Focus on Marestail

Biology, herbicide resistance make marestail a tough competitor 

Marestail (horseweed) competes with soybeans throughout the season and can cut crop yields significantly if it’s not controlled before planting. The Weed Science Society of America calls marestail one of the 10 worst herbicide-resistant weed species. This annual weed is a threat for several reasons:  

  • It is resistanttofive different classes of chemistry. Marestail populations resistant to glyphosate or ALS-inhibiting herbicides are widespread, and biotypes resistant to both chemistries have been found in several states. 
  • Marestailgerminatesin the fall or spring. Winter-annual marestail emerges from late summer through late fall, overwinters as a rosette and grows rapidly in early spring. Summer-annual marestail emerges from late March through June. Its long germination period requires effective residual control in soybeans. 
  • It thrives in no-till.In anIndiana survey, marestail was found in 61 percent of no-till fields, compared to 8 percent of conventionally tilled fields. 
  • Marestailis a prolificseed producerPlants mature in late summer or early fall and produce up to 200,000 seeds per plant. These seeds are readily dispersed by wind, helping to spread herbicide-resistant biotypes.  

Consider the following practices to control marestail:  

  • Use a fallherbicidetreatment to reduce marestail density and size in the spring. Aaron Hager, University of Illinois weed scientist, notes that an herbicide tankmix applied between mid-October and late November can provide more effective and consistent control of emerged marestail than spring-applied herbicides.  
  • Applyan effectivepreplant burndown to ensure the field is free of marestail at soybean planting.  
  • Include effective residual herbicideswith the burndownto extend control for several weeks. In a 2010 Ohio study, soybeans yielded 57 bu. per acre with an effective burndown and 65 bu. per acre when a burndown was stacked with a residual herbicide.  
  • Spraysmallplants. Marestail is most easily controlled in the seedling or rosette stage. The spring burndown should be applied before stem elongation. 
  • Usepreplanttillage to control marestail whenever feasible.  
  • Remove surviving plants manually before they produce seeds. 

Photo Credit: © Big Stock 82741832

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