Livestock grazing for weed management - Ecological Management of Agricultural Weeds - page 429

herbicides for the control of C. vulgare damaged clover and reduced liveweight
gains by an amount equivalent to the reduction from 1.7 C.vulgare plants m 2 .
In the second year of the study, live-weight gain was not affected by C. vulgare
density.
These studies indicate that the impact of weeds on grazing land productiv-
ity depends on weed density and the level of forage use. Weeds reduce animal
gains primarily when weed infestations are severe and animal use and forage
availability are closely matched (Hartley, 1983 b ). Less palatable species do not
reduce animal gains at low infestation levels and when surplus forage is avail-
able. Even when they are not grazed, weedy species still protect soil, add
organic matter, and recycle nutrients. The decision to control weeds in
grazing lands should incorporate not only weed control costs, but also the
likelihood that additional forage will be converted to animal products and soil
will not be left unprotected. Unless a weed is replaced by a more productive
forage species which in turn is converted into animal products, direct weed
control expenditures may be uneconomic. However, numerous experiments
have shown that low-cost or income-generating practices such as changing
the grazing regime, adding additional fencing, or introducing a different
animal species may shift the floristic composition in a pasture or rangeland to
more productive, less weedy species, reduce the severity of weed outbreaks,
and maintain soil cover.
Weed control through altered grazing regimes
Simple changes in grazing rates and schedules may in some cases
effectively reduce pasture weeds.
Hordeum glaucum , H. murinum , and H. leporinum are annual grasses that
become unpalatable after stem elongation and flowering, and later produce
seeds with spine-like awns that damage young stock (Popay & Field, 1996).
Hartley et al .(1978) tested lax and hard grazing treatments in autumn,winter,
and spring for Hordeum spp. control. They found that adjusting sheep
numbers to maintain desirable pasture species at 2-4 cm during all grazing
seasons effectively prevented flowering and seed production of Hordeum spp.
Maintaining desirable pasture species at 2-4 cm also guaranteed adequate
pasture cover in the late summer,which limited Hordeum spp.germination.By
the third year of grazing, Hordeum spp. had virtually disappeared from the
flexible stocking treatment which maintained sward height of 2-4 cm and
from the treatment with hard spring and autumn grazing, but light summer
and winter grazing. In this later treatment hard spring grazing prevented
flowering and light summer grazing assured abundant pasture cover during
the period of potential Hordeum spp.germination.