Field Notes talks post-emergence weed control

Angie Peltier and Ryan Miller, UMN Extension crops educators and Debalin Sarangi, UMN Extension weed scientist

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 Eric Yu.

Weed growth progressing

While recent cool and cloudy weather may have temporarily delayed both crop and weed development, folks that were able to get their crops planted in mid-May may now be seeing 4-6 inch tall giant ragweed plants. Waterhemp, the prolific and multiple herbicide-resistant bane of many Minnesota agronomic crop producers, and lambsquarters are now also reaching 4 inches in height. This information is the equivalent of a giant banner reading, “if you didn’t start post-emergence herbicide applications yesterday, start them today.”

Depending on your location and planting dates, timely rains may have come after planting and pre-emergence herbicide applications, resulting in good activation. Pre-emergence herbicides act on germinating weed seeds as the active ingredient is taken up by the seed along with the water into which it has dissolved. In the event of delayed rains after planting and pre-emergence herbicide applications some weeds likely emerged unaffected by pre-emergence herbicides requiring sooner than anticipated post-emergence applications.

Herbicides often require an actively growing plant for good efficacy and those products with some residual activity will require a timely, activating rainfall. Now that most crops are up and so are many weeds, the forecasted good growing conditions with warmer temperatures followed by rain mean that post-emergence herbicide applications are likely to work well.

The Label is the Law: Herbicide Cutoffs and Max Use Rates

While we apply them to manage weeds, ideally herbicide applications don’t also ‘manage’ our crops. Herbicide labels have various restrictions that help us to avoid injuring both this year’s and next year’s crop. This time of year, it is important to revisit label restrictions related to the current crop’s stage of growth and development. After corn has gotten 12 inches tall, making an atrazine application would be off-label (aka: illegal). A careful perusal of labels of those products you plan to use is also required for those premix products such as Resicore and Acuron that might contain atrazine or some other active ingredient with a crop growth stage-related restriction. 2,4-D herbicide formulations also have an 8-inch height restriction. The University of Minnesota Extension Weed Science team is currently finishing up an article that will be posted on MN Crop News detailing herbicide crop-development-related application restrictions.

For some corn products that can be used both pre- and post-emergence, it is easy to get awfully close to the maximum use rate in any given growing season. Using more than the maximum label rate is not economical, is illegal and can increase the chances of your 2026 crops suffering carryover injury. While folks can use products containing fomesafen or lactofen for soybeans (ex. Flexstar and Cobra, respectively) throughout most of June, careful attention to the calendar is key as fomesafen containing products have a 10-month corn rotation restriction and as we head into July we need to consider making product changes to avoid carryover injury. Dry or droughty weather can also lengthen the time period needed for active ingredients to break down in the soil through either microbial metabolism or hydrolysis, exacerbating carryover issues.

Water quality to improve efficacy

Most Minnesota crop producers are likely using well water to fill their spray tanks. Well water in general tends to be hard water, with high concentrations of dissolved salts. Getting your water tested and then adding water conditioners such as ammonium sulfate (AMS) as needed to deal with any issues you’re your test results uncover can make a large difference in improving herbicide efficacy. Weed scientists at UMN Extension recently wrote an article about pH adjustments for improved efficacy and weed scientists at Purdue University have written about overcoming water quality issues for herbicide efficacy. For those in southeast Minnesota interested in seeing for themselves how water pH can impact efficacy, join UMN Extension for a crops day in Rochester on July 1, 2025.

Best Practices for Enlist One

Throughout the countryside, conversations inevitably shift to the herbicide-tolerance traits people used for soybeans in 2025. Many people have moved to the E3 trait package, meaning that folks can use products from three herbicide groups: glyphosate (Group 9), 2,4-D choline (Group 4) and/or glufosinate (Group 10).

Last year, when the UMN Extension weed science group used Liberty Ultra (glufosinate) with Enlist One, the ever-changing, online label required folks to use a drift-reducing agent. In 2025, in order to ensure the correct droplet size, the Liberty Ultra online label instructs folks to use a methylate seed oil (MSO), or high surfactant methylated crop oil concentrate (HSMCOC) or high surfactant crop oil concentrate (HSCOC) adjuvant to tank mix Liberty Ultra with Enlist One. Paying attention to which spray tips are required will also both keep you on the right side of the law and improve efficacy. Link for approved tank mixes and qualified nozzles.

Weed Management as a Long-Term Endeavor

We can learn from the weed management successes and failures of our friends and neighbors that raise sugarbeets. Rather than thinking about sugarbeet weed management as simply taking place during their 2025 sugarbeet cropping season, preparations for managing broadleaf weed species in 2025 sugarbeets took place in the years preceding 2025. Managing broadleaf weeds in a spring wheat crop provides additional active ingredient options that wouldn’t be available in a broadleaf crop. In addition, the numerous herbicide-tolerance trait packages available to manage broadleaf weeds in soybeans can provide additional chemical weed control options. When tackled aggressively, this multi-pronged, muti-year strategy helps to reduce the density of the weed seed bank encountered by sugarbeets, as not only are additions not being made to the seed bank in non-sugarbeet years, but some of the preexisting seeds in the bank have time to be eaten by soil-dwelling insects or lose viability through natural attrition.

Over a 3-year period, one of Debalin Sarangi’s graduate students managed weeds either normally or aggressively in soybeans planted before sugarbeets and then monitored waterhemp seedling emergence patterns and density over time in the subsequent sugarbeet crops. They found that those plots that had better weed control in soybean had less weed pressure overall in sugarbeets; conversely those treatments that had more waterhemp in the soybean year also had more in the sugarbeet year. The valuable lesson being that even a single year of stellar weed management can result in fewer weeds to contend with in the following year.

In crops such as sugarbeets that have few chemical weed management options, weed managers can still start with a solid foundation of a pre-emergence herbicide in a preceding soybean crop such as a Group 15 herbicide and Verdict. Post-emergence herbicide options can include Cobra, Enlist or Liberty, tank-mixed with Warrant. Folks can also incorporate non-chemical weed management tactics such as hand rouging weeds, starting with a weed-free seed bed due to pre-plant cultivation and using a rotary hoe to ensure that there are no additions to the seed bank.

Successful Use of Contact Herbicides

Contact herbicides have activity on the plant tissue that they come into contact with, not moving far from the point of deposition, making application coverage key for good product efficacy. Also key is making the application when weeds are no larger than 3 inches tall as larger weeds can overlap and provide cover for more newly emerged weeds that won’t be properly controlled. Larger weeds also tend to have more growing points that need to die in order for the plant to die. Making an application in conditions in which plants are actively growing (mid-day, sunny weather, 80 degree plus temperatures and higher humidity) can also help to improve contact herbicide activity.

Using properly conditioned water to mix up a tank of Liberty, for example, is essential. For Liberty in particular, 3 pounds of AMS is needed per acre. UMN Extension demonstration projects have shown quite clearly that using less than 3 lb of AMS per acre can result in less than adequate weed control. Being sure of the concentration of AMS in the product that you plan to use can also help to ensure that you have adequate AMS in your tank mix.

Lower than required AMS additions resulted in subpar common lambsquarters control.

Two musts for getting good coverage include adequate carrier volume and proper droplet size. Demonstrations have shown that even in a ‘train wreck’ sort of field plagued with high densities of lambsquarters and waterhemp seedlings, the more water applied per acre with Liberty (for example), the better, with 10 gal/A providing poor control, 15 gal/A better control and 20 gal/A nearly 100% control. Photos below are from a 2024 research and demonstration trial at the Rochester research and demonstration site.

Liberty herbicide applied with appropriate adjuvants at 10 gallons per acre.

Liberty herbicide applied with appropriate adjuvants at 15 gallons per acre.

Liberty herbicide applied with appropriate adjuvants at 20 gallons per acre.

Audience questions

Sarangi and Miller answered numerous audience questions including, “I’m noticing more iron deficiency chlorosis symptoms this year. Could corn herbicides used in the year before my soybean crop cause increased symptoms in my soybean crop?”, “With the forecasted rain for southern Minnesota, can you comment on the rain fast period of various active ingredients and would there be any need for adjuvants?”.

Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.