Field observations suggested that control of Canada thistle variety horridumby hexazinone was greater than that of variety integrifolium. To confirm these observations, the response of these two varieties to hexazinone and 13 other herbicides was examined under controlled environment conditions. The tolerance of the variety integrifoliumto hexazinone was 40% greater compared with the variety horridum. Differences in shoot growth response between the two varieties were not detected from any of the other herbicides.
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Beckie et al. (2002) studied this question.