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Invasions by exotic plants are occurring at an increasing rate and are considered a serious threat to both agricultural systems as well as native communities (Drake et al. 1989). Many of the most harmful exotics in North America were introduced intentionally (Ruesink et al. 1995, Williams 1980). Exotic plants such as tamarisk (Tamarix spp.), Russian olive (Elaeagnus angustifolia), purple loosestrife (Lythrum salicaria), African lovegrasses (Eragostis lehmanniana, E. curvula) and smooth bromegrass (Bromus inermis) were originally brought to North America for agricultural or horticultural purposes but have displaced native species and threaten biological diversity of native communities (Malecki et al. 1993; Robinson 1965; Romo and Grilz 1990; Bock et al.1986; Shafroth et al. 1995).
Many Eurasian grasses have been intentionally introduced throughout temperate North America, primarily for hay and pasture. The most commonly planted exotic grass in western North America is crested wheatgrass (Agropyron cristatum, A. desertorum). There are between 15 and 26 million acres of crested wheatgrass on this continent (Holchek 1981; Rogler and Lorenz 1983). The conversion of native prairie to crested wheatgrass primarily occurred after the drought of the late 1920s and 1930s when large areas of marginal cropland were abandoned and then seeded with non-native grasses to reduce soil erosion potential (Lorenz 1986). Today, crested wheatgrass continues to be planted over large areas of the Northern Great Plains. Since 1985 several million acres of crested wheatgrass have been planted on idled cropland as part of the Conservation Reserve Program.
There are many reasons for the popularity of crested wheatgrass. It is tolerant of cold and drought and suited to most soils (Knowles and Buglass 1980). Crested wheatgrass establishes easily from seed, responds well to fertilizer (Box 1986) and has high nutritive value (Box 1986). It generally yields better than native grasses of the Great Plains (Dormaar et al. 1978; 1995; Smoliak and Dormaar 1985; McWilliams and VanCleave 1960), and usually provides greater livestock weight gains (Smoliak 1968; Hofmann et al. 1993). Seeded stands of crested wheatgrass resist invasion by weeds (Knowles and Buglass 1980). The advantages of crested wheatgrass are obvious, and it is not uncommon for livestock managers to break native prairie in order to plant it.
Although crested wheatgrass is generally considered a beneficial introduction, there are several, often overlooked...