Prescribed Sheep Grazing to Suppress Spotted Knapweed on Foothill Rangeland

Defoliation Timing Effects on Spotted Knapweed Seed Production and Viability

“…Sheep in moderate to heavy spotted knapweed infestations typically consume less graminoids and more spotted knapweed when prescribed sheep grazing occurs in the late-bud-early-flower stage rather than the bolting stage (Thrift et al 2008). The late-bud-early-flower stage and the full-flower stage of spotted knapweed usually occur after rangeland graminoids have set seed and become less palatable and less vulnerable to defoliation (Olson et al 1997;Thrift et al 2008), but while the forage volume and nutritive value of spotted knapweed remain high (Olson and Wallander 2001;Thrift et al 2008). It should be noted that 22% of mature spotted knapweed seeds remain viable after passing through the digestive system of a sheep (Wallander et al 1995).…”

“…Eighty single-stem spotted knapweed plants between 23 cm and 36 cm in height were selected each year (2006 and 2007) on a site moderately infested with spotted knapweed (36% of vegetative composition; Thrift et al 2008). The areas containing the 80 plants (286 m 2 ) were fenced with 1.8-m-tall welded wire panels to exclude ungulate grazing.…”

“…Plants clipped at all other timings had 100% of their buds and flowers removed, plus 3 cm of stem below each bud or flower. The 35-40% relative utilization level during the bolting stage was determined by a previous study at the same study site, in which the relative utilization of spotted knapweed by sheep grazing on a landscape scale was 35-40% during the bolting stage (Thrift et al 2008). The 3 cm of foliage clipped beneath the buds or flowers was based on personal observations of how sheep graze spotted knapweed buds and flowers.…”

“…Sheep grazing is well suited for spotted knapweed control because sheep usually prefer forbs over grasses (Hanley 1982), sheep are adapted to grazing steep topography (Olson and Lacey 1994), and sheep have a cleft upper lip and narrow muzzle, allowing greater selectivity of plant parts (Arnold and Dudzinski 1978). For example, relative utilization of spotted knapweed by sheep averaged 35% to 50% in June or July, whereas relative utilization of desirable graminoid forage averaged only 15% except under exceptionally hot and dry conditions (Thrift et al 2008). Sheep grazing of this noxious weed may decrease its competitiveness with native grasses (Olson and Wallander 2001).…”

Changing Climate and Advances on Weeds Utilization as Forage: Provisions, Nutritional Quality and Implications

“…micranthos), a weed of rangelands in Northern America was reported to displace local plant species, degraded wildlife habitats, altered biogeochemistry of soil and triggered soil erosion and thus its control through grazing was found to be biologically and economically viable. Similarly, broom snakeweed and medusa-head (Taeniatherum caput-medusae) were effectively controlled through controlled grazing with reasonably good palatability [11][12][13][14][15][16][17].…”