Livestock grazing for weed management - Ecological Management of Agricultural Weeds - page 432

Changing the grazing species to control weeds
The addition of an animal species specifically for weed control may
require new management skills for the farmer and increased investment in
facilities, fencing, and animals. However, mixed-species grazing can be effec-
tive for diminishing weed problems at low cost and under certain circum-
stances also produce additional income.
The addition of sheep to a bull rotational grazing system to control Senecio
jacobaea , a weed highly toxic to cattle, but less toxic to sheep, was tested by
Betteridge et al . (1994). Herbicides are effective for S. jacobaea control, but also
reduce forage legume growth. Timely mowing eliminates S. jacobaea seed pro-
duction. However, mowed plants become perennial and produce seed the fol-
lowing year if mowing is not continued.Young bulls were rotationally grazed
at 10.5 stocking units ha 1 on 15 1-ha paddocks, completing the rotation
every 30 to 50 days. Sheep were grazed with the bulls at 1.5 or 3.0 stocking
units ha 1 on the same rotation or were mob-grazed for four days four times
annually at two similar rates. In mob-grazing a large number of sheep are
grazed in a small paddock for short periods. By the end of the first year 82% of
S. jacobaea in the bulls-only plots were flowering, while only 32% to 36% and
0% to 5% of the S. jacobaea in the bulls 1.5 and 3.0 stocking units of sheep

ha 1 were in flower. Senecio jacobaea plants were largest with bulls-only
grazing. Senecio jacobaea plants grazed with bulls

3.0 stocking units of sheep

were smaller than either bulls 1.5 stocking

units of sheep. The density of new S. jacobaea seedlings increased when sheep
were grazed rotationally with young bulls, but decreased with either level of
bulls

sheep mob-grazing or bulls

sheep mob-grazing. Betteridge et al . (1994) suggested that infrequent
sheep mob-grazing did not open the sward as much to new S. jacobaea seedling
establishment, although mob-grazing was less effective in reducing the size
of already established S. jacobaea .
The use of cattle to graze Nardus stricta -infested sheep pastures was studied
by Grant et al . (1996). This weedy perennial tussock grass has a lower feed
value,lower growth rate,and lower palatability than most other grasses of the
Scottish hill region. It also provides poor habitat for birds compared to the
heather communities that it often replaces.In the study,grasslands with more
than 50% N. stricta cover were continuously grazed by sheep at two intensities
(grasses between N. stricta tussocks maintained at 3.5 or 4.5 cm) and one cattle
grazing routine (4.5 cm). Over a five year period, N. stricta cover increased 86%
in the 4.5-cm sheep treatment and 72% in the 3.5-cm sheep treatment, but
declined 30% in the 4.5-cm cattle treatment (Figure 9.7). Nardus stricta was not
highly competitive, showing lower leaf expansion rates and lower tiller pro-