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

The South American Campos ecosystem 211
main problem in developing an optimum stocking rate criterion is the need to
preserve forage to be used when grass growth is limited by moisture stress or
low temperatures.
Liveweight gain s are highly variable, being a function of weather and
forage availability. When pastures have winter species, autumn deferment is
recommended to supply forage for winter. When winter grasses are scarce,
forage accumulation must be achieved in other periods because of fast declining
forage quality in autumn; under conditions of low autumn growth and quality
loss as the resting period extends, the accumulated forage is inadequate to
supply animal requirements and provide the desired liveweight gains (Ayala
et al., 1999).
Inadequate grazing management – such as overgrazing , inadequate
subdivision and continuous grazing – prevent flowering and seed production of
winter grasses, leaving reliance on vegetative mechanisms alone for persistence.
This may be the main reason for decreased cover of winter grasses in natural
grasslands of the Campos .
Raising the fertility level of the soil by means of N and P fertilization
increases the production and quality of natural grassland . The process is
relatively slow, with increased responses as more nutrients are applied.
Fertilization “disturbance” leads vegetation to a new equilibrium point, with
botanical changes consisting of an increase in productive species frequency,
and therefore increased secondary production. This technology complements
grassland improvement with legume introduction, as well as cultivation of
perennial and annual forages. Natural grassland fertilization allows increased
production and quality of vegetation on soils too shallow for more productive
forages. At the same time, the residual benefits of N and P fertilization must
also be considered.
N and P additions, particularly the latter, should help to return to natural
grassland something that has been extracted during centuries of grazing , from
livestock introduction at the beginning of the seventeenth century, besides
its contribution to plant and animal biodiversity maintenance on natural
grasslands. It is important to conserve this natural resource without degrading
it, maintaining an awareness of the economic, ecological and social aspects
implicit in sustainable development .
The introduction of legumes, coupled with fertilizing at sowing, yearly
maintenance phosphorous dressings and grazing management , move vegetation ,
in a slow biotic process, to a new equilibrium point where yield and quality
are higher than the original. Grazing management and fertilization have
to be closely controlled to maintain the pasture at this higher equilibrium
point. The result should be a sward dominated by winter species, where high
quality native perennial species are outstanding. This is an alternative route
to increasing annual primary production without using herbicides while
conserving productive species of the natural grassland .