Grasslands of the World.pdf - Disasters and Conflicts - UNEP

Grasslands of central North America 241
summer-season grazing (Launchbaugh and Owensby, 1978). However, Olson,
Brethour and Launchbaugh (1993) cautioned against using early-season grazing
at high stocking rates when vigour of cool-season plant species is a concern.
McCollum et al. (1990) reported that total beef production was increased
19 percent under increased early-season stocking compared with traditional
season-long stocking.
Grazing systems
Other stocking plans for the Great Plains include several types of rotational
grazing systems (Holechek, Pieper and Herbel, 2001). The objective of many of
these plans is to increase individual plant vigour and overall plant productivity .
The basic design for rotation systems is to defer grazing on different pastures
or portions of the entire area at different seasons in different years (Vallentine,
1990). This objective is met by adjusting the number of pastures and herds to
ensure that the same area is not grazed at the same period each year. Research
results comparing specialized grazing systems to continuous grazing have
been mixed (Herbel, 1974; Herbel and Pieper, 1991; Hickey, 1969; Van Poolen
and Lacey, 1979). In some cases there has been little difference in either cattle
production or vegetation status between continuous grazing and some form
of rotation grazing in the central Great Plains (Hart et al., 1988; Lodge, 1970;
Rogler, 1951; McIlvain and Shoop, 1969; McIlvain and Savage, 1951; McIlvain
et al., 1955). Generally, rotational grazing systems, whereby livestock are
concentrated in one pasture for short periods, decreased livestock performance
(gain per head), presumably because of lower selection, lower nutritive quality
of forage selected and lower digestibility (Malechek, 1984; Pieper, 1980). Field
studies in some cases confirmed this (Fisher and Marion, 1951; Heitschmidt,
Kothmann and Rawlins, 1982; Pieper et al., 1991; Smith et al., 1967). Other
studies indicated that there was some vegetation improvement , including
higher production, increases in abundance of plant species desirable for
grazing, or increases in plant cover (Herbel and Anderson, 1959; Smith and
Owensby, 1978; Pieper et al., 1991) under specialized grazing systems. Van
Poollen and Lacey (1979) reported that six studies in the northern Great Pains
showed virtually no difference in herbage yields under continuous grazing
and rotation systems, while in tall-grass prairie (Flint Hills in Kansas) herbage
yields were 17 percent higher under specialized grazing systems than under
continuous grazing. In some cases, improved management and more uniform
grazing distribution under specialized grazing systems are confounded with
the grazing system.
During the 1970s and 1980s, interest in short-duration or time-controlled
grazing , as advocated by Savory (1999), peaked. This grazing approach is based
on having a large number of paddocks and moving livestock rapidly through
the paddocks, especially during periods of rapid plant growth (Savory, 1983,
1999; Savory and Parsons, 1980). The grazing period is often only a matter