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  • Conversion of Smooth Brome (Bromus inermis) to Switchgrass (Panicum virgatum) on Untilled Prairie in Northwest Iowa
  • Michael L. Sundall, Lora B. Perkins, and Troy W. Grovenburg

Since European settlement, vegetation and disturbance regimes in the North American prairies have dramatically changed. Native tall and mixed-grass prairies once covered a majority of the central and eastern Great Plains region of the United States, but less than 4% of the original 60 million ha of tallgrass prairie remains (Steinauer and Collins 1996). Smooth brome (Bromus inermis) is an introduced cool-season perennial, sod-forming grass that invades both native cool- and warm-season grasslands throughout North America (Dibbern 1947, Wilson 1991, Stubbendieck et al. 1994). The first documented importation of smooth brome to the United States was to the California Agricultural Research Station around 1880 as a cold and drought tolerant forage species (Newell and Keim 1943). In 1897, 12 tons of seed were imported from the Volga River region, divided and sent out to state research facilities across the United States (Dunn 1985). Because smooth brome has become an extremely successful invasive grass, it is problematic for managers working to restore and conserve native plant communities.

Many methods for smooth brome control have been evaluated including herbicide, fire, mowing, and grazing. A variety of herbicides, such as glyphosate, atrazine, imazapic, imazapyr, and sulfometuron have been evaluated for smooth brome control but have limited efficacy (Bahm et al. 2011). Fire is occasionally effective because smooth brome is a rhizomatous species that is sod forming and grows tillers, making it vulnerable during the dominant tiller stages (Wilson 2000). The two main tiller stages are December to mid-March, when the tillers are at or below ground level and mid-March through early May, when tillers are emerged and not elongated (Wilson 2000). Timing also is important when considering mowing; mowing prior to flower development causes greater damage than waiting until flowers had developed, with a height of 3.8 cm causing more damage than successively higher cuttings (Lawrence and Ashford 1964). Grazing may provide greater potential than burning or herbicide for reducing smooth brome without harming natives in cool-season grasslands (Stacy et al. 2005). No single treatment method has been found that provides consistent or complete smooth brome control; however combinations of methods may be more successful (Bahm et al. 2011). This project examines combinations of treatments with the goal of converting a monotypic stand of smooth brome to a stand dominated by native switch-grass (Panicum virgatum).

The study area was a 4.05 ha field located in Palo Alto County, Iowa (43°0755.00 N, 94°4959.50 W). The study area is remnant prairie that has never been tilled or plowed. The field was used as pasture 30 years ago and was mowed twice for hay 15 years ago, but since has been unmanaged. Pre-treatment vegetation in the plot was approximately 80% smooth brome with almost 20% Kentucky bluegrass (Poa pratensis) and small amounts of cleavers (Galium aparine) and goldenrod (Solidago canadensis). [End Page 355]

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Figure 1.

Mean % cover of Smooth brome and Switchgrass at the end of the growing season (September 2014) in Iowa, US. Error bars are ±1 SE.

We used a split-plot design with the main plots arranged as a randomized complete block design (RCBD). The four main factors included control, fire (applied by the landowners), herbicide (glyphosate as Durango® DMA® Herbicide, DOW AgroSciences, Indianapolis, IN, at a rate of 0.5 gallon acre−1), and fire and herbicide; main plot treatments were randomly assigned to plots. Subplot treatments were randomly applied within each main plot; treatment sub-plots were 3 × 3-m square, and the entire design was replicated twice. Sub-plot treatments were control, seed only, and mycorrhizal fungal inoculant (Root Growth Enhancer Mycorrhizal Fungi, Down to Earth Distributors Inc., Eugene, OR, at 1.5 × the recommended rate) and seed. Switchgrass was seeded at a rate of 6 pounds per acre. We burned 15 days before planting and applied herbicide one week after the fire (8 days before planting); the fungal inoculant was applied the same day as the seeds were planted...


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