Livestock grazing for weed management - Ecological Management of Agricultural Weeds - page 431

and movement and often correlated with abiotic site factors,was the appropri-
ate scale for the improvement of grazing management.
The difficulties in controlling grazing pressure in highly heterogeneous
paddocks were demonstrated in a study by Gillen, Krueger & Miller (1984).
They documented the distribution of cattle grazing within a diverse paddock
using a preference index. This was calculated as the proportion of time the
animals spent grazing a large relatively uniform patch divided by the relative
physical space represented by that patch within the paddock.The paddocks in
the study were over 3500 ha. Cattle had low preference indices for slopes
greater than 20%, and spent more time on increasingly gentler slopes. They
also grazed more heavily closer to water. Preference indices fell below 1.0 on
areas 600 m or more from water sites. Cattle grazing distribution was not
altered by the location of salt in their study.Hart et al .(1993) compared contin-
uous grazing in a 24-ha paddock with rotational grazing on 20 paddocks of 7
ha. In smaller paddocks use did not vary, whereas in the large paddock forage
use declined with distance from water.In large paddocks,animal grazing time
was lower,animal travel time increased,and cow/calf weight gains were lower.
To facilitate timely and flexible grazing, large, internally diverse paddocks
can be subdivided into smaller paddocks which are less variable internally.
With smaller paddocks, grazing and other management inputs can be
adjusted to the weed infestations and sward composition and vigor in each
area.Animals can be concentrated in certain paddocks for short periods,while
other paddocks recover.
Electric fencing has increased the practicality of permanent or temporary
small paddocks. The recent development of lightweight temporary fence
posts,high-tensile wire,solar-powered chargers,and portable meshes has also
reduced the cost and made small paddocks feasible in areas remote from
electrical installations.
A farmer's options to increase or decrease grazing animal numbers for
weed control are greatly facilitated by an agile marketing system, adequate
off-farm infrastructure for animal transport, and the availability of off-farm
feed supplies (Medd, Kemp & Auld, 1987; Stafford Smith, 1996). For these
reasons,in remote range- or pasture-based livestock areas in developing coun-
tries, farmers can not easily vary their herd or flock numbers or feed resources
at the farm level in a short period. On the other hand, in temperate humid
pasture regions,many options are readily available to farmers to permit short-
term expansion or contraction of grazing pressure on specific paddocks.