Got Weeds? These Sheep Will Make House Calls Chilled by an autumn wind, Enrique Marquez watched from horseback as the sheep gamboled down the mountain. A border collie nipped the heels of wayward ewes. All summer and into the fall, the flock grazed on noxious weeds infesting about 1,000 acres of public lands above the Missoula Valley as part of this city’s effort to restore its native prairie grasses. Nationwide, sheep grazing is gaining popularity as a low-cost, nontoxic tool in the battle to control leafy spurge, knapweed, dalmatian toadflax and other invasive weed species. The approach is catching on in places like Nantucket, Civil War battlefields in Virginia, ski slopes in Vermont and vineyards in California. Tom McDonnell, a staff consultant with the American Sheep Industry Institute, called this kind of grazing a “growth industry.” Mr. McDonnell cited a study by the College of Agriculture and Life Sciences at Cornell University that indicated nonnative weeds had invaded 40 percent to 50 percent of America’s croplands, pasture and public lands and were spreading at a rate of 1.75 million acres per year. Sheep grazing is a long-term solution best used in conjunction with other methods, like beneficial insects, controlled burns, herbicides and hand pulling, officials said....