Canada thistle management systems for sustainable and organic farms

Canada thistle (Cirsium arvense) is a vigorous, competitive perennial weed that is difficult to control in both conventional and organic farming systems. This study was conducted to examine the effects of summer annual cover crops and defoliation on Canada thistle suppression and to identify mechanisms of control. In field experiments conducted on two existing thistle populations, the effects of cover cropping and mowing on Canada thistle suppression were investigated. Whole plot treatments were fallow, buckwheat, sudangrass-cowpea mixture (MIX), and sudangrass with mowing frequencies (0 to 2 times) as subplots. MIX and sudangrass produced more standing biomass, greater regrowth, and more surface mulch following mowing than the buckwheat. Sudangrass and MIX reduced Canada thistle shoot density and biomass. Mowing was effective in suppressing Canada thistle shoot density and biomass only on the site with greater initial density. A sudangrass or MIX cover crop alone or combined with mowing could reduce survival of Canada thistle in sustainable or organic farming systems.;Greenhouse experiments examined the mechanisms causing Canada thistle suppression by defoliation and a sudangrass cover crop. Multiple defoliations 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 shoot mass and number. The combination of sudangrass interference, defoliation, and surface mulch provided the greatest Canada thistle suppression compared to thistles subjected to only defoliation, only interference, or the combination of interference and defoliation. Sudangrass allelopathy did not suppress Canada thistle root or shoot mass, but thistle allelopathy or interference appears to have affected or altered Sudangrass root-to-shoot ratio. Sudangrass cover crop interference, environmental modifications from a sudangrass surface mulch, and less Canada thistle photosynthesis from defoliation can work in unison to suppress Canada thistle and reduce long-term fitness. All mechanisms are believed to have decreased photosynthetic capacity and thistle root systems, suppressing Canada thistle plant growth and potential for spread. Canada thistle shoot mass and numbers may serve as an aboveground proxy for root mass and overall plant health, enabling managers to quantify the effectiveness of control strategies.;Additionally, field studies were conducted on populations of USDA landrace and commercial sorghum accessions to evaluate morphological traits, regrowth potential, and suitability for cover cropping and biofuel applications. Canonical variates in Canonical Discriminant Analysis (CDA) indicated that the traits of unmowed tiller number, regrowth tiller number, and regrowth biomass were the most significant in discriminating between accessions. The accessions clustered into three subgroups. Three multi-stemmed accessions with an ability to regrow clustered away from the bulk of the USDA sorghums in CDA and scatterplots. Multi-stemmed accessions may be useful for breeding improved summer annual cover crops that are tall, produce copious amounts of biomass, and rapidly regrow after defoliation; although propensity to lodging, poor germination, and allelopathic potential may be problems. Additionally, several landrace sorghum accessions in the USDA germplasm collection may be useful for breeding cover crop and biofuel feedstocks, due to great height and biomass production, although it will be necessary to select for improved regrowth potential. Crosses between USDA landraces and the multi-stemmed accessions could lead to a sorghum cover crop and biofuel plant with great unmown biomass and height and ability to regrow following mowing.