Submitted to: Weed Science Society of America Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 5/15/2002
Publication Date: 8/15/2002
Citation: Weed Science Society of America Abstracts. 2002. v. 42. Abstract p. 10. Interpretive Summary: In a study to determine what is the best time to graze sheep and at what stocking rate to control spotted knapweed, a troublesome invasive weed, sheep were grazed in a pasture near the United States Sheep Experiment Station 10 miles north of Dubois, ID that is infested with spotted knapweed. After two years of grazing trials spotted knapweed has declined in this sagebrush steppe environment. This decline was very large among small plants (rosette stage) and larger plants. With over 75% of the small plants and 50% of the large plants disappearing. The number of medium sized plants increased slightly and was probably due to some of the smaller plants not being grazed. Grazing when the plants are just starting to grow a flower stalk (bolt stage) resulted in the fewest number of flowers being produced. Spotted knapweed cover, after two years of sheep grazing trials, declined from 13.3 to 11.8%. Cover of forbs increased from 4 to 8% and cover of grasses increased from 13 to 23%, which means the quality of the vegetation was improving. Bare ground declined from 49 to 46%. There were two stocking rates, high and low. The high and low stocking rates resulted in equal amounts of spotted knapweed decline but the low stocking rate resulted in more forbs and grasses and less bare ground. A light stocking rate of sheep when spotted knapweed is just growing its flower stalk gives the greatest reduction of flowers and has the least impact on native vegetation.
Technical Abstract: After two years of grazing a spotted knapweed infested pasture 10 miles north of Dubois, ID, spotted knapweed has declined but other vegetation components have changed as well. Vegetation in the area is typified as sagebrush steppe with three-tip sagebrush (Artemisia tripartita), bluebunch wheatgrass (Pseudoroegnaria spicata), and hawksbeard (Crepis acuminata) as the dominant species. Historically the pasture was gazed by cattle. Sheep grazing experiments were conducted using three grazing seasons (rosette, bolting, and flowering of spotted knapweed), two stocking rates (12 and 20 sheep days per paddock) and three replications of each 0.043 ha treatment. Twenty-four permanent plots (20 by 50 cm) were placed in each paddock to measure spotted knapweed numbers and percent cover of various vegetation components. Generally, numbers of small (rosette) and large (> 5cm basal diameter) spotted knapweed declined (190 to 43 and 2.4 to 1.2 m-2, respectively) in all treatments including the control whereas numbers of medium spotted knapweed increased (19 to 25 m-2). Number of flowers also declined in all treatments (82 to 27 m-2) however the decline in the control plots was least (195 to 130 m-2) and there were more flowers in the control plots at the end of the second year than in all other treatments (p= 0.02). Grazing at the bolt stage resulted in the fewest number of flowers (2.5 m-2). In all treatments spotted knapweed cover declined from 13.3 to 11.8%, cover of forbs increased (3.9 to 8%), annual grasses increased (0.3 to 0.7%), perennial grasses increased (13 to 23%), shrubs remained the same at about 14.4%, and bare ground decreased (49 to 46%). Perennial grasses increased the most in the control plots (14 to 29%), whereas bare ground decreased the most in the control plots (49 to 38%). Bare ground did not change in the higher intensity grazing but it did decrease from (49 to 44%) in the more lightly grazed plots. The two years of this experiment have come in drought years, which helps to explain the decline in several of the vegetation components, especially in the control plots. In addition some of the small spotted knapweed plants died or grew to be placed in the medium category. Light grazing by sheep did not result in any pasture degradation except for a smaller decline in bare ground compared with the controls. Grazing at the bolt stage was most effective in reducing flower production. This experiment will be grazed for another two years.