ISSUE 12   July 30, 2008


The following was written by Dr. Chris Boerboom, Weed Scientist, U of WI.

Considering the cost of nitrogen, we can ill afford to allow weeds to use this expensive input. We conducted an experiment for the past 2 years that provided results that truly surprised us in regard to weeds and nitrogen. Basically, we confirmed that weeds are highly effective consumers of nitrogen. As a consequence, the potential exists for weeds to capture some of the nitrogen that the corn needs for optimum yields. We just didnít expect the effect to be so large. In term of competition, the biggest risk would seem to be when weeds are allowed to grow too large before being controlled postemergence. Therefore, our experiment was designed to determine how delayed weed control might affect optimum nitrogen rates in corn.

In the experiment, we applied increasing rates of nitrogen over the range of 0 to 200 lb/a. Within these increasing nitrogen rates, we controlled weeds either preemergence, at a recommended 4-inch height, at a 12-inch height, and a weedy check. Weeds were controlled postemergence with glyphosate in the Roundup Ready corn. The 12-inch height was used to mimic a time delay that might occur because of rainy or windy weather or other delays. The time difference between the 4-inch height and the 12-inch height was 8 days in both years.

A few key findings:

  • When weeds were sprayed at 4 inches, they had accumulated an average of 12 lb/a N. However, by 8 days later, they had accumulated an average of 25 lb/a N when they were 12 inches tall.
  • The nitrogen uptake by the weeds reduced the nitrogen that the corn was able to accumulate. Corn had 15 lb/a less nitrogen (or 18% less) at tasseling when weeds were controlled at 12-inches tall as compared to when weeds were controlled preemergence. 
  • Surprisingly, corn yields were similar when 200 lb/a N was applied, regardless of whether the weeds were controlled preemergence or when they were 12-inches tall. How could that be? Apparently, we replaced the nitrogen that the weeds robbed from the corn early in the season to regain this yield. (When the weeds were controlled preemergence, the corn had excess nitrogen that it did not use efficiently.) The economic optimum nitrogen rate was at least 100 lb/a lower when weeds were controlled preemergence or by 4-inches tall compared to when weeds were controlled at 12-inches (Table 1).
  • Table 1. Economic optimum nitrogen rates in corn with four weed management treatments at a 0.15 nitrogen:corn price ratio.


    Economic optimum nitrogen rate (lb N/a)

    Weed management treatment



    Weed-free (preemergence)



    4-inch weed control timing



    12-inch weed control timing



    Weedy (nontreated)



    - Corn yield loss averaged 15 bu/a or 7% in 2006 and 25 bu/a or 11% in 2007 when weeds were not controlled until they were 12-inches tall compared to the preemergence timing. This yield loss would not be acceptable at $2/bu corn and certainly would be outrageous at $5/bu corn. The bulk of this yield loss must have been caused by competition for nitrogen.

    - At the end of the season, the best weed control (or fewest weeds) may not always provide the best corn yield. In 2006, the plots sprayed at the 12-inch timing had the fewest weeds at the end of the season, but not the highest yield. Remember, the goal of weed management is to protect crop yield.

    Rich Zollinger
    Extension Weed Specialist

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