Mechanisms Explaining Sudangrass Interference and Defoliation Suppression of Canada Thistle

Canada thistle (Cirsium arvense) has become the most problematic weed of sustainable and organic farmers in the Midwestern United States. Previous field studies found that sudangrass (Sorghum sudanese) and defoliation suppressed Canada thistle.  Our objective was to understand the factors resulting in suppression of Canada thistle from defoliation and a cover crop of sudangrass. Defoliation (1 to 4 times) reduced Canada thistle shoot and root mass, root-to-shoot ratio, number of shoots, and final height compared to no defoliation. Canada thistle root mass was positively correlated with Canada thistle shoot mass and number. The combination of sudangrass interference, defoliation, and surface mulch suppressed Canada thistle suppression more than only defoliation, only interference, or the combination of interference and defoliation. Sudangrass grown so only allelopathy was possible did not suppress Canada thistle mass. Canada thistle decreased the Sudangrass root-to-shoot ratio. Overall, less Canada thistle photosynthesis from interference or shoot removal suppressed the weed. Additionally, we found that Canada thistle shoot mass and numbers may serve as a proxy for root mass and overall plant health, enabling managers to quantify the effectiveness of control strategies.