Ranking Weed Competitiveness in Soybean

Ranking Weed Competitiveness in Soybean

May 29, 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%-50% 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.

Nebraska Field Research

We conducted field studies at two eastern Nebraska sites in 2002 and 2003 to determine and compare weed competitiveness as influenced by soybean row spacing and the timing of weed emergence relative to crop growth stage. This study was part of a Master's Degree project for Shawn Hock.


  • 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.
Table 1. Relative competitiveness of weeds in 2002 and 2003 field trials.
  1. Common sunflower
  2. Common cocklebur
  3. Giant ragweed
  4. Velvetleaf
  5. Common waterhemp
  6. Redroot pigweed
  7. Common lambsquarters
  8. Giant foxtail
  9. Banyardgrass
  10. Fall panicum
  11. Yellow foxtail


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:


  1. It's important to properly identify weed species, their competitiveness and the weed composition of weedy areas before making weed management decisions.


  2. Planting soybean in narrower rows will reduce the competitiveness of most weed species, providing a competitive advantage to the crop.


  3. 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.


This study was partially funded by a North Central Regional Weed Science grant.

Stevan Knezevic, Extension Integrated Weed Management Specialist
Haskell Agricultural Laboratory, Concord



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