Submitted to: Weed Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/28/2011
Publication Date: 4/1/2012
Citation: Bicksler, A., Masiunas, J., Davis, A.S. 2012. Canada thistle (Cirsium arvense) suppession by sudangrass interference and defoliation. Weed Science. 60:260-266. Interpretive Summary: Canada thistle, more than any other weed species, is emblematic of the difficulty organic growers face in maintaining effective weed control. Reproducing by both seeds and rhizomes, patches of this perennial weed quickly spread, ravaging crop yields. Attempts to combat Canada thistle through tillage usually exacerbates the problem by cutting rhizomes into many pieces and dispersing patches even further. Previous work demonstrated that competition from a sorghum sudangrass crop or defoliation by mowing both reduced growth of Canada thistle. Our aim was to determine the mechanism by which sorghum sudangrass suppresses thistle. We established sorghum sudangrass and Canada thistle in a greenhouse competition study, in combination with various levels of defoliation. A second greenhouse experiment tested the potential for allelopathic interactions between sorghum sudangrass and Canada thistle. Sudangrass interference was effective by itself in suppressing thistle growth; combining interference with defoliation did not further reduce thistle growth. We found no evidence of a living sudangrass cover crop having a phytotoxic effect on Canada thistle. Sudangrass competition or frequent shoot removal minimizes the photosynthetic capacity of Canada thistle, suppressing the growth of this troublesome perennial weed.
Technical Abstract: Canada thistle is difficult to manage in farming systems with reduced reliance on herbicides, including organic and low-external input systems. Previous field studies found that defoliation or sudangrass interference suppressed Canada thistle. Our objective was to understand the factors causing suppression of Canada thistle. Greenhouse studies found increased defoliation frequency (up to 4 defoliations) decreased Canada thistle shoot height, shoot and root mass, and root-to-shoot ratio. Plants with larger root mass had greater shoot mass and shoot number (r = 0.87 and 0.73, respectively), indicating a probable interdependence of root size (carbohydrate reserves), bud density and subsequent shoot growth. Canada thistle shoot dry mass was 38.7, 2.76, and 0.39 g/pot in the defoliation only, sudangrass interference only, and defoliation + interference + surface mulch treatments, respectively. Sudangrass interference was effective by itself in suppressing thistle growth; combining interference with defoliation did not further reduce growth (i.e. 2.76 and 2.83 g/pot, respectively) In an experiment minimizing interspecific competition, we found no evidence of sudangrass having a phytotoxic effect on Canada thistle. Sudangrass competition or frequent shoot removal minimizes the photosynthetic capacity of Canada thistle, suppressing growth of troublesome perennial weed.