It has been recognized that grazing animals can influence the mix of plants in the vegetation community. Cattle tend to prefer grass and over time move the plant community towards more woody shrubby plants (browse) and forbs. Sheep tend to graze forbs and goats prefer browse, which over time will increase the grass component of the plant community (See diagram below for further explanation). However, each animal species will utilize all these groups of plants seasonally. Depending on the weed species and the season, it is possible to develop a system to suppress infestations. In recent years, there has been an increased effort to integrate grazing with herbicides and biological controls.
The use of sheep and goats for weed control has gained considerable interest recently due to the increasing problems caused by broadleaf weeds (mainly spotted knapweed and leafy spurge) in the Northwest. Although grazing doesn't kill these plants immediately it will stress the plants and can reduce the soil seed bank over time if animals are introduced while the plants are in flower. In order for grazing to be effective it must continue for multiple years. Intensive grazing can cause soil disturbance and damage to desirable plants so it's good not to let animals stay in one area for too long. Animals are also good at carrying weed seeds from one area to another, either in their fur or in their gut, so caution should be taken when transporting animals that have been grazing noxious weeds to a weed-free area. For detailed information on using sheep and goats for noxious weed control contact the M.S.U. Sheep institute.