Title: Spotted knapweed utilization by sequential cattle and sheep grazing
Abstract: Prescribed (or targeted) sheep grazing can effectively suppress the invasive perennial forb spotted knapweed (Centaurea stoebe L. ssp. micranthos (Gugler) Hayek). Some ranchers and other natural resource managers, however, resist using this weed management tool over concerns that sheep may consume too much of the graminoid standing crop, thereby decreasing its availability to cattle and wildlife and possibly harming graminoids with excessive utilization. One potential approach to address these concerns is to graze spotted knapweed infestations first with cattle, immediately followed by sheep. We evaluated this sequential grazing strategy on foothill rangeland in western Montana, comparing sequential grazing at a moderate stocking rate in mid-June (spotted knapweed in bolting stage) versus mid-July (spotted knapweed in late-bud/early-flowering stage). Pastures (0.81 ha) were grazed with 3 yearling cattle for 7 days, immediately followed with 7 days of grazing by 7 yearling sheep. Combined relative utilization of graminoids by cattle and sheep averaged 40% in June and July, safely within sustainable grazing utilization levels recommended for the site. Combined relative utilization of spotted knapweed by cattle and sheep also did not differ between June and July, averaging 62%. Previous research indicates that this degree of utilization is sufficient to suppress spotted knapweed. Our results indicate that prescribed sheep grazing can be applied immediately following cattle grazing in either June or July to suppress spotted knapweed without over-utilizing desirable graminoids. Cattle and sheep will eat less graminoids and more spotted knapweed if cattle and sheep graze sequentially when spotted knapweed is in its late-bud/early flowering stage (mid-July) rather than its bolting stage (mid-June). [Henderson L. Stacee, Tracy K. Mosley, Jeffrey C. Mosley, & Rodney W. Kott (2012). Spotted knapweed utilization by sequential cattle and sheep grazing. Rangeland Ecology & Management, In-Press.dx.doi.org/10.2111/REM-D-09-00194.1]