Study of various herbicides effect on two Canada thistle (Cirsium arvense L.) varieties

The response of two varieties of Canada thistle (Horridum, and Integrifolium) to 14 herbicides was examined under controlled environment conditions in 2000, at Saskatoon, Saskatchewan. Each herbicide constituted a separate experiment. Each experiment was arranged in a completely randomized design with four replications (one pot per replicate). Each herbicide was applied at 0, 0.125, 0.25, 0.5, 1, and 2 times the recommended rate (in g a.i. ha-1): metsulfuron, 4.44; 2,4-D, 876; 2,4-DB, 1400; clopyralid, 152; dicamba, 128; MCPA, 876; MCPB, 1700; MCPB+MCPA (15:1, respectively, by volume), 1700; mecoprop, 926; hexazinone, 1008; bentazon, 840; bromoxynil, 336; glyphosate, 880; glufosinate, 500. Twenty-one days after treatment, shoots were cut at soil level, and dry matter was determined. For statistical analysis of the dose-response data were fitted to a log-logistic model. Shoot dry matter responses of the Canada thistle varieties to the herbicides were described well by log-logistsic model. Results indicated that bromoxynil, and hexazinone had maximum effect, and metsulfuron minimum effect on Canada thistle (based on control effect index). Varieties horridum and integrifolium responsed differently to increasing rates of hexazinone only. Variety integrifolium was 40% less sensitive to the herbicide than variety horridum. If can not attribute the lack of control to soil properties or environment, it should ascertain the varietal type.