Sequential cattle and sheep grazing for Spotted Knapweed control
Henderson, Stacee Lyn.
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Spotted knapweed (Centaurea stoebe L.) infests millions of hectares of native rangeland in North America. Spotted knapweed creates large monocultures, which decreases biodiversity, reduces livestock and wildlife forage, and increases surface water runoff and soil erosion. Sheep are an effective tool for controlling spotted knapweed and have been widely used on cattle ranches for weed control. However, cattle producers are concerned that sheep will over-utilize desirable graminoids. Therefore, research is needed to determine an effective grazing strategy using cattle and sheep that will adversely affect spotted knapweed, while minimizing over-use of desirable graminoids across the landscape. This 2-year study quantified graminoid and spotted knapweed utilization and diet composition and foraging behavior of cattle and sheep sequentially grazing spotted knapweed-infested rangeland in western Montana. Twenty-one Targhee yearling wethers and 9 Black Angus yearling cattle were used. Animals were randomly assigned to one of 3, 0.81-ha pastures that were grazed in either mid-June or mid-July (n=6 pastures). Cattle grazed each pasture for 7 days, immediately followed by sheep grazing for 7 days in each month. Analysis of covariance was used to determine differences in diets, relative preference indices, foraging behavior, and utilization between June and July for cattle and sheep to determine the optimal month for implementing prescribed sheep grazing. Relative utilization of spotted knapweed did not differ between June and July and averaged 61.5%. Graminoid utilization was moderate (<45%). Cattle preferred forbs in June, spotted knapweed and forbs in July, and avoided graminoids in July. Sheep avoided graminoids in June and July, preferred forbs in June, and showed no preference or avoidance of spotted knapweed. Cattle ranches with large spotted knapweed infestations can effectively use prescribed sheep grazing immediately following cattle grazing in June or July to achieve high levels of use on spotted knapweed, thus reducing viable seeds incorporated into the soil, while maintaining optimal utilization levels on desirable graminoids.