Effect of Milestone® on Canada Thistle and the Native Plant Community in a Restored Tallgrass Prairie

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Native tallgrass prairies are diverse ecosystems that evolved with periodic disturbances such as fire and grazing pressure and are dominated by species that include big bluestem (Andropogon gerardii Vitman), Indiangrass [Sorghastrum nutans(L.) Nash], and switchgrass (Panicum virgatum L.). Currently, less than one percent of the original tallgrass prairie ecosystem in North Dakota and Minnesota is left in native or restored condition. Fire suppression and removal of native nomadic grazers has increased invasion from woody plants, and the native and restored prairie that remains is threatened by continued human disturbance. Invasion by noxious weeds has become a major threat to the conservation of biodiversity and ecosystem sustainability in these remnant and restored prairies.

In 2006, North Dakota State University in cooperation with The Nature Conservancy established a field study in restored tallgrass prairie within the Glacial Ridge Preserve in northwestern Minnesota. The objectives of the study were to evaluate Canada thistle (Cirsium arvense L.) control with Milestone® and measure effects on the plant community.

The study site was dominated by restored native perennial grasses such as big bluestem and Indiangrass. The site also contained many native forb species including purple prairie clover (Dalea purpurea Vent.), maximilian sunflower (Helianthus maximiliani Schrad.), Canada goldenrod (Solidago canadensis L.), and white panicled aster (Aster simplex Willd.). Areas within the restoration had varying densities of Canada thistle infestation, but a majority of the restoration site was not invaded. Other weeds were also present in the restoration area such as absinth wormwood (Artemisia absinthium L.), perennial sowthistle (Sonchus arvensis), and field bindweed (Convolvulus arvensis L.).

Canada thistle-infested plots were paired with non-infested restored native sites. The restored native plant communities had a similar composition compared with the corresponding Canada thistle–infested plant communities, with the exception of Canada thistle presence, and were used to determine the effect of  Milestone on native species. The changes in plant species composition (richness, evenness, and diversity) and cover in both Canada thistle-infested and native restored plant communities were determined after Milestone treatment.

Milestone was applied in the fall at 7 fluid ounces per acre (fl oz/A) to Canada thistle-infested sites when Canada thistle was at the rosette fall-regrowth stage, and was also applied to corresponding native restored prairie plots. Canada thistle control was determined by counting stem density in July before treatment and 10 and 22 months after treatment (MAT). Plant community composition was evaluated by visually assessing the plant foliar cover. Bare ground, litter, and individual plant species cover were estimated in mid-July prior to herbicide treatment and 10 and 22 MAT.

Results of the study showed that Milestone at 7 fl oz/A applied in fall effectively reduced Canada thistle stem density more than 99% (16 stems per m2 to 0.1 stems per m2) at 10 MAT (Table 1). Long-term control of Canada thistle was excellent with a 90% reduction in stem density at 22 MAT. Milestone® also removed or reduced several undesirable forb species from the restored prairie communities, such as absinth wormwood and perennial sowthistle. 

Overall plant community composition in both the Canada thistle-infested and native plant communities was altered by Milestone. The major change was a decrease in the total number of high (desirable) and low seral (undesirable/weedy) forbs in the treated communities compared with non-treated communities. A number of high seral forbs were reduced or removed by Milestone, including maximilian sunflower and purple prairie clover. Foliar cover of high seral forbs in the native plant community was reduced from 12.2 to 7% at 22 MAT. However, most of the forbs that decreased 1 year after treatment recovered and were present by the second year following treatment. The cover of high seral grass species, such as big bluestem, slender wheatgrass (Elymus trachycaulus [Link] Gould ex Shinners),  and Indiangrass increased 46% following Milestone application in both the Canada thistle–infested and native plant communities, with the greatest increase in Canada-thistle infested plots. The total foliar cover of high seral grasses in Canada thistle–infested communities nearly tripled, averaging 51.3% at 10 MAT compared with only 19% before treatment (Figure 1). 

Species richness, evenness, and diversity were reduced after Milestone application in both Canada thistle–infested and native plant communities (Table 2). However, the benefits of Canada thistle control, removal of undesirable species, and the increase in native grass cover should lead to an overall improvement in the long-term stability and composition of the restored prairie plant community. This will likely outweigh the reduction of high seral forb species resulting from a Canada thistle control program at the Glacial Ridge Preserve.

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