the Minnesota Department of Agriculture has reported acetochlor (Group 15) in surface water in southern Minnesota. One strategy to minimize this risk would be to rotate PREs and save the acetochlor for the layered residual treatment with the postemergence application when runoff is less likely to occur. Other Group 15s (e.g., Dual II Magnum, Zidua), Group 14 (Valor) or Group 27 (Callisto) herbicides are examples that could be rotated with acetochlor.
Use the cash crop to a competitive advantage
Row spacing
Use cultural practices and the crop’s competitive edge to race against weeds later in the season. In a study comparing 12- and 22-inch soybean rows, it took an additional two weeks for the 22-inch beans to reach 90% canopy cover compared to the 12-inch row soybeans. Narrower row spacing can lead to earlier canopy closure, better competition with weeds, and less waterhemp germination late in the season.Planting date
An early planting date can also give crops a competitive edge. In a Rosemount study comparing three soybean planting dates - May 1, May 15, and June 1 - canopy growth and closure was significantly earlier with the May 1 planting date. The earliest planting date also yielded the highest at 62 bushels, while soybeans planted a month later on June 1 yielded about 50 bushels.Crop rotations
Confuse the weeds, urges Dr. Peters. For example, consider corn, soybeans, and wheat in a rotation. Differences in planting dates, row spacings, tillage practices, how and when fertilizer is applied changes the environment for the weeds and makes it less predictable. Small grains in the rotation are also effective at suppressing broadleaf weeds according to many studies over the past 50 years. If processing vegetables or sweet corn crops are grown in your area, including these in your rotation can impact weeds due to the different planting and harvest dates. An expanded crop rotation also brings diversity to the herbicide sites of action that are available.Cover crops as a weed management tool
Cover crops provide several benefits including the ability to suppress weeds. The degree of weed suppression largely depends on the amount of biomass that can be produced. In general, as biomass increases, weed suppression also increases. However, there is a balancing act between producing biomass and maintaining crop yield.In his recent graduate program, Dr. Yu studied the potential for cereal rye (CR) cover crop to suppress weeds in soybeans. The cereal rye had been seeded the previous fall (late Sep to early Oct) following corn silage. The study included two soybean planting dates (May 15 and June 1) and four termination treatments:
- 7 days before planting (DBP)
- At planting (AP)
- At planting with a preemergence application of Verdict (AP+PRE)
- 7 days after planting (DAP)
CR biomass increased as the termination was delayed. The greatest amount of biomass, about 6000 lb/a, was produced in the late-planted soybeans when the CR was terminated 7 days after planting (June 8). In contrast, roughly half that amount (2600-3000 lb/a) was produced in the early planted soybeans at a similar 7 DAP termination time (May 22).
Weed suppression increased with CR biomass, but complete weed reduction didn’t occur until biomass production reached 4500 lb/a. This level of biomass production most closely corresponded to the June 8 termination time in the late-planted soybean treatment. However, complete weed suppression came at a price as soybean yields took a hit in these late-planted treatments.
In the early planted soybean, waterhemp suppression reached 80% when CR was terminated at planting, but suppression increased to 90% when a PRE herbicide was added. Similar results were achieved for yellow foxtail. Soybean yields were greatest in the early planted soybeans when CR was terminated at or after planting.
If new to a cereal rye cover crop, Yu recommends planting soybeans early and terminating the cover crop at planting with or without a preemergence herbicide or 7 days after planting.
Cover crop planting dates matter, too. In an earlier study out of Lamberton, CR biomass production was greater with September seeding dates vs. later October seedings. For more information see https://blog-crop-news.extension.umn.edu/2020/09/planting-date-matters-for-cover-crops.html
Weed electrocution
Electrocuting weeds is not a new concept. In 1979, the Lasco Lightning Weeder made its debut in sugarbeet country in northern Minnesota. Fast forward to the Weed Zapper today, which is essentially a generator pulled behind the tractor. It has the potential to eliminate late-season herbicide-resistant weed escapes and reduce both the number and viability of weed seeds that might be returned to the weed seed bank in the soil.How does it work? The electricity that the Weed Zapper produces is transferred through the boom. When weeds touch the boom, the heat destroys plant cell integrity and the plant immediately wilts. Within hours, cross-sections of the apical and basal stems of a broadleaf weed turn brown and necrotic.
Dr. Kevin Bradley, University of Missouri, studied electrocution control of several weeds, including waterhemp, at growth stages from 12-inches through seed set. He found that electrocution tended to be more effective at later growth stages (pollination and seed set) when weeds are taller. Visual control of waterhemp 42 days after treatment reached 82% with two electrocution passes. Some waterhemp survival may have been due to electricity reaching only part of the plant. Since waterhemp can easily reestablish growing points, some plants may branch and regrow from the lower part of the plant. Despite this potential, a significant advantage of electrocution is that it also impacts seed viability. In the case of late-season waterhemp, seed viability was reduced 59% in this study.
Weed electrocution may fit well into an integrated program to eliminate escapes. Keep in mind that the bar must remain above the crop canopy or damage will occur. Control is inconsistent and less effective on grasses and kochia, because, unlike waterhemp, they don’t have a good conduit for the electricity to pass through.
How many passes are recommended? Peters suggests two passes but separate them by 5-7 days rather than coming back on the same pass immediately. This would be particularly beneficial where weed pressure is high. If kochia is an issue, it may take several times to set the weed back.
Tillage
When planning tillage, what is its purpose? Your strategy will vary based on whether your objective is to prepare a seedbed or control weeds. Finally, interrow cultivation as a tool is still a viable weed management tool.Strategies to maintain Group 15 herbicides
Group 15 herbicides, such as Dual, Warrant, Surpass, Outlook, and Zidua, are important for waterhemp control, particularly when populations are becoming resistant to so many postemergence herbicides. The Group 15 herbicides are very effective on small-seeded broadleaf and grass weeds as both a preemergence application and as a layered residual with a postemergence application. This approach has provided extended control for difficult-to-control weeds like waterhemp.However, we have been relying heavily on this group and this selection pressure could result in resistance. Illinois has already reported the presence of waterhemp that is resistant to Group 15 herbicides.
The key is to rotate both crops and herbicides. For residual waterhemp control, rotate the Group 15 herbicides with those from Groups 5, 14, and 27. Using herbicides from these other groups will provide diversity in the of sites of action and slow resistance development. To demonstrate how frequently Group 15 herbicides are used, Peters urges growers to map out pigweed control programs across the rotation. How many times is a Group 15 herbicide being used? Are Group 14, 5, and 27 herbicides represented? Mix it up and take advantage of the chemical diversity that’s available to maintain Group 15 effectiveness.
Think long-range economics
With current ag economics and uncertain world events, it’s a difficult time for budget decision-making. What should priorities look like? Peters contends it’s important to take the long view on weed management. While it may be tempting to back off on weed management in 2025, that decision could affect yields not only this year but for 5 or 10 years or even longer. That’s because weed seeds remain viable in the soil: kochia, 1-2 years; pigweeds, 4-6 years; and ragweeds 10 to 20 years. Controlling the weed seed bank now pays off with better yields and fewer weed management headaches in the future.Integrated weed management depends on individual farm operations and cropping systems. There is no "one-size-fits-all" approach to weed management. If you're interested in a new tactic, try it on a small portion of the farm. If it's successful, it can be expanded.
Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program.
Products are mentioned for illustrative purposes only. Their inclusion does not mean endorsement and their absence does not imply disapproval.
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