Pay Attention to Timing

Remove weeds before they interfere with yield 

Are you killing weeds at the right time — before they start cutting into yield?  

Research shows that good weed control within the first four to six weeks after crops are planted is critical to avoid yield losses from weed interference. Iowa State University weed experts remind growers that “the effectiveness of any weed control program depends largely on timeliness of the control.” 

Every crop has a critical period when weeds must be removed to prevent yield losses, says Stevan Knezevic, University of Nebraska Extension integrated weed management specialist:  

  • In soybeans, the critical period of weed control (CPWC) begins at the V1 to V3 growth stage, or about nine to 19 days after crop emergence depending on row spacing. When weed removal is delayed beyond that time, yields drop 2 percent per leaf stage until canopy closure.  
  • In cornweeds can begin reducing yield as early as V2 to V3, when corn is about 6 to 8 inches tall. As with soybeans, when weeds are allowed to grow after that time, corn yields fall 2 percent per leaf stage until canopy closure. University of Minnesota research shows that corn yields in Minnesota test fields dropped by 40 bushels per acre when weed removal was delayed just five days beyond the critical control period.  
  • Cottonanother crop that is very sensitive to early-season weed competition, can suffer yield losses from weed interference as early as the V2 growth stage.  

In addition to crop sensitivity to weed competition, several other factors influence the critical weed-control period. These include: 

  • Cropping practicesIn both soybeans and cotton, narrower row spacings can delay the CPWC. In corn, increased nitrogen fertilizer boosts the crop’s weed tolerance, delaying the CPWC. 
  • Weed spectrumSomeweeds are particularly competitive or hard to control. For example, fast-growing giant ragweed that emerges with the crop will nudge the CPWC earlier. 
  • Weed sizein relation to crop stage. Weeds that emerge before the crop also push the CPWC earlier 
  • Pre-emergence weed control. An effective pre-emergence application lengthens the CPWC, giving you more time for postemergence weed control, weed experts say. It also cuts the density and variety of species that need to be removed with your post application. In addition, weeds will be smaller and more uniform in size, making them easier to kill.  

Photo Credit © iStock AleksandarDickov 2017

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