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Controlling Canada thistle (Cirsium arvense) is one of the most pressing concerns of organic field crop growers. Not only can it limit yield but it can determine what it is possible to grow in a field and even force producers out of organic production. Currently Canada thistle management relies on intensive tillage, summerfallow, and/or rotation away from grains to perennial forages for several years. However, the focus on tillage and summerfallow can have negative repercussions for soil fertility and quality, and while perennial forages are able to suppress perennial weeds, they are not always an economically feasible option.

There is research to suggest that competitive green manures can be managed to gain control of Canada thistle. Two possible strategies are apparent. The first uses perennial legumes such as white clover and alfalfa, which are periodically mowed (usually for forage production) based on either calendar date, thistle height, or bud formation (Cormack, 2002; Graglia and Melander, 2005). A potential deterrent to producers using this strategy is that perennial legumes such as alfalfa take two to four years to establish (Meyer, 1999), meaning that the infested land is taken out of production during that period. A biennial legume crop such as yellow sweet clover may be a more financially viable alternative to perennial legumes. The second green manure management technique has been recently developed by CETAB+ in Québec. Multiple tillage operations are performed at one- or two-week intervals in May and June, followed by late, dense seeding of annual legumes such as soybean or forage pea (Weill, 2013). The annual green manure is terminated the same year it is established, allowing field crops to be grown the following year. Both of these strategies reduced Canada thistle populations by 75% or greater. Ìý

Methods that mechanically damage Canada thistle can reduce thistle populations outside the green manure phase. After crop harvest, shallow secondary tillage with a skimmer plough or chisel plough, followed several weeks later by deep primary tillage using either deep mouldboard plough or double-layer plough allowed thistle density and biomass to be maintained at very low levels over a seven year period (Gruber and Claupein, 2009). This technique was particularly effective if a perennial grass-clover mix was included in the crop rotation (Gruber and Claupein, 2009). Selective in- or above-canopy clipping of Canada thistle stems has also shown potential, reducing Canada thistle coverage by more than 60% in corn (Valand and Ã…nestad, 2014). An integrated approach that uses strategic tillage combined with competitive green manures, mowing, and above- or in-canopy weed clipping may allow better weed control, while reducing reliance on tillage for Canada thistle control.

The objective of this research is to develop a strategy to control Canada thistle organically. To meet this objective the following sub-objectives will be addressed:

1) Identify practices that reduce density and patch size of Canada thistle infestations

2) Measure the cumulative effects of different management systems on Canada thistle density and patch size

3) Measure the impacts of Canada thistle management systems on soil quality

We anticipate that this study will produce a comprehensive strategy that balances perennial weed control needs with soil quality objectives.ÌýÌýÌýÌýÌý

Cormack, WF. 2002.Ìý Effect of mowing a legume fertility-building crop on shoot numbers of creeping thistle (Cirsium arvense (L.) Scop.). In Powell, J et al. (eds), Proceedings of the UK Organic Research 2002 Conference, Organic Centre Wales, Institute of Rural Studies, University of Wales Aberystwyth, p. 225-226.

Graglia, E, Melander, B.Ìý 2005.Ìý Mechanical control of Cirsium arvense in organic farming.Ìý Paper at 13^th European Weed Research Society Symposium, Bari, Italy, 20-23 June 2005.

Gruber, S, Claupein, W. 2009.Ìý Effect of tillage intensity on weed infestation in organic farming.Ìý Soil and Tillage Research 105: 104-111.

Meyer, D.Ìý 1999.Ìý Alfalfa: Seed germination, seedling growth, vegetative development.Ìý North Dakota State University.Ìý .

Valand, S, Ã…nestad, SI. 2014.Ìý Testing of thistles horror.Ìý .

Weill, A, Bournival, M, Leblanc, M, Chauvette, S. 2013.Ìý Development of two methods to combat perennial sow thistle (Sonchus arvensis L.) and Canada thistle (Cirsium arvense) in organic field crop production.Ìý Quebec Ministry of Agriculture, Fisheries and Food Project Number CETA-1-LUT-11-1538.

Canada thistle (Cirsium arvense L.) is a highly adaptable and persistent noxious weed that is one of the most pressing concerns of organic field crop producers.  Current Canada thistle management relies on intensive summerfallow, and/or rotation to perennial forages for several years.  Tillage and summerfallow can have negative repercussions on soil fertility and quality, and perennial forages are not always an economically feasible option... 

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