Angie Peltier and Ryan Miller, UMN Extension crops educators and Debalin Sarangi, UMN Extension weed specialist
The following information was provided during a 2025 Strategic Farming: Field Notes session. Use your preferred podcasting platform or listen online to a podcast of this Field Notes session hosted by UMN Extension crops educator Dave Nicolai.
In southeast Minnesota, field work and planting progress have been delayed in comparison with western Minnesota. While early April allowed for field work, limited crop planting occurred in a window from April 11 to 16. Some crops – soybean and corn alike - seeded at that time are beginning to emerge and in some instances emergence and stand development problems are becoming evident. Now is the time to assess soybean and corn stands to determine whether crops experiencing emergence issues may need to be replanted in full or part. Most corn and soybean planting has occurred since May 4 and has yet to emerge.
Throughout much of the state, particularly in the west, soil moisture has been a limiting factor, with IPM specialist emeritus Bruce Potter reporting that southwest Minnesota has been “desert-dry.” The low relative humidities, sustained, drying winds and lack of rainfall have led to red flag warnings issued day upon day throughout much of the state and wildfires near Duluth.
Weeds wait for no one; we are beginning to see weeds in need of management at this early point in the growing season. In southern Minnesota where there was either no pre-emergence herbicide applied or the pre-emergence applied hasn’t been activated, lawns of waterhemp seedlings at the cotyledon stage and 2-inch tall lambsquarters and ragweeds can be observed. In western and northwestern Minnesota, 1.5-3-inch diameter kochia plants have also been observed.
While individual pre-emergence herbicides require varying amounts of water for activation, a general rule of thumb is that ½ to 1 inch of rain within a week of application and 1-2 inches total within 2 weeks of application is needed for activation.
For those that have used or plan to use pre-emergence herbicides, if weed seedlings have already begun to emerge, make sure that your herbicide mixture contains chemistries with some burndown activity that can take care of already-emerged seedlings. If weed seedlings have already emerged before one has made a pre-emergence herbicide application, there is still time to add burndown products to the mix. If you have made your pre-emergence herbicide applications and weeds have started to emerge due to a lack of activating rainfall, make plans for an earlier than normal post-emergence herbicide application. Don’t hesitate and don’t delay once weeds have reached three inches tall, it is time to start spraying.
Some multiple mode-of-action premixes used widely in corn have both post-emergence (foliar activity) and pre-emergence (residual activity). Rather than making the full label rate application at one time and risking the product not activating due to a lack of rain, some farmers have chosen to apply half after planting and half as a post-emergence application. Please consult the specific herbicide label’s “split application” section for rates and other information. If the label does not allow a split application, that should be avoided. Especially in situations where grass weeds are an issue, folks will also need to add additional active ingredients such as glyphosate or glufosinate to the mix. It is important to make certain that the planted variety, hybrid or hybrid blend has the appropriate herbicide tolerance trait(s) prior to making your herbicide product choice. Other herbicide options are available for farmers growing conventional crops that lack herbicide tolerance traits.
Soybean pre-emergence herbicides do not provide reach back. If one planted soybeans without using a pre-emergence herbicide, it is recommended that one add products with residual activity with a timely post-emergence application.
Contact herbicides, such as Flexstar, Cobra and Liberty Ultra, do not move systemically within weeds; only the tissue onto which the herbicide is deposited will be killed. Therefore, adequate coverage is key to getting good control with contact herbicides. Including all required or recommended adjuvants, conditioning water, having adequate carrier volume (the more the better), using the full label rate, and targeting 3-inch-tall weeds are keys to better weed control.
Systemic herbicides move from the site of deposition to a plant’s growing sites and also tend to work best when applied to smaller weeds. Research has shown that targeting 3-inch-tall weeds with Enlist will result in better weed control than targeting weeds that are more than 4 inch tall. Plants 8 inches tall may not be killed even when applying 2,4-D in a tank mix with other herbicide active ingredients.
Seriously consider layering a Group 15 herbicide with residual properties when you make your post-emergence applications to target germinating weed seedlings and prolong your residual weed control.
For soils that didn’t receive rain after planting for sufficient pre-emergence activation but have adequate moisture at the seeding depth of the crop, larger-seeded weeds like velvetleaf and giant ragweed may be able to germinate and push through the dry layer to emerge. In these instances, be prepared for careful, periodic scouting and an earlier-than-planned post-emergence application.
Dry conditions can lead to dry roadways, and on many of our rural township roads those roads are paved with crushed rock. Driving on these rural roads can kick up significant clouds of dust that can move into crop fields. Be aware that research has shown that herbicide efficacy can decrease in plants closest to these roads due to the dust covering weed tissue. Certain herbicides (e.g. glyphosate) bind to soil or dust particles and will become less effective; however, the use of adjuvants (such as ammonium sulfate and nonionic surfactants) helps overcome this issue to some extent.
To get better weed control, some folks may have decided to use a pre-plant, incorporated herbicide in soybean, such as a dinitroaniline product. While this approach can work well in some instances, including in dry conditions, there is a risk of incorporating the product below the depth of weed seedling emergence, thereby diluting the herbicide’s efficacy by applying some of it where it is not likely to work. Caution and carefully following label directions is urged when using this approach, as some herbicides will explicitly state that pre-plant incorporation is not recommended because of the risk of crop injury due to direct contact with germinating crop seeds.
In addition, the dry fall 2024 through spring 2025 period means that carryover injury symptoms might be seen in soybeans or corn. If in 2024 you used herbicides containing Stinger, Flexstar, or HPPD’s there is an increased chance of seeing carryover injury.
Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.
Disclaimer: Note that inclusion or exclusion of herbicide active ingredients or trade names are for illustration purposes only and neither implies endorsement nor lack of endorsement.
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| Early season weed flush. Photo: Dave Nicolai |
Crops and weeds are emerging
According to the USDA National Ag Statistics Service’s planting progress and crop condition report, as of this past weekend corn planting was 75% complete; this was 6 days ahead of 2024 and also ahead of the 5-year average. Thirty percent of the corn crop has emerged, about 5 percentage points ahead of average. Soybeans were 52% planted, 8 and 6 days ahead of both 2024 and the 5-year average, respectively. Fourteen percent of soybeans had emerged, running about a week ahead of the 5-year average.In southeast Minnesota, field work and planting progress have been delayed in comparison with western Minnesota. While early April allowed for field work, limited crop planting occurred in a window from April 11 to 16. Some crops – soybean and corn alike - seeded at that time are beginning to emerge and in some instances emergence and stand development problems are becoming evident. Now is the time to assess soybean and corn stands to determine whether crops experiencing emergence issues may need to be replanted in full or part. Most corn and soybean planting has occurred since May 4 and has yet to emerge.
Throughout much of the state, particularly in the west, soil moisture has been a limiting factor, with IPM specialist emeritus Bruce Potter reporting that southwest Minnesota has been “desert-dry.” The low relative humidities, sustained, drying winds and lack of rainfall have led to red flag warnings issued day upon day throughout much of the state and wildfires near Duluth.
Weeds wait for no one; we are beginning to see weeds in need of management at this early point in the growing season. In southern Minnesota where there was either no pre-emergence herbicide applied or the pre-emergence applied hasn’t been activated, lawns of waterhemp seedlings at the cotyledon stage and 2-inch tall lambsquarters and ragweeds can be observed. In western and northwestern Minnesota, 1.5-3-inch diameter kochia plants have also been observed.
Rain is needed to activate pre-emergence herbicides
With the widespread problem of herbicide-resistant weed species, UMN Extension has long recommended using pre-emergence herbicides to target weeds at a vulnerable point in their lifecycle – germination. Pre-emergence herbicides need to be in the soil solution, dissolved in water for germinating weed seeds to imbibe and be controlled by them. This means that rain is needed to activate pre-emergence herbicides.While individual pre-emergence herbicides require varying amounts of water for activation, a general rule of thumb is that ½ to 1 inch of rain within a week of application and 1-2 inches total within 2 weeks of application is needed for activation.
For those that have used or plan to use pre-emergence herbicides, if weed seedlings have already begun to emerge, make sure that your herbicide mixture contains chemistries with some burndown activity that can take care of already-emerged seedlings. If weed seedlings have already emerged before one has made a pre-emergence herbicide application, there is still time to add burndown products to the mix. If you have made your pre-emergence herbicide applications and weeds have started to emerge due to a lack of activating rainfall, make plans for an earlier than normal post-emergence herbicide application. Don’t hesitate and don’t delay once weeds have reached three inches tall, it is time to start spraying.
Some multiple mode-of-action premixes used widely in corn have both post-emergence (foliar activity) and pre-emergence (residual activity). Rather than making the full label rate application at one time and risking the product not activating due to a lack of rain, some farmers have chosen to apply half after planting and half as a post-emergence application. Please consult the specific herbicide label’s “split application” section for rates and other information. If the label does not allow a split application, that should be avoided. Especially in situations where grass weeds are an issue, folks will also need to add additional active ingredients such as glyphosate or glufosinate to the mix. It is important to make certain that the planted variety, hybrid or hybrid blend has the appropriate herbicide tolerance trait(s) prior to making your herbicide product choice. Other herbicide options are available for farmers growing conventional crops that lack herbicide tolerance traits.
What about “reach back”?
“Reach back” is when a pre-emergence herbicide applied in a crop field is not activated by a timely rainfall, but once activated, can “reach back” and kill emerged weeds. Reach back is an elusive and unpredictable phenomenon, with some weed scientists likening the odds of reach back taking place to winning the lottery. Reach back is more likely to occur the shorter the time period between weed emergence and activation. HPPD inhibitor herbicides (Group 27) when applied in a premix with other chemistries can sometimes result in reach back.Soybean pre-emergence herbicides do not provide reach back. If one planted soybeans without using a pre-emergence herbicide, it is recommended that one add products with residual activity with a timely post-emergence application.
Application strategies for contact and systemic herbicides
DO NOT WAIT for weeds to grow larger than 3 inches before beginning your post-emergence herbicide applications. Larger weed species have more growing points that will need to be killed in order for that plant to not re-grow. In addition, larger weeds shade out smaller weeds, making it more difficult to get good coverage as more recently emerged seedlings may be physically shielded by those larger, earlier-emerged seedlings.Contact herbicides, such as Flexstar, Cobra and Liberty Ultra, do not move systemically within weeds; only the tissue onto which the herbicide is deposited will be killed. Therefore, adequate coverage is key to getting good control with contact herbicides. Including all required or recommended adjuvants, conditioning water, having adequate carrier volume (the more the better), using the full label rate, and targeting 3-inch-tall weeds are keys to better weed control.
Systemic herbicides move from the site of deposition to a plant’s growing sites and also tend to work best when applied to smaller weeds. Research has shown that targeting 3-inch-tall weeds with Enlist will result in better weed control than targeting weeds that are more than 4 inch tall. Plants 8 inches tall may not be killed even when applying 2,4-D in a tank mix with other herbicide active ingredients.
Seriously consider layering a Group 15 herbicide with residual properties when you make your post-emergence applications to target germinating weed seedlings and prolong your residual weed control.
Dry soils and weeds
Many of the waterhemp seedlings that have already emerged have done so in very dry soils and, as a result, have not grown much since. All it will take is another rainfall event and some heat for them to grow very quickly, outgrowing our ability to control them chemically.For soils that didn’t receive rain after planting for sufficient pre-emergence activation but have adequate moisture at the seeding depth of the crop, larger-seeded weeds like velvetleaf and giant ragweed may be able to germinate and push through the dry layer to emerge. In these instances, be prepared for careful, periodic scouting and an earlier-than-planned post-emergence application.
Dry conditions can lead to dry roadways, and on many of our rural township roads those roads are paved with crushed rock. Driving on these rural roads can kick up significant clouds of dust that can move into crop fields. Be aware that research has shown that herbicide efficacy can decrease in plants closest to these roads due to the dust covering weed tissue. Certain herbicides (e.g. glyphosate) bind to soil or dust particles and will become less effective; however, the use of adjuvants (such as ammonium sulfate and nonionic surfactants) helps overcome this issue to some extent.
To get better weed control, some folks may have decided to use a pre-plant, incorporated herbicide in soybean, such as a dinitroaniline product. While this approach can work well in some instances, including in dry conditions, there is a risk of incorporating the product below the depth of weed seedling emergence, thereby diluting the herbicide’s efficacy by applying some of it where it is not likely to work. Caution and carefully following label directions is urged when using this approach, as some herbicides will explicitly state that pre-plant incorporation is not recommended because of the risk of crop injury due to direct contact with germinating crop seeds.
In addition, the dry fall 2024 through spring 2025 period means that carryover injury symptoms might be seen in soybeans or corn. If in 2024 you used herbicides containing Stinger, Flexstar, or HPPD’s there is an increased chance of seeing carryover injury.
Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.
Disclaimer: Note that inclusion or exclusion of herbicide active ingredients or trade names are for illustration purposes only and neither implies endorsement nor lack of endorsement.
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