Livestock grazing for weed management - Ecological Management of Agricultural Weeds - page 430

Carduus pycnocephalus and C. tenuiflorus are also vulnerable to an altered
grazing regime (Bendall, 1973). These weeds germinate in autumn, over-
winter as rosettes, and seed in the spring in Tasmania,Australia.Withholding
grazing in the autumn created increased competition from desirable forage
species in the pasture. Seedlings of both Carduus species became etiolated and
less prickly compared to fall-grazed treatments in which seedlings were
compact and had hardened spines.During winter or spring grazing following
no grazing in autumn, sheep preferentially grazed C. pycnocephalus and C.
tenuiflorus . They also consumed the growing points, which were several centi-
meters above the soil surface. The growing points of these weeds on autumn-
grazed pastures were below ground level and inaccessible to grazing sheep.
With autumn grazing Carduus spp. densities were 4 to 13 plants m 2 com-
pared with 0.8 to 2.0 plants m 2 without grazing. The reduction in Carduus
spp.numbers did not carry over to the following year.While the seed bank was
not measured in the study, this was probably the source of new Carduus seed-
lings. Over time, impacts of the different grazing regimes may have become
more evident.
Chrysanthemum leucanthemum is unpalatable to cattle. Olson, Wallander &
Fay (1997) noted that severe infestations are the product of season-long
grazing at low cattle-stocking densities and suggested that while sheep might
graze C. leucanthemum better than cattle, most cattle ranches are not equipped
to manage sheep.They proposed that intensive grazing by cattle might expose
C. leucanthemum to non-selective grazing, trampling damage, and untimely
germination and seedling mortality. In a two-year experiment comparing
intensive cattle grazing to ungrazed controls, the grazed treatment had lower
densities of C. leucanthemum in the seed bank, as seedlings, and as rosettes
(Olson, Wallander & Fay, 1997). Adult C. leucanthemum densities were not dif-
ferent in grazed and ungrazed treatments, although cattle both trampled and
pulled out C. leucanthemum stems. The investigators concluded that grazing
and trampling impacts on the initial C. leucanthemum life stages could be
expected to lead to longer-term declines in adult infestations.
Effects of paddock size and uniformity on weed control
Increasing grazing pressure when weeds are vulnerable or decreasing
grazing pressure to reduce weed establishment sites can be achieved when
paddocks contain relatively uniform vegetation on similar soil and slope con-
ditions. In their reviews of the spatial heterogeneity of plant-large herbivore
interactions in grazing systems, Coughenour (1991) and Bailey et al . (1996)
concluded that the scale of large patches, corresponding to animal grazing