Canada thistle [Cirsium arvense (L.,) Scop.] control in winter wheat

Field trials have been conducted since 1983 to evaluate herbicides for crop tolerance and Canada Thistle control in winter wheat. Winter wheat has shown excellent tolerance to a variety of herbicides, both fall and spring applied. Stage of development of shoot apices does not correlate with leaf number in winter wheat as it does in spring wheat. The Zadok scale of development may be preferable to leaf counts in winter wheat in establishing preferable time for treatment with herbicides. Preliminary indications are \nthat the timing of application of systemic herbicides such as 2,4-D is not as critical in winter wheat as it is in spring wheat. Sampling adjacent to and in untreated Canada thistle patches demonstrated a competitive relationship between Canada thistle density and winter wheat yield. Good to excellent top growth control of Canada thistle has been obtained with clopyralid, chlorsulfuron, and metsulfuron applied in late spring. In general, top growth control of Canada thistle has been acceptable with 2,4-D and dicamba until late July. Significant yield increases of winter wheat with effective top growth control from spring-applied herbicides have not been obtained.