by Liz Stahl, Extension educator – crops, Tom Peters, UMN & NDSU Extension sugarbeet agronomist and weed scientist, and Dr. Jeff Coulter, UMN Extension corn agronomist
Weed control issues during the 2025 season and status of the corn crop were topics of discussion on the August 13 University of Minnesota Extension Strategic Farming: Field Notes program. Dr. Tom Peters, UMN & NDSU Extension sugarbeet agronomist and weed scientist & Dr. Jeff Coulter, UMN Extension corn agronomist discussed these issues and more, with moderator UMN Extension crops educator, Dave Nicolai.
Common ragweed, giant ragweed, and common lambsquarters are among the first weeds to germinate and emerge. Waterhemp emergence follows. Waterhemp was out of the ground by early May this year, which was typical. What was not typical, was the consistency of rains this year, especially during June and July. Waterhemp germinates when we have a cooling cycle followed by a warming cycle. A rain event typically brings cooler temperatures, resulting in cooler, moist soils. When the sun comes out and soils start to dry, this can generate a flush of weeds.
Where crop stands were thin or where the crop canopy didn’t close, this created a perfect storm for late waterhemp germination and emergence. Waterhemp that emerges in early August still has the potential to flower and produce seed. Late-emerging common ragweed and giant ragweed have been observed this year as well, particularly on field edges. Conditions have been ripe this year for robust weed germination and emergence.
Glufosinate performs best under humid, sunny, and warm conditions. Peters would rank humidity as the number one need for consistent waterhemp control with glufosinate. Humidity helps by preventing the spray droplet from drying quickly. Humidity also helps keep the leaf cuticle hydrated, which improves movement of Liberty through the cuticle. Inconsistencies in control this year may be related to dry air masses in June and early July.
Weed height at application and coverage impact glufosinate effectiveness as well. Applications should be made to small weeds (target less than 3 inches in height). A key reason for this is that as weeds grow, the number of growing points on the weed increases. A contact herbicide like glufosinate will need tremendous coverage as weed size increases. Nozzle selection, water volume, and droplet size also contribute to overall waterhemp control. Peters recommends applying glufosinate at 20 gpa at least for adequate control.
Mesotrione products, such as Callisto or premixes like SureStart and Triple Flex, have been staples in broadleaf control in corn. Inconsistencies in control have been reported this year, however, and this is something that continues to be investigated.
It is way too late in the season (and off label) based on crop stage to be applying products like Liberty, Enlist, or Roundup on soybean to try and prevent waterhemp seed production. Hand weeding and harvesting weedy areas separately are a couple strategies that remain at this point. Plan for a robust weed management program in 2026 in these areas.
Temperatures have not been too high, and it’s been quite humid lately, so plants haven’t been using as much water as they normally would this time of year. This factor, along with the precipitation many areas have received, should carry us through to the end of the season without moisture stress across most of the state.
Wind events in late July did result in greensnap and lodging in parts of southern MN. At this time in the season, if plants snapped below the primary ear, this will likely result in a total loss in yield for the affected plants. If snapping occurred above the ear, yield loss of around 50% could be expected. Any fields damaged by hail should be targeted for early harvest.
Several crop diseases including gray leaf spot and southern rust have been found, but usually at low enough levels so as not to cause a yield loss. Tar spot has also been found, mostly at low levels to date. Keep disease issues in mind when selecting hybrids for 2026. Coulter also recommends selecting hybrids that are consistent top yielders (top 20%) for your area.
Listen to the full program through your preferred podcasting platform or at the Strategic Farming: Field Notes website. This session wraps up the 2025 Strategic Farming: Field Notes program. Stay tuned for the 2026 Strategic Farming: Let’s Talk Crops series which will run Jan – March!
Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.
Weed control challenges relate to weed biology
There has been an uptick in questions and concerns around weed escapes this year, particularly with waterhemp, and what we should do in setting up for 2026. Peters notes we should look first to weed biology when trying to address these issues.Common ragweed, giant ragweed, and common lambsquarters are among the first weeds to germinate and emerge. Waterhemp emergence follows. Waterhemp was out of the ground by early May this year, which was typical. What was not typical, was the consistency of rains this year, especially during June and July. Waterhemp germinates when we have a cooling cycle followed by a warming cycle. A rain event typically brings cooler temperatures, resulting in cooler, moist soils. When the sun comes out and soils start to dry, this can generate a flush of weeds.
Where crop stands were thin or where the crop canopy didn’t close, this created a perfect storm for late waterhemp germination and emergence. Waterhemp that emerges in early August still has the potential to flower and produce seed. Late-emerging common ragweed and giant ragweed have been observed this year as well, particularly on field edges. Conditions have been ripe this year for robust weed germination and emergence.
Waterhemp continues to be a challenge
Being a dioecious plant (separate male and female plants), there is tremendous potential for diverse and unique genetic combinations in waterhemp as pollen from a male plant could fertilize a female a half a mile away. As the prevalence of herbicide resistance continues to increase in waterhemp, farmers are being left with fewer effective postemergence herbicide options. Peters notes weed scientist Dr. Alan Dexter would say “dead weeds don’t make seeds”, and that they also don’t make new mutations in plants. This reinforces the need to double down on waterhemp control and use an integrated approach in controlling waterhemp and preventing seed production at the end of the season.Herbicide performance and application conditions
This season there have been reports of reduced herbicide performance in waterhemp, including with glufosinate (a.i. in Liberty) and mesotrione (a.i. in Callisto). Considering that waterhemp germination and emergence can be continuous over a season, it can be very difficult to time postemergence herbicide applications for effective, full-season control. An application of an effective preemergence (PRE) herbicide is critical for a good start to the season. PRE herbicides can also shorten the window for when we need to deal with later-emerging weeds. As we look to 2026, PRE herbicides should continue to be a key part of our weed management plans.Glufosinate performs best under humid, sunny, and warm conditions. Peters would rank humidity as the number one need for consistent waterhemp control with glufosinate. Humidity helps by preventing the spray droplet from drying quickly. Humidity also helps keep the leaf cuticle hydrated, which improves movement of Liberty through the cuticle. Inconsistencies in control this year may be related to dry air masses in June and early July.
Weed height at application and coverage impact glufosinate effectiveness as well. Applications should be made to small weeds (target less than 3 inches in height). A key reason for this is that as weeds grow, the number of growing points on the weed increases. A contact herbicide like glufosinate will need tremendous coverage as weed size increases. Nozzle selection, water volume, and droplet size also contribute to overall waterhemp control. Peters recommends applying glufosinate at 20 gpa at least for adequate control.
Mesotrione products, such as Callisto or premixes like SureStart and Triple Flex, have been staples in broadleaf control in corn. Inconsistencies in control have been reported this year, however, and this is something that continues to be investigated.
It is way too late in the season (and off label) based on crop stage to be applying products like Liberty, Enlist, or Roundup on soybean to try and prevent waterhemp seed production. Hand weeding and harvesting weedy areas separately are a couple strategies that remain at this point. Plan for a robust weed management program in 2026 in these areas.
Corn crop status
Coulter reports the corn crop overall is looking very good across the state and that corn is about 2 to 5% ahead of normal in development. Most corn is in the dough stage, so kernel number is set - stress at this point would reduce kernel weight. There has not been a loss of lower leaves that we have been seeing in recent years with N or drought stress.Temperatures have not been too high, and it’s been quite humid lately, so plants haven’t been using as much water as they normally would this time of year. This factor, along with the precipitation many areas have received, should carry us through to the end of the season without moisture stress across most of the state.
Wind events in late July did result in greensnap and lodging in parts of southern MN. At this time in the season, if plants snapped below the primary ear, this will likely result in a total loss in yield for the affected plants. If snapping occurred above the ear, yield loss of around 50% could be expected. Any fields damaged by hail should be targeted for early harvest.
Several crop diseases including gray leaf spot and southern rust have been found, but usually at low enough levels so as not to cause a yield loss. Tar spot has also been found, mostly at low levels to date. Keep disease issues in mind when selecting hybrids for 2026. Coulter also recommends selecting hybrids that are consistent top yielders (top 20%) for your area.
Listen to the full program through your preferred podcasting platform or at the Strategic Farming: Field Notes website. This session wraps up the 2025 Strategic Farming: Field Notes program. Stay tuned for the 2026 Strategic Farming: Let’s Talk Crops series which will run Jan – March!
Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.
The use of pesticide trade names is for educational purposes only. No endorsement is implied, and no discrimination is intended.
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