Weed control and corn crop assessment wraps up UMN Strategic Farming Field Notes

Weed control issues and status of the corn crop were topics of discussion on the season wrap-up of the University of Minnesota Extension Strategic Farming: Field Notes program on August 13.

Weed control update

Consistent rains during June and July were a likely contributor to an uptick in weed escapes this year, particularly with waterhemp.  Consider waterhemp biology – waterhemp germinates when we have a cooling cycle followed by a warming cycle.  Rainfall events typically bring cooler temperatures, resulting in cooler, moist soils.  When the sun comes out and soils start to dry, a flush of waterhemp is generated.

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 into August has the potential to flower and produce seed, although less than plants that emerge earlier.

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.  An integrated approach to control waterhemp and prevent seed production is needed for successful long-term management.

This season there have been reports of reduced herbicide performance in waterhemp, including with glufosinate (a.i. in Liberty).  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.  Preemergence (PRE) herbicides can shorten the window for when we need to deal with later-emerging weeds.  An effective PRE herbicide application provides a good start to the season and should be a critical part of weed management programs.

Note that glufosinate performs best under humid, sunny, and warm conditions.  Weed height at application and coverage also impact glufosinate effectiveness.  Applications should be made to small weeds (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 needs tremendous coverage as weed size increases.  Nozzle selection, water volume (at least 20 gpa), and droplet size also contribute to overall control.

It is way too late in the season (and off-label) based on crop stage to be applying products like Liberty, Enlist, or Roundup in 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

The corn crop overall is looking very good across the state.  Temperatures have not been too high, and it’s been quite humid, so plants haven’t been using as much water as they normally would this time of year.  Considering this along with the amount of precipitation most areas have received, the crop should make it to the end of the season with little to no moisture stress.

Several crop diseases including southern rust, tar spot, and gray leaf spot have been found in the state, although usually at low enough levels so as not to cause a yield loss.  Check UMN resources https://extension.umn.edu/corn/corn-pest-management for more information and 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 at z.umn.edu/strategicfarming.  Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.