In California's Angeles National Forest, northeast of Los Angeles, sheep keep 13,000 acres of ridge-top firebreaks free from flammable chaparral and other brush. Sheep act as brush-control on private lands in northern and southern California, as well as the Pacific Northwest. In Vermont and Alberta, Canada, sheep are used to control brush invasion on ski slopes. Noxious weeds are a major threat to both public and agricultural lands, killing surrounding vegetation and triggering soil erosion. The weeds not only make the land unfit for agriculture and cattle, they threaten to drive out native plant species and destroy wildlife habitat.
Sheep are unique in that they readily consume plants other animals avoid or find toxic.
As a result, sheep are used extensively to control noxious plants. The problem is particularly severe in Montana, Wyoming, and the Dakotas. While leafy spurge is toxic to most animals, sheep thrive on the weed. In Oregon's Baker County, the first-ever spurge outbreak has been controlled for four seasons by sheep. In Montana, alone, spotted knapweed has infested 810,000 acres, including parts of Glacier National Park.