Canada thistle (Cirsium arvense) is a widespread noxious weed throughout the prairie regions of the Midwestern United States. Selective herbicides provide effective control if applied at the early bud growth stage (late June) or at fall re-growth stage (mid-September to October), preferably after a light frost. However, there are few recent studies evaluating the effect of herbicide application time on Canada thistle control.
Field studies were established on Canada thistle in two counties in eastern South Dakota. Herbicide treatments included a fall application of Milestone® herbicide at 5 and 7 fluid ounces per acre (fl oz/A) and Tordon® 22K herbicide at 1 pint per acre (pt/A) applied in September, October or November in Brookings and Clark Counties. A summer application of Milestone at 5 and 7 fl oz/A was also applied at multiple times from May through October in Clark County. Plot design was a randomized complete block with three replications of each treatment. Evaluations of herbicide treatments were made one year following treatment. Control was determined from visual estimates of Canada thistle shoot growth reduction relative to the non-treated control. Treatment means were compared using analysis of variance (P=0.10).
Results of these studies showed that Canada thistle control was slightly more consistent with Milestone at 7 fl oz/A compared to Milestone at 5 fl oz/A.
Fall Application Timing: Milestone and Tordon 22K resulted in similar or greater control when applied in September compared to October or November indicating that September may be the optimal time for fall applications (Figure 2 and 3).
Summer Application Timing:
Canada thistle control with Milestone® herbicide at 5 fl oz/A in Clark County was more consistent when applied in July and August compared to May, September and October. Canada thistle control with Milestone at 7 fl oz/A was significantly lower when applied in May compared to applications made from July through October (Figure 4). Canada thistle plants are not fully emerged in May, which reduces effectiveness of herbicide treatments.
Results of these studies indicate that the optimal time to control Canada thistle in the summer with Milestone herbicide is after Canada thistle plants are fully emerged in June through August. For fall applications, the optimal application time for Canada thistle control with Milestone or Tordon 22K herbicides is September when green basal re-growth is present. Milestone at 7 fl oz/A was more consistent than lower rates of Milestone® or Tordon 22K at later fall application timings of October and November. Additional articles on Canada thistle control with Milestone herbicide are available at www.techlinenews.com.
This article summarizes a poster presented at the Western Society of Weed Science Annual Meeting. Reviewed and updated April 2019. The entire poster can be viewed on-line.
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Milestone herbicide is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. When treating areas in and around roadside or utility rights-of-way that are or will be grazed, hayed or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas or rotating the treated area to sensitive crops. See the product label for details. State restrictions on the sale and use of Milestone apply. Consult the label before purchase or use for full details.
Tordon 22K is a federally Restricted Use Pesticide. Always read and follow label directions.