Percent Spotted Knapweed (Centaurea stoebe) in the Diets of Grazing Sheep

Abstract The objective of this study was to use fecal near-infrared reflectance spectroscopy (NIRS) to estimate the percent spotted knapweed in the diet of grazing ewes to fine tune grazing prescriptions. In trial 1, five ewes were randomly selected from a band of 900 ewes to estimate weekly variation in percent spotted knapweed in the diet. Fecal samples from these ewes were collected weekly from June 22 through August 17 (nine collections per ewe for a total of 45 observations). In trial 2, fecal samples were collected from 89 ewes in the band to estimate variability in percent spotted knapweed in the diet among sheep at two sampling dates corresponding to the bud (July 13) and postflowering (August 15) stages of spotted knapweed phenology. In trial 1, the percent spotted knapweed in the diet was similar (P > 0.05) on June 22, June 29, July 13, and July 20 but was greater (P < 0.05) on July 6 than June 22 and 29. Percent spotted knapweed in the diet was also greater (P < 0.05) from July 27 through August 17 than from June 22 through July 20. On July 13 in trial 2, 55% of ewes had 0 to 5% spotted knapweed in their diets, whereas 44% of ewes had 5 to 20% spotted knapweed in their diets. On August 15, only 1% of ewes had < 10% spotted knapweed in their diets, whereas 44% of ewes had 20 to 25% spotted knapweed in their diets. The estimated percent spotted knapweed in the diet of the 89 ewes in trial 2 was similar to that of the five ewes during the same time period in sampling trial 1 (5.3% compared with 5.0% for sampling trials 1 and 2, respectively, on July 13 and 22.0% compared with 20.7% on August 15 and 17 for trials 1 and 2, respectively). This is the first study to estimate percent spotted knapweed in the diet of individual grazing ewes. These data suggest that the best time to graze spotted knapweed–infested pastures would be in late July or early August, when spotted knapweed is flowering, but before viable seed production. Nomenclature: Spotted knapweed, Centaurea stoebe L. ssp. micranthos (Gugler) Hayek Interpretive Summary: Spotted knapweed is an aggressive invasive species that has replaced native perennial grasses on many foothill range and pasture lands in the United States and Canada. Herbicides, insects, pathogens, and fire have not effectively contained the spread of this weed. In recent years, there has been a renewed interest in using small ruminant grazers in integrated weed management programs. This has led to the need to characterize differences in diet preferences more specifically throughout the grazing period and between individual animals under typical grazing conditions. Ours is the first study to estimate percent spotted knapweed in the diets of individual sheep throughout the grazing season. Percent spotted knapweed in the diet of grazing sheep was lower in June and July than August. Percent spotted knapweed in the diet of grazing ewes averaged 5% in mid-July and 21% in mid-August. A subsample of five ewes was adequate for estimating percent spotted knapweed in the diets of ewes in the band. Our data suggest that sheep will consume spotted knapweed when herded to areas with high concentrations and that the best time to graze spotted knapweed–infested pastures would be in late July or early August, when spotted knapweed is flowering but before viable seed production.