Submitted to: Western Society of Weed Science Research Reports
Publication Type: Research Notes
Publication Acceptance Date: 12/1/2002
Publication Date: 3/13/2003
Citation: Anderson, R.L., Beck, D. 2003. Sequencing crops to reduce weed community density. Western Society of Weed Science Research Reports. pp. 222-223. Interpretive Summary: Producers in the semiarid Great Plains are rapidly changing their crop rotations. This change provides an opportunity for producers to develop ecologically-based crop sequencing, where the benefits of natural ecological processes are incorporated into production systems. Based on weed community data collected from a long-term crop rotation study near Pierre, SD, we determined that aranging crops in a sequence of two winter annual crops followed by two summer annual crops has the greatest impact on weed density. The difference between rotations was 12-fold, demonstrating the effect rotation design can have on weed density. Producers in the Great Plains are fortunate in that both winter and summer annual crops are viable options; thus, producers can reduce weed community density considerably with appropriate crop sequencing.
Technical Abstract: Rotations are rapidly changing in the Great Plains because of no-till systems. In place of winter wheat-fallow, producers are seeking rotations comprised of a diversity of crops. To help producers plan alternative rotations, a cropping systems study was initiated near Pierre, SD in 1990. Sixteen rotations comprised of various combinations of crops are being evaluated in a no-till production system. After 10 years, weed communities among rotations differed considerably. To quantify this difference, we recorded weed densities among four rotations: winter wheat-fallow (W-F), winter wheat-chickpea (W-CP), winter wheat-corn-chickpea (W-C-CP), and winter wheat-corn-soybean-dry pea (W-C-SB-Pea). The rotations include a range of winter and summer annual crops, with winter wheat and dry pea being winter/spring annual (cool season) crops, whereas corn, soybean and chickpea are summer annual crops. Weeds in 10 randomly-placed 0.1 square meter quadrats were counted in July of 2001 and 2002. Weed density averaged 31 plants/square meter in W-F, with downy brome and Japanese chess being the main weed species. When chickpea was included in the rotation (W-CP), weed density increased to 60 plants/square meter, with summer annual weeds such as green foxtail, witchgrass and redroot pigweed as well as the brome species comprising the weed community. When a second summer annual crop, corn, was added to the rotation (W-C-CP), brome species were eliminated in the weed community, but summer annual weeds remained at a density of 25 plants/square meter. In the four-year rotation of W-C-WB-Pea, with a balance of two winter/spring crops (winter wheat and dry pea) followed by two summer annual crops (corn and soybean), weed density was only 5 plants/square meter, a 12-fold difference compared to weed density in W-CP and a 5-fold difference compared to W-C-CP. Designing rotations in a cycle-of-four with two winter annual crops followed by two summer annual crops will minimize the density of the weed community.