Livestock grazing for weed management - Ecological Management of Agricultural Weeds - page 418

bunch grass Panicum maximum and three legumes were stocked with weaned
bull calves at three rates (two, four, and six animals ha 1 ). Even at the lowest
stocking rate, the trailing legume, Pueraria phaseoloides , virtually disappeared.
This species' greater palatability increased the frequency with which it was
grazed. Its trailing growth habit also increased its susceptibility to treading.
Both factors contributed to slower P. phaseoloides regrowth than for other
species.The other two legumes, Centrosema pubescens and Stylosanthes guianensis ,
were favored differentially by the low and moderate stocking rates respec-
tively. Centrosema pubescens has a climbing growth habit that allowed it to grow
above grasses at lower stocking rates. Erect-growing S. guianensis was shaded
by taller grasses at lower grazing rates, but prospered when grass was grazed
more heavily.As the stocking rate increased,the frequency of weedy volunteer
species increased. The increased bare ground and more open sward favored
germination and establishment of weeds from seeds brought in from outside
each field or in the soil. By the end of the third year at the highest stocking
rate, the planted grass and legumes were being replaced by weedy species.
These included grazing-resistant grasses, such as Paspalum conjugatum and
Axonopus compressus , and unpalatable broadleaf species, such as Sida acuta and
Lantana cinerea . These species were able to maintain or increase their leaf area,
roots, buds, and seeds, in spite of the high stocking rate.
The states and transitions framework,proposed by Westoby,Walker & Noy-
Meir (1989) for the analysis of non-equilibrium rangelands,provides a similar
perspective on shifts in vegetation composition.Non-equilibrium rangelands
under the influence of large abiotic variability are often characterized by
abrupt, threshold-like changes or transitions from one vegetation type or
state to another. For example, in southern Texas, perennial grasslands shift
abruptly to dense Prosopis shrublands, although there may be intermediate
states such as grasslands infested with shrub seedlings (Archer, 1989).
Westoby, Walker & Noy-Meir (1989) suggested that even when transitions are
gradual and linear (e.g., in grasslands that receive more abundant and regular
precipitation), the states and transitions framework can usefully organize
information on vegetation composition and factors contributing to the
change from one state to another.
A states-and-transitions diagram defines the different possible alternative
persisting states for the vegetation in a grazing region (Figure 9.3). Dominant
forage and weed species present, vegetation structure, or soil condition may
be used to define states. The different states are the result of grazing manage-
ment, fertilization, overseeding of forages, and discrete events like fires.
Variability in weather frequently plays a critical role. A change in vegetation
represents a different state, if it alters livestock production or signals an