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Title: Variable Impacts of Imazapic on Downy Brome (Bromus Tectorum) and Seeded Species in Two Rangeland Communities

item Monaco, Thomas
item Rigby, Craig

Submitted to: Invasive Plant Science and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/16/2008
Publication Date: 4/1/2009
Citation: Morris, C., Monaco, T.A., Rigby, C.W. 2009. Variable Impacts of Imazapic on Downy Brome (Bromus Tectorum) and Seeded Species in Two Rangeland Communities. Journal of Invasive Plant Science and Management 2:110-119.

Interpretive Summary: Combining herbicide application with seeding desirable perennial species has emerged as an effective technique for the restoration of weed-infested wildlands. The development of selective herbicides for fast-growing annual grasses has made this technique an option for the millions of acres dominated by downy brome within the Great Basin desert of North America. However, when selectivity is based on herbicide rate, success depends on the ability of the seeded species to tolerate the herbicide. Our objective was to evaluate how downy brome and seeded perennial species responded to variable rates of imazapic herbicide. A salt desert shrub and a Wyoming big sagebrush plant community, both seriously impaired by downy brome, were chosen for this study. Imazapic was applied at five different rates in autumn 2003, and five perennial plant materials were seeded. Downy brome cover and establishment of seeded plant material were subsequently evaluated in spring 2004 and 2006. Imazapic treatments in the Wyoming big sagebrush community greatly reduced downy brome cover; however, this site had lower establishment for seeded species. In contrast, imazapic had lower control of downy brome, but seeded species had higher establishment, in the salt desert shrub community. Our results indicate that both sites experienced trade-offs between downy brome control and injury/mortality of seeded species. The differences in results at the two sites can primarily be attributed to difference in precipitation, which can affect herbicide effectiveness through a number of mechanisms, including foliar and root uptake of herbicide, soil organic matter, and resource competition between seeded species and downy brome. Results also indicated that downy brome treated with imazapic began to recover within two years after treatment. This effect was more dramatic at the mesic Wyoming big sagebrush site, where downy brome was reduced to 12% cover one year after imazapic was applied, but rebounded at all rates to levels greater than the plots that received no herbicide. At the drier salt desert shrub site, downy brome cover also increased two years after application, though not as dramatically. It is not known at what level of downy brome cover combined with desirable species establishment is required in order to maintain low downy brome cover over the long term. Clearly, lower levels are required than what was achieved in this study, suggesting that at least one additional application of herbicide may be necessary. While the use of imazapic herbicide combined with seeded perennial species shows promise in shifting downy brome-dominated sites to perennial species, proper dosage based on site-specific conditions is critical, as is follow-up treatment.

Technical Abstract: In the Great Basin desert of North America, over 20 million acres of land have been invaded by downy brome. A common control measure includes herbicide use combined with seeding of desirable species. The herbicide imazapic is registerred for use on rangelands and provides effective short-term control of certain invasive annual grasses. However, details about optimal application rates for downy brome and susceptibility of simultaneously seeded species are lacking. Thus, we investigated downy brome and seeded species responses to variable rates of imazapic (0, 35, 70, 105 and 140 g ai/ha ) in two plant communities (Wyoming big sagebrush and salt desert shrub). In autumn 2003, plots were treated with imazapic and seeded with one of five perennial plant materials (Siberian wheatgrass ['Vavilov' and the experimental source 'Kazak']; prostrate kochia ['Immigrant' and the experimental source '6X'], and Russian wildrye ['Bozoisky II']). Downy brome cover and seeded species establishment were evaluated in spring of 2004 and 2006. We developed a simple conceptual model to assist in explaining the dynamic and complex interactions that affected our results. Downy Brome cover in 2004 decreased with increasing imazapic rate at both sites, although more so at the Wyoming big sagebrush site. In 2006, no difference in downy brome cover existed among herbicide rates at the Wyoming big sagebrush site. At the salt desert shrub site, the high rate of imazapic reduced downy brome cover by about 25% compared to untreated lots. 'Vavilov' Siberian wheatgrass was the only seeded species with lower downy brome cover in 2006 than 2004. Seeded species establishment increased with imazapic rate in the salt desert shrub community.