University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources

June 1, 2007

When planning control measures

Rank weed species competitiveness in soybean

Weed scientists have developed the concept of competitive indices as a scale for ranking competitiveness of different weed species. Competitive indices are usually based on the dry matter produced by weed plants. Weed competitiveness is highly influenced by cropping practices, including crop row spacing. For example, narrower crop rows can reduce the competitiveness of weed species by 20-50% compared to wider rows. Weed competitiveness also depends on the weed emergence time relative to the crop growth stage. In general, later emerging weeds are much less competitive than earlier emerging ones.

Field studies were conducted at two locations in eastern Nebraska in 2002 and 2003 to determine and compare the values for competitive indices among weed species as influenced by soybean row spacing and the weed emergence time relative to the crop’s growth stage. This study is also a Master’s degree project for Shawn Hock.

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

The most competitive weed found in this study was common sunflower, producing twice as much dry matter than any other species. Common cocklebur was the next most competitive weed followed by giant ragweed and then velvetleaf. Common waterhemp was more competitive than redroot pigweed but less competitive than velvetleaf. Common lambsquarters was the next competitive and slightly more competitive than the grassy species. Giant foxtail was the most competitive grass, followed by barnyard-grass, fall panicum and yellow foxtail. In general, competitive indices were affected by row spacing and emergence date. Weeds growing in 30-inch rows were more competitive than those in 7.5-inch rows. Weeds also were more competitive when emerging with the crop than when emerging a week or two later.

The major practical implications of this study are:

  1. It’s important to properly identify weed species and their composition before making weed management decisions since weed species differ in their competitiveness,
  2. Planting soybean in narrower rows will reduce the competitiveness of most weed species, providing a competitive advantage to the crop.
  3. Scout fields to determine weed emergence times relative to the crop stage, because data shows that 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 the North Central Regional Weed Science grant.

Stevan Knezevic
Extension Weeds Specialist
NEREC Haskell Ag Lab

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© 2007 by the University of Nebraska Board of Regents. All rights reserved.
Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.
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