After a dry winter and early growing season start, rain has turned this into a tale of two springs. In west central and northwestern Minnesota, early-seeded small grains and corn were planted before the frequent, heavy rains started in mid-May. Since then, planting soybeans and other crops has been a challenge, according to Dr. Joe Ikley, Extension weed specialist at North Dakota State University.

Although corn and soybean planting is on track with the 5-year state averages, cool weather, rain and a lack of sunshine have limited crop and weed growth in the entire region.  When sunshine and warm temperatures return, weeds like waterhemp can grow an inch every day.  It won’t take long for them to exceed recommended treatment heights, so fields need to be watched closely. 

“Weeds will continue to emerge because we have so much soil moisture,” states Dr. Debalin Sarangi, University of Minnesota Extension weed specialist, “so adding a Group 15 residual herbicide like Warrant, Dual or Outlook to the postemergence herbicide tank mix will help with control.” 

Where growers had an opportunity to apply preemergence (PRE) herbicides they have been working very well. In contrast, where only early postemergence herbicides were applied, herbicide activity has been slow and fields tend to be weedy.  

With a contact herbicide, you should have a sense of how well it’s working within 3 to 4 days. Weeds may not be dead, but you can still gauge effectiveness.  Systemic herbicides work best when weeds are actively growing, so conditions that delay growth also delay translocation and control.  If you normally expect activity within 5 days, you may need 7-10 days or more to gauge how well it’s working.

Herbicide drift is a concern with wind speeds trending upward, especially in this tale of two springs where crops are at varying stages of development. There are still relatively good, although small, application windows at either end of the day when wind speeds are low enough. When spraying near sunrise or sunset, it is important to pay attention to low level temperature inversion conditions as they increase your risk of herbicide drift.

While end-of-day applications minimize drift, herbicide activity is less effective. If wind speeds allow, it’s best to apply herbicides near midday when it’s sunny and weeds are growing fast.  Check your weather app to find the best time for postemergence applications. 

Where PRE application windows were limited in corn, growers will be relying on early POST applications. While many products recommend application at V2 or 12-inch corn, height restrictions vary depending on the premix. Read the label for growth stage cutoffs.

Many corn herbicide products can be applied either as a PRE or POST application. Because of this, growers can likely stick with their weed control program. In contrast, if PREs didn’t get applied in soybeans and the field was rained out, herbicides may need to be switched.

Dr. Sarangi’s team surveyed waterhemp populations for herbicide resistance and what they found was alarming. Multiple herbicide resistance is spreading in Minnesota, with some waterhemp populations surviving six different sites of action. Liberty is the only postemergence herbicide that waterhemp is not resistant to, so it’s important to preserve its effectiveness: Spray smaller weeds, use multiple herbicide sites of action, rotate crops and herbicide traits, and consider using cover crops and tillage. 

“Most kochia populations have glyphosate resistance and there are increasing pockets of dicamba resistance,” states Dr. Ikley. He also focuses on Group 14 resistance, since these herbicides are important for burndown in no-till and kochia control in dry beans and sugarbeets. They still have good activity, but kochia control decreases rapidly when weeds exceed two inches. “For the best control, at least three effective active ingredients should be applied,” he adds. 

Neither weed scientist is a fan of rescue treatments. The herbicide application will kill off the most susceptible of the tall weeds, but given their size, there will likely be survivors. They would  at least be more herbicide-tolerant, which can become problematic long term as populations shift toward resistance.

For more information from University of Minnesota Extension, visit extension.umn.edu/crop-production.

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

Article contributed by Phyllis Bongard, U of M Extension content development and communications specialist, Farmington, bonga028@umn.edu.