Integrating herbicides and revegetation to restore rangeland infested with spotted knapweed (Centaurea stoebe) and downy brome (Bromus tectorum)

By Jane Mangold*, Noelle Orloff*, Hilary Parkinson*, Mary Halstvedt^. *Montana State University, Bozeman, MT, US,, ^Dow AgroSciences, Billings, MT, US. In: Proceedings Western Society of Weed Science, 2014.


Some rangeland plant communities previously comprised of native grasses and forbs are now co-dominated by a complex of invasive forbs and annual grasses. Management often focuses on controlling the invasive forb(s) with little regard to annual grasses.  If remnant native perennial grasses are no longer present to re-occupy the site following invasive forb control, annual grasses may proliferate. We applied a variety of combinations of herbicides that would control the invasive forb spotted knapweed (Centaurea stoebe) and the annual grass downy brome (Bromus tectorum) followed by re-seeding with desirable grasses in an attempt to restore degraded rangeland.  We applied eight herbicide treatments and six re-seeding treatments in late summer and fall 2009 at two sites in western Montana. Four years post-treatment, we sampled density and biomass of established seeded grasses and cover of spotted knapweed and downy brome. Of the seeded grasses, tall wheatgrass (Agropyron elongatum) and bluebunch wheatgrass (Agropyron spicatum) were established at densities of about 1.1 and 1 plant/m2, respectively, averaged across all herbicide treatments, but herbicide treatment did not influence establishment. Herbicide and seeding interacted to influence spotted knapweed cover; applying aminopyralid and revegetating with tall or bluebunch wheatgrass reduced spotted knapweed to 0% cover, while non-seeded plots had about 5% spotted knapweed cover.  Downy brome cover four years after treatment was only influenced by herbicide treatment. Aminopyralid reduced downy brome cover to about 1% compared to the non-sprayed treatment at about 4%.  Our data support the integration of herbicides and revegetation to decrease spotted knapweed cover, but downy brome cover appears to be more variable and unpredictable.