Weed Control and Nitrogen

Are early-season weeds stealing your nitrogen? 

Weeds compete with crops for sunlight, water and nutrients — especially nitrogen (N). You might be shocked at just how much N early-season weeds can steal, even when they are small.  

Many research studies have demonstrated this insidious loss. A study by the University of Wisconsin in 2006 and 2007, for example, documented what happened to optimum N rates when weeds were allowed to compete with corn for different lengths of time.  

In moderate weed density, if weeds were killed at or before they reached four inches in height, corn yields did not suffer, the Wisconsin researchers found. But when post-emergence weed control was delayed for eight days and weeds reached 12 inches tall before treatment, corn yields fell by 7 to 11 percent.  

In those eight days, weeds gobbled up an alarming amount of N, causing the economic optimum N rate to jump by more than 100 pounds per acre compared to the weed-free treatment. The economic optimum N rate is the point at which the last increment of N returns a yield increase large enough to pay for the additional N.  

Likewise, a study at North Carolina State University in 2011 and 2012 found that weeds allowed to grow 9 inches tall in corn took 80 to 100 pounds per acre of N away from the crop. This occurred regardless of N fertilizer sources or rates.  

Winter annual weeds can also steal enough soil N to affect yields. A 2010–2011 study at Kansas State University showed weeds such as pennycress and henbit in no-till fields took up an average of 16 pounds per acre of N by May. Corn yields fell by 7.6 bushels per acre when herbicide applications were delayed until April or May versus when winter annual weeds were killed in November or March. 

Two lessons growers should take away from these findings include: 

  • Early-season weed competition forNcontributes to crop yield loss. 
  • Weed-controltimingaffects optimum N rates. 


Photo Credit: © iStock: Ptaiwun 2015

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