By Liz Stahl, Extension Educator – Crops, Rodrigo Werle, University of Wisconsin Extension Cropping Systems Weed Scientist, and Debalin Sarangi, University of Minnesota Weed Scientist
Waterhemp is widely considered the number one most troublesome weed across the North Central Region of the US, and experiences in MN and WI are no exception. Surveys conducted in Wisconsin show there has been rapid evolution of herbicide resistance in waterhemp to 2,4-D, dicamba, and atrazine. In screenings of populations in 2018, these herbicides could kill about 89 to 92 out of every 100 waterhemp plants. In contrast, follow-up testing of populations collected in the fall of 2023 revealed that about half of the waterhemp could survive 2,4-D and 80% survived atrazine. Farmers are seeing reductions in control with Enlist One, the Group 14 herbicide fomesafen (a.i. in Flexstar), and the Group 27 herbicide mesotrione (a.i. in Callisto). Surveys conducted in MN show widespread herbicide-resistance issues in waterhemp as well, and glufosinate-resistant waterhemp in MN has now been officially confirmed. The challenges posed by herbicide resistant weeds and recommended management strategies were the topics of discussion on the March 11, 2026 Strategic Farming: Let’s Talk Crops episode on “Strategies for winning the war against waterhemp.” Dr. Rodrigo Werle, University of Wisconsin Weed Scientist, and Dr. Debalin Sarangi, University of Minnesota Weed Scientist, joined moderators Dave Nicolai, and Dr. Eric Yu, UMN Extension Educators in Crops for the discussion.
What would we do for weed management in corn if resistance to Callisto (a HPPD inhibitor), for example, became widespread in waterhemp? Werle posed this question, emphasizing the importance of being a good steward of this chemistry. If the novel soybean technology with resistance to the HPPD herbicides become widely used, mesotrione could be used year after year, putting significant selection pressure on populations to select for resistance to this chemistry.
Resistance in waterhemp is not only being found to herbicides applied postemergence (POST) but to herbicides applied preemergence (PRE) as well. One issue observed in WI is that the length of residual activity for soil-applied Group 14 herbicides (PPO inhibitors) is shortening. In WI, resistance to herbicides applied PRE and POST has been found with atrazine and Group 14 herbicides. More recently, resistance to the Group 15 herbicide metolachlor (e.g. in the active ingredient in Dual II Magnum) has been documented. It is important to note that resistance issues are being seen mainly when these herbicides are being applied alone, which emphasizes the importance of using an effective mixture for effective control.
For more information and details on herbicide classification groups, refer to the Herbicide Classification Chart.
Many growers are using the Enlist E3 system. Werle reports seeing more effective POST control when Liberty is used with Enlist One, compared to either alone.
The key is to make sure POST applications are made to weeds no larger than 3 inches tall (we know, easier said than done, but that is what it takes for effective POST control). The more applications are delayed, the more the crop will intercept the spray solution. When weeds are shielded by the crop canopy, weeds get exposed to sublethal rates, resulting in poor weed control and selection for tougher to control weeds, according to WI research.
When using glufosinate, AMS is a must to include in the tank. First, AMS is a water conditioner, and second, it is a source of N that increases the uptake and translocation of foliar herbicides. Third, when AMS is included in a glufosinate application, you are adding additional ammonia. Glufosinate controls weeds in part due to the accumulation of ammonia to toxic levels in the plant.
Werle also highlighted research demonstrating how a clogged nozzle can impact coverage and ultimately control. Getting good coverage is particularly important with a contact herbicide like glufosinate. A new set of nozzles can go a long way in optimizing coverage.
See Best practices for using Liberty herbicide and Don't break glufosinate (Liberty)! for additional details on optimizing glufosinate efficacy.
For growers of non-GMO soybean, it is recommended to go extra heavy on the residuals up front. Another consideration, planting very early may not be the best thing to do with non-GMO soybeans as POST options are more limited. Planting soybeans around mid-May should not impact yield potential much, but it will allow for the burndown of a significant flush of weeds typically before crop establishment.
After screening waterhemp populations for the last five to six years, Sarangi officially confirmed that glufosinate-resistant waterhemp has been found in MN. This finding resulted after getting a lead from a farmer with control issues in his field. This work came to fruition thanks to efforts from his grad student Navjot Singh, and support from the MN Soybean Growers.
In the glufosinate-resistant waterhemp population, a target site mutation was not detected through DNA sequencing. It is important to note that the population is also showing reduced sensitivity to other herbicides, including 2,4-D, atrazine, fomesafen, glyphosate, imazethapyr, and mesotrione at 1x and 3x rates. Although there was some survival with dicamba at a 1x rate, this was the only herbicide that killed all of the plants at a 3x rate.
Sarangi’s team is evaluating herbicide programs on this population, using strong PRE herbicides and various POST options. He reiterated the importance of not using reduced rates as this will promote the development of resistant populations.
Sarangi noted that non-target site resistance in weeds, particularly metabolic resistance, is very challenging to deal with (see Why care about metabolic herbicide resistance?) and that metabolic resistance could be the beginning of the end of the chemical weed control era. Using only herbicides to deal with this challenge is unlikely to solve the problem. Non-chemical strategies such as the use of cover crops, weed seed destructors at harvest, cultivation, and planting in narrow rows to encourage canopy closure sooner are just some of the non-chemical tactics we will need to use for weed management into the future.
Both Sarangi and Werle emphasized that when herbicides are used, however, we need to make sure we are doing it right (including using the right rate, adjuvants, timing, gallonage, and applying to small weeds) and mixing up our MOAs as much as possible.
Watch a recording of the full program at the Strategic Farming YouTube channel.
Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.
U of WI: Waterhemp Research Summary: What we have learned from 5 years of chemical waterhemp control in soybean.
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| Waterhemp in soybean. Photo: Liz Stahl |
Good stewardship is needed to help preserve options
Waterhemp is a small-seeded broadleaf weed, and most emergence comes from the top 1 inch of soil or less. It is a very prolific seed producer, with the potential to produce more than one million seeds per plant. It emerges over a long period of time and late into the season, with precipitation and temperature driving weed flushes. Being a dioecious plant (male and female reproductive structures are on separate individuals), there is much potential for genetic diversity within the species.What would we do for weed management in corn if resistance to Callisto (a HPPD inhibitor), for example, became widespread in waterhemp? Werle posed this question, emphasizing the importance of being a good steward of this chemistry. If the novel soybean technology with resistance to the HPPD herbicides become widely used, mesotrione could be used year after year, putting significant selection pressure on populations to select for resistance to this chemistry.
Resistance in waterhemp is not only being found to herbicides applied postemergence (POST) but to herbicides applied preemergence (PRE) as well. One issue observed in WI is that the length of residual activity for soil-applied Group 14 herbicides (PPO inhibitors) is shortening. In WI, resistance to herbicides applied PRE and POST has been found with atrazine and Group 14 herbicides. More recently, resistance to the Group 15 herbicide metolachlor (e.g. in the active ingredient in Dual II Magnum) has been documented. It is important to note that resistance issues are being seen mainly when these herbicides are being applied alone, which emphasizes the importance of using an effective mixture for effective control.
Strategies for success
Starting with a strong PRE herbicide is important in both corn and soybean, and reduced rates are not going to cut it. Werle’s research is showing that starting with a reduced rate of a PRE is a mistake – he recommends front-loading with a PRE program containing multiple effective ingredients at full rates. Their research also shows the most effective options on waterhemp are from the Group 14 herbicides (including sulfentrazone and flumioxazin) and from the Group 15 herbicides (including pyroxasulfone, s-metolachlor, and dimethenamid). Metribuzin can also be a good option (a rate of at least 8 oz/a was needed for good control), but restrictions based on soil type and soil characteristics limit where this product can be applied (refer to the pesticide label for specifics). Although Group 2 herbicides are no longer effective on most waterhemp populations due to widespread resistance issues, adding a Group 2 to a Group 14 or 15 herbicide may help with other broadleaf weeds like common ragweed and giant ragweed, and annual grasses. Front-loading with a strong PRE at a full rate may reduce or eliminate the need to include an additional residual herbicide with a timely POST application.For more information and details on herbicide classification groups, refer to the Herbicide Classification Chart.
Many growers are using the Enlist E3 system. Werle reports seeing more effective POST control when Liberty is used with Enlist One, compared to either alone.
The key is to make sure POST applications are made to weeds no larger than 3 inches tall (we know, easier said than done, but that is what it takes for effective POST control). The more applications are delayed, the more the crop will intercept the spray solution. When weeds are shielded by the crop canopy, weeds get exposed to sublethal rates, resulting in poor weed control and selection for tougher to control weeds, according to WI research.
Tips for best performance with glufosinate
Formulation does matter, and it can greatly impact weed control. Using a low-cost generic version of glufosinate without any additives, for example, will not provide control. Be sure to check the label of the product you are using to make sure what adjuvants are needed. Glufosinate also performs best in sunny conditions and high temperatures.When using glufosinate, AMS is a must to include in the tank. First, AMS is a water conditioner, and second, it is a source of N that increases the uptake and translocation of foliar herbicides. Third, when AMS is included in a glufosinate application, you are adding additional ammonia. Glufosinate controls weeds in part due to the accumulation of ammonia to toxic levels in the plant.
Werle also highlighted research demonstrating how a clogged nozzle can impact coverage and ultimately control. Getting good coverage is particularly important with a contact herbicide like glufosinate. A new set of nozzles can go a long way in optimizing coverage.
See Best practices for using Liberty herbicide and Don't break glufosinate (Liberty)! for additional details on optimizing glufosinate efficacy.
Herbicides alone will not solve our herbicide-resistant weed troubles
A cereal rye cover crop can be very helpful in suppressing waterhemp. Research has shown that about 4,500 lb/a in biomass is needed to suppress waterhemp by at least 50% when no other herbicide is used (See: How we believe cereal rye cover crop suppresses waterhemp). This amount of biomass correlates well to about 30-inch-tall cereal rye. When soybeans are planted early, around mid to late April, research in WI has shown that cereal rye can be terminated about 2 weeks after planting without hurting soybean yield. The current recommendation in soybean is to terminate cereal rye at 30-inch height or when soybean is at the emergence state, whichever occurs first.Dicamba questions and recommendations for conventional soybean producers
Werle reports that he has been getting calls from growers who are having trouble with control with dicamba in corn. His biggest recommendation is to make sure when someone is spraying Status in corn is to use a minimum of 5 oz/a for more consistent control and to consider adding another effective tank mix partner.For growers of non-GMO soybean, it is recommended to go extra heavy on the residuals up front. Another consideration, planting very early may not be the best thing to do with non-GMO soybeans as POST options are more limited. Planting soybeans around mid-May should not impact yield potential much, but it will allow for the burndown of a significant flush of weeds typically before crop establishment.
Glufosinate-resistant waterhemp in MN is here
Surveys of 90 waterhemp populations across MN in 2020 and 2021 found a wide range of herbicide-resistance combinations in populations, including four populations with resistance to six different MOAs. Glufosinate was the only herbicide found to work on these populations.After screening waterhemp populations for the last five to six years, Sarangi officially confirmed that glufosinate-resistant waterhemp has been found in MN. This finding resulted after getting a lead from a farmer with control issues in his field. This work came to fruition thanks to efforts from his grad student Navjot Singh, and support from the MN Soybean Growers.
In the glufosinate-resistant waterhemp population, a target site mutation was not detected through DNA sequencing. It is important to note that the population is also showing reduced sensitivity to other herbicides, including 2,4-D, atrazine, fomesafen, glyphosate, imazethapyr, and mesotrione at 1x and 3x rates. Although there was some survival with dicamba at a 1x rate, this was the only herbicide that killed all of the plants at a 3x rate.
Sarangi’s team is evaluating herbicide programs on this population, using strong PRE herbicides and various POST options. He reiterated the importance of not using reduced rates as this will promote the development of resistant populations.
Sarangi noted that non-target site resistance in weeds, particularly metabolic resistance, is very challenging to deal with (see Why care about metabolic herbicide resistance?) and that metabolic resistance could be the beginning of the end of the chemical weed control era. Using only herbicides to deal with this challenge is unlikely to solve the problem. Non-chemical strategies such as the use of cover crops, weed seed destructors at harvest, cultivation, and planting in narrow rows to encourage canopy closure sooner are just some of the non-chemical tactics we will need to use for weed management into the future.
Both Sarangi and Werle emphasized that when herbicides are used, however, we need to make sure we are doing it right (including using the right rate, adjuvants, timing, gallonage, and applying to small weeds) and mixing up our MOAs as much as possible.
Watch a recording of the full program at the Strategic Farming YouTube channel.
Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.
Additional Sources and Resources
U of WI: 2025 Weed Science Research ReportU of WI: Waterhemp Research Summary: What we have learned from 5 years of chemical waterhemp control in soybean.
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