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Agronomy Day 2008

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Suppressing Canada thistle Using Sudangrass Cover Crop

Abram Bicksler, John Masiunas, and Dan Anderson Graduate Research Assistant, Associate Professor, and Agricultural Research Specialist Department of Natural Resources and Environmental Sciences
Contact: (217) 333-1588 (aslant@illinois.edu) or (217) 244-4469 (masiunas@uinuc.edu)
Web site: http://asap.sustainability.uiuc.edu/members/dananderson/documents/canada_thistle

Canada thistle (Cirsium arvense) is a vigorous, creeping perennial weed that forms dense patches. In organic production, systems minimizing herbicides, or integrated with translocated herbicides you can use a Sudangrass (a warm season forage grass) cover crop to suppress Canada thistle. The aim of a Sudangrass cover crop is to outcompete the thistle when its roots are replenishing carbohydrates.

We recommend combining spring tillage, Sudangrass cover crops, and mowing or grazing. Concentrate early season (March to May) tillage on areas with Canada thistle. The goal is to eliminate the first flushes of Canada thistle and cut-up and bring roots to the soil surface. Early season tillage does not eliminate root pieces in the upper soil depths and or control thistle roots deeper in the soil.

Sudangrass can suppress shoots emerging from those remaining roots. Three months after seeding Sudangrass, the thistle was 3% of initial numbers. The following year, areas formerly with Sudangrass had stunted Canada thistle and numbers only 2% of those the previous spring. Use a forage variety of Sudangrass (i.e. ‘Special Effort’, ‘Piper’) or a sorghum-Sudangrass hybrid (i.e. ‘Greengrazer V’, ‘BMR’). Broadcast or drill Sudangrass at 20 to 35 lb/ acre.

Mowing when Sudangrass is three to four feet tall can further suppress Canada thistle and manage Sudangrass shoot production. Mowing once reduced the number of Canada thistle shoots more than mowing twice. Mowing alone only suppressed thistle for a single growing season. Sudangrass combined with mowing controls Canada thistle due to competition for resources, creation of a surface mulch, and release of inhibitory compounds.

Mowing when Sudangrass is three to four feet tall can further suppress Canada thistle and manage Sudangrass shoot production. Mowing once reduced the number of Canada thistle shoots more than mowing twice. Mowing alone only suppressed thistle for a single growing season. Sudangrass combined with mowing controls Canada thistle due to competition for resources, creation of a surface mulch, and release of inhibitory compounds.

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