Submitted to: Society for Range Management Meeting Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 12/1/2003
Publication Date: 1/24/2004
Citation: Johnson, D.E., Louhaichi, M., Richman, L.M., Carpinelli, M.F., Knox, V. 2004. Population dynamics of selected forbs treated with oust herbicide. In: Society for Range Management Meeting Proceedings, 57th Annual Meeting. 2004 CDROM.
Interpretive Summary: Oust® (sulfometuron methyl) herbicide is commonly used on Great Basin rangelands to control medusahead, an invasive annual grass. There is a concern that efforts to control medusahead may negatively impact desirable plant species coexisting with medusahead. This study assessed the impacts of fall applications of Oust® on native forbs. Seven of the thirteen forb species studied suffered significant density reductions over a two-year period, while six species were unaffected. More research is needed to assess the overall benefit of using Oust® to control medusahead where desirable, native species coexist.
Technical Abstract: Medusahead (Taeniatherum caput-medusae [L.] Nevski), a nonnative, winter-annual grass, is invading rangelands throughout the western US. Oust® (sulfometuron methyl) has been used to control medusahead. Oust® interferes with enzymes (acetohydroxy acid synthase [AHS]) that are necessary for plant cell division and pre-emergence treatments control weeds through root uptake. We assessed the impacts of fall applications (pre-emergent) of Oust® (October 2001) on native forbs on the sagebrush/bunchgrass steppe of central Oregon over two growing seasons. We evaluated the population dynamics of selected native plant species on plots that had been treated with Oust® and on untreated controls. The experimental design was a split-plot in time (repeated measures) with randomly applied treatments. To ensure that native plant populations were represented across treatments and controls, we employed plot pairing with pairs having similar initial species composition, density, and vigor of selected native plants. The application rate was 70.0 g ai/ha (1.0 oz. ai/acre) for Oust®. Seven of 12 forbs tested had a significant reduction in density (P ' 0.10) after two years: Achillea millefolium L. 28% survival (P = 0.0219), Agoseris glauca (Pursh) Raf. 28 % survival (P = 0.1007), Astragalus purshii Dougl. ex Hook. 25 % survival (P = 0.0051), Balsamorhiza serrata A. Nels. & J.F. Macbr. 15 % survival (P = 0.023), Crepis modocensis Greene 36 % survival (P = 0.0012), Lupinus saxosus T.J. Howell 83 % survival (P = 0.0612), Minuartia nuttallii (Pax) Briq. 44 % survival (P = 0.096), Phlox hoodii Richards. 39 % survival (P = 0.0044). This experiment was not designed to evaluate the effectiveness of Oust® treatments on the control of unwanted plants. Photographic analysis suggested that Oust® treated plots had 5% less vegetative cover than control plots after 1 year (P = 0.0286). Two years after treatment there was 1.8% (P = 0.0537) more cover of vegetation in treated plots.