Native perennial grasses such as bluebunch wheatgrass (Pseudoroegneria spicata) are ecologically and economically important in the Intermountain, Great Basin and Northern Great Plains Regions. In grassland and shrubland habitat, bluebunch wheatgrass may occupy as much as 60 percent of the plant community. These habitat types are also highly susceptible to invasion by noxious weeds including knapweeds (Centaurea sp.) and leafy spurge (Euphorbia esula). Herbicides are often used to control these and other invasive broadleaf weeds; therefore, understanding the tolerance of desirable grass species to herbicide treatments is important for land managers.
A field experiment established on native rangeland in western Montana measured the tolerance of cool-season bunchgrass to applications of Milestone® herbicide compared to Perspective herbicide. Perennial native grasses on the site were dominantly bluebunch wheatgrass (45%), Idaho fescue (Festuca idahoensis) (15%), and Junegrass (Koleria cristata) (10%). Noxious weeds were absent within the study area; however, native forbs including lupine (Lupinus sp.) and buckwheat (Eriogonum sp.) occupied about 25 percent cover. Grazing livestock were excluded from the site during the season of herbicide application and for two years following treatment. Soils are sandy loam and elevation is 4,320 feet.
Individual plots were 10 by 20 feet in size and arranged in a randomized complete block design, with three replications per treatment. Herbicides were applied with a CO2-pressurized backpack sprayer at 13.5 gallons per acre in late spring (June) or fall (September) (Table 1). Application timing was typical for broadleaf weed control in the Intermountain Region.
Milestone herbicide was applied at two rates: the maximum broadcast label rate of 7 fluid ounces per acre* (fl oz/A) applied in spring and fall, and the spot treatment rate of 14 fl oz/A (spring only). Perspective herbicide was applied in both spring and fall at the noxious weed control rate of 4.75 ounces of product per acre* (oz/A) and 9.5 oz/A, which is less than the maximum broadcast label rate of 11 oz/A. Herbicides were applied with a non-ionic surfactant (NIS) at 0.25% v/v.
(*Note: 7 fl oz/A of Milestone® herbicide is equal to 1.75 ounces active ingredient per acre (oz ai/A) of aminopyralid. 4.75 oz product/A of Perspective herbicide is equal to 1.88 ai/A aminocyclopyrachlor plus 0.75 oz ai/A chlorsulfuron.)
Visual evaluations of perennial grass injury were collected approximately 30 and 60 days after spring application, and one and two years after treatment (YAT). Herbicide symptoms that were evaluated included epinasty (twisting, drooping, weakened leaves or stems), chlorosis (yellowing), and visual stunting compared to plants in non-treated plots.
Perennial grass was also harvested at peak growth by clipping a 0.5 square meter frame within each plot about one and two years after treatment. Bluebunch wheatgrass was separated from other perennial grasses during clipping because of significantly higher visual injury to this species. The second year after treatment, bluebunch wheatgrass was selectively harvested from a 0.5 square meter frame in each plot. Harvested grasses were dried for seven days in a greenhouse, weighed to the nearest gram, and weight converted to pounds per acre (lb/A). Data were analyzed by analysis of variance.
Season of application
No injury was noted on grasses 30 days after application with any herbicide treatment. Slight injury, including weakened stems and bending seed heads, was observed on bluebunch wheatgrass and Junegrass (<7% injury) 60 days after spring application with Milestone® at 14 fl oz/A and Perspective at 9.5 oz/A. No visual injury was noted on Idaho fescue the year of treatment with any herbicide.
One and two years after treatment
Bluebunch wheatgrass was significantly impacted by Perspective herbicide one and two years after treatment. Visual injury to bluebunch wheatgrass was greatest with Perspective at 9.5 oz/A (Table 2); however, even the recommended noxious weed rate of 4.75 oz/A of Perspective herbicide significantly reduced bluebunch wheatgrass biomass one year after treatment (Figure 1). Injury and biomass reduction to bluebunch wheatgrass was significantly greater with Perspective than with Milestone herbicide. Injury symptoms on bluebunch wheatgrass included a reduction in plant growth height, overall plant vigor, and reduced plant density with applications of Perspective herbicide. There was no visual injury noted on either Idaho fescue or Junegrass one year following treatment for any herbicide.
The application of Perspective herbicide shifted the plant community from a site dominated by bluebunch wheatgrass to one dominated by Idaho fescue, Junegrass, and tolerant forbs (primarily buckwheat). The change in plant community structure caused by removal of bluebunch wheatgrass could have long-term ecological implications to the rangeland resource, and result in overall reduction in productivity of the site.
Published March 2015; updated June 2019
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