Control of Medusahead (Taeniatherum caput-medusae) and Other Annual Grasses with Imazapic Guy B. Kyser, Joseph M. Ditomaso, Morgan P. Doran, Steven B. Orloff, Robert G. Wilson, Donald L. Lancaster, David F. Lile, and Marni L. Porath* Invasive annual grasses, such as medusahead, can reduce forage production capacity and interfere with revegetation projects in California rangelands. Because of the taxonomic similarity to other more desirable grasses, achieving selective control of invasive annual grasses can be difficult. In selectivity trials conducted in Yolo and Siskiyou counties, CA, the herbicide imazapic gave control of many nonnative annual grasses yet provided some level of selectivity to specific perennial grasses used in revegetation projects throughout the western United States. The selectivity difference between newly seeded perennial and annual grasses was greater with PRE applications than with POST treatments. Both perennial and annual grasses within the tribe Hordeae were more tolerant to imazapic than other grass species. In addition, field experiments were conducted at three sites in northern California (Yuba, Yolo, and Lassen counties) and one in southern Oregon (Lake County) to test the response of imazapic to varying management conditions. Imazapic was applied PRE in fall (and also spring in Lake County) at rates from 35 to 210 g/ha on undisturbed rangeland, in comparison with rangeland cleared of standing plant material and thatch by either tillage, mowing and raking, or burning. Imazapic generally showed enhanced weed control when applied following disturbance. Rates as low as 70 g/ha, if combined with thatch removal, provided significant suppression of medusahead. In addition, disturbance alone generally reduced medusahead cover in the following year. Although imazapic showed potential for control of medusahead and other annual grasses, its selectivity window was relatively narrow. Nomenclature: Imazapic; medusahead, Taeniatherum caput-medusae (L.) Nevski, ELYCM. Key words: Grassland, integrated weed management, invasive weed, native species, rangeland, restoration, selectivity. In ; restoring rangeland and grassland infested with noxious weeds, it may be necessary to incorporate revegetation efforts along with weed control strategies. Revegetation is critical for both reestablishing rangeland utility and for preventing weeds from quickly reinvading. Concurrently, some form of weed control is necessary to ensure success of revegetation plantings. In California’s Mediterranean climate, with most pre- cipitation occurring from October to April, revegetation plantings are most successful when made in fall or late winter, depending on the location. However, invasive plants, such as yellow starthistle (Centaurea solstitialis L. CENSO) and many noxious annual grasses, germinate over an extended period during the rainy season, thus limiting the effectiveness of POST herbicides used at the time of planting. Because no herbicide currently registered for use in California rangelands selectively controls annual grasses, PRE or POST, without injuring perennial grass seedlings, grass weeds are the greatest obstacle to perennial grass revegetation efforts. One of the most problematic annual grass weeds in California and EQ the western United States is medusahead, which is estimated to infest about 1 million ha in the 17 western states (Duncan and Clark 2005). Medusahead is a winter annual native to the Mediterranean region. It matures after most other annual vegetation has senesced. Thus, during its seed production phase, in late spring to early summer, medusahead can access soil moisture and sunlight without competition from other annual grasses. In addition, medusa- head produces a thick, silica-rich thatch that decomposes slowly, suppressing germination and establishment of desir- able plants (Young 1992). Because medusahead is adapted to germinate through thatch (Harris 1977; Young et al. 1971a), it can form dense stands, excluding nearly all competitors. Additionally, medusahead is considered to be a poor forage plant owing to its high-silica content (. 10% of dry weight), low-nutrient value, and rough unpalatable texture (George 1992; Lusk et al. 1961). Management of medusahead has proven difficult. Control can be achieved by burning in some locations (Furbush 1953; Murphy and Lusk 1961), but not others (Young et al. 1971b), or by a combination of tillage, herbicide treatment, and perennial grass reseeding (Young et al. 1969). Imazapic 1 is a PRE or POST herbicide that has been used in native-plant restoration projects in many parts of the western and midwestern United States (Barnes 2004; Masters et al. 2001). Its average persistence in soil is about 120 d (Vencill 2002), so it has the potential to control weeds throughout the initial establishment of a revegeta- tion planting. Imazapic is most effective against annual grasses. For example, both fall and spring applications of imazapic at 70 g/ha controlled downy brome (Bromus tectorum L.) (Dewey et al. 2003; Sebastian and Beck 2004). A higher rate of 140 g/ha gave good control of medusahead in other studies (Monaco et al. 2005; Shinn and Thill 2002). Weed Technology wete-21-01-07.3d 30/11/06 17:32:28 1 Cust # WT-06-027R1 DOI: 10.1614/WT-06-027.1 * Staff Research Associate and Extension Specialist, Department of Plant Sciences, University of California, One Shields Avenue, Davis, CA 95616; third author: Farm Advisor, Cooperative Extension Solano County, 501 Texas Street, Fairfield CA 94533-4498; fourth author: Farm Advisor, Cooperative Extension Siskiyou County, 1655 South Main Street, Yreka, CA 96097; fifth and seventh authors: Farm Advisors, Cooperative Extension Lassen County, 707 Nevada Street, Susanville, CA 96130; sixth author: Farm Advisor, Cooperative Extension Modoc County, 202 West 4th Street, Alturas, CA 96101; eighth author: Farm Advisor, Cooperative Extension Malheur County, 710 Southwest 5th Avenue, Ontario, OR 97914. Corresponding author’s E-mail: jmditomaso@ucdavis.edu. ; Weed Technology 2007 21:000–000 0 N Weed Technology 21, January–March 2007