Invasive plants such as leafy spurge and spotted knapweed kill off surrounding vegetation, trigger soil erosion, decrease available water, and cost agriculture millions of dollars each year. The weeds not only make the land unfit for crops and cattle, they threaten native plant species and drive off wildlife. For example, a U.S. Forest Service study concludes that forage loss due to knapweed invasion in western Montana's Lolo National Forest could result in the loss of 220 elk annually by 1998. Noxious plants typically are imports that have few, if any, natural enemies in North America. Managing these weeds with herbicides is costly, while manual clearing is largely ineffective.
Sheep are unique in that they readily consume plants that other animals avoid or find toxic.
As a result, land managers are turning to sheep grazing as an environmentally sound and cost-effective way to control noxious weeds. The invasion is particularly severe in the Dakotas, Montana, and Wyoming. While leafy spurge is toxic to most animals, sheep thrive on the weed. Wyoming studies found that sheep grazing effectively controls leafy spurge far less expensively than other control measures. The BLM, U.S. Forest Service, and Bureau of Reclamation have allowed sheep to graze free of charge on spurge-infested public recreational lands and cattle allotments.