An estimated 400 million ha or 42% of the total land area of the United States is used for pasture and grazing (Bovey 1987). Most wildlife and livestock depend upon rangelands for survival. In the United States, rangeland is represented by a variety of ecosystems, including desert and rich alluvial valleys, coastal and inland foothills, high mountain meadows, and inland plains. Domestic livestock, particularly Bos (cattle), Ovis (sheep) and Equus (horses), have grazed many of these ranges for nearly 200 years. As a result, the plant composition has changed greatly from the original ecosystems (Murphy 1986). A number of species can cause significant problems on rangelands (Table 1). The vast majority of these weeds have been introduced from other continents. However, management practices, particularly fire suppression or overgrazing, have increased the proportion of some native species. These natives can reduce overall forage quality or quantity (e.g., Juniperus spp., Artemisia tridentata, and Gutierrezia spp.) or poison livestock (e.g., Delphinium spp., Astragalus spp., and Amsinckia menziesii var. intermedia). No particular life cycle is typical to noxious weeds of rangeland. Noxious range weeds can be annuals (e.g., Centaurea solstitialis, Crupina vulgaris, Bromus tectorum), biennials (e.g., Carduus nutans, Conium maculatum, Onopordum acanthium), long-lived herbaceous perennials (e.g., Convolvulus arvensis, Centaurea maculosa, Cirsium arvense), shrubs (e.g., Gutierrezia spp., Artemisia tridentata), or trees (e.g., Juniperus spp., Prosopis glandulosa). Although these species are represented by several plant families, the largest number of noxious species belong to the Asteraceae (sunflower) family.
