ISSUE 9   July 9, 2009


Weed scientists have developed a scale for ranking the competitiveness of weed species, allowing producers to identify the most problematic species and develop a comprehensive control plan tailored to fit their weed problem.

Competitive ratings are usually based on the dry matter produced by weeds. Weed competitiveness is highly influenced by cropping practices, including crop row spacing. For example, narrower crop rows can reduce weed competitiveness by 20 to 50 percent compared to crop fields planted in wider rows. Weed competitiveness also depends on when weeds emerge relative to the crop growth stage. In general, later emerging weeds are much less competitive than earlier emerging ones.

Field Research:

Field studies were conducted at two locations in the mid-west in 2002 and 2003 to determine and to compare weed competitiveness as influenced by soybean row spacing and the timing of weed emergence relative to crop growth stage. The study was part of a Master's Degree project for a graduate student.

  • Soybeans were planted in 7.5- and 30-inch rows.
  • Seven broadleaf and four grass weed species were planted: common lambsquarters, redroot pigweed, common waterhemp, common sunflower, common cocklebur, Pennsylvania smartweed, giant ragweed, yellow foxtail, giant foxtail, fall panicum, and barnyardgrass.
  • Weeds were planted at three soybean growth stages - crop planting (VP), crop emergence (VE), and 2nd trifoliate (V2).
  • Soybean yield data, weed biomass, and weed seed production were collected at the season end.
  • Findings:

    The most competitive weed was common sunflower, producing twice as much dry matter as any other species.

    In general, competitive ratings were affected by row spacing and emergence date. Weed species growing in 30-inch crop rows were more competitive than weeds in 7.5-inch rows. Weeds emerging with the crop were more competitive than those emerging a week or two later.

    Applying the Research:

    The major practical implications of this study are:

  • It's important to properly identify weed species, their competitiveness and the weed composition of weedy areas before making weed management decisions.
  • Planting soybean in narrower rows will reduce the competitiveness of most weed species, providing a competitive advantage to the crop.
  • Scouting fields regularly will help you determine weed emergence relative to crop stage. Weeds emerging a week or two after the crop are much less competitive than those emerging with the crop.


    A new publication, "Identification and Control of Invasive and Troublesome Weeds in North Dakota," by Rodney G. Lym and Andrea Travnicek is available from NDSU free of charge. This 74-page publication contains numerous color photographs and descriptions of 32 weeds. The current list of 12 noxious weeds are included as well as species listed by various counties as noxious. Other species included are either invasive weeds found in bordering states with the potential to move into North Dakota or are commonly misidentified native species that do not require control efforts such as the native thistles. The publication is intended to help land managers identify and control noxious and invasive weeds. The publication is a collaborative effort of NDSU and the North Dakota Department of Agriculture. To obtain the publication, contact the NDSU Agriculture Communication Distribution Center by calling (701) 231-7882 or e-mailing The publication also is available through county Extension offices and the Department of Agriculture.

    Rich Zollinger
    Extension Weed Specialist

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