Livestock grazing for weed management - Ecological Management of Agricultural Weeds - page 427

Grazing for weed control in pastures and rangelands
Weeds in grazing lands may be grasses, forbs, shrubs, or trees. Weed
problems may result from temporary phenomena dependent on pasture age
or weather, from a slow response to inadequate management, or from the
invasion of exotic species.
When pastures are established in arable crop land, most weeds originate
from the previous land use through the soil seed bank.While special measures
are needed for their control, these primarily annual weeds are seldom an
important part of the plant community for more than a short period. The
increased availability of nitrogen under intensive pastures with highly pro-
ductive legumes may also provide the conditions for an increase in herbaceous
nitrophilous weeds, many of which are annuals (Matthews, 1981; Tothill,
Mott & Gillard, 1981).
Shrub encroachment can result from either overgrazing or undergrazing.
An increase of woody species on semiarid lands results from the overgrazing
of the grass layer and greater infiltration of water to the subsoil. This favors
the establishment and growth of shrubs which then shade out the overgrazed
grass (Coppock, 1993).This situation is associated more often with cattle than
sheep (Tothill, Mott & Gillard, 1981). New Zealand hill pastures established
after forest clearing in wetter areas suffered shrub encroachment as soil fertil-
ity declined . Uneven grazing of low palatability forages at declining stocking
rates allowed the establishment of shrubs such as Ulex europaeus tolerant of
low fertility and acid soils (Daly, 1990). Many pastures were eventually
abandoned.
An increase of low-palatability herbaceous vegetation occurs under a
variety of conditions. Tothill, Mott & Gillard (1981) proposed that in drier
zones more palatable species are under both grazing and environmental
stress, whereas less palatable species are only under environmental stress. In
the wet tropics, overgrazed planted grasses lose vigor as soil fertility declines,
and are displaced by less palatable species or weeds. Grazing-tolerant grasses
such as Paspalum conjugatum , Paspalum notatum , and Axonopus compressus , and
native legumes produce a relatively stable, although not highly productive,
pasture (Serrao & Toledo, 1990). However, planted pastures may also be
invaded by unpalatable broadleaf and grass weeds such as Pseudoelephantopus
spicatus or Homolepsis aturensis under the same conditions.With erratic manual
or chemical weed control and low grazing pressure, these pastures revert to
patchy shrub and secondary forest (Serrao & Toledo, 1990).
Introduced species can also become weeds in grazing lands, altering the
existing vegetation composition, before monitoring and management