The effect of grazing is especially important in an environment with variable rainfall. Temporary droughts may change perennial vegetation into one dominated by ephemerals. Excessive grazing may slow or prevent the redevelopment of the perennial vegetation component once rainfall increases (Skarpe 1992).
Since detailed descriptions of grazing systems are provided by Tainton (1999), Savory & Butterfield (1999) as well as numerous other authors, they will not be dealt with in detail here. Rather, the actual response of the vegetation to grazing is considered.
Walker et al. (1981) and Ruess (1987) report that light grazing may stimulate grass growth, and lead to a higher biomass. This increase is probably due to the induction of tiller formation of perennial grasses (Tainton 1999). On the other hand, if grazing is too light, a significant amount of dry, moribund material may remain at the end of the season, causing the grasses to shade themselves.
Heavy grazing, or too fast rotational grazing, on the other hand, causes a decrease in grass production in the long run, since the amount of photosynthetic area is reduced before it may grow to any extent. Under very heavy grazing pressure plants may die altogether. The threshold between light and heavy grazing is not fixed, but needs to be viewed in terms of the ability of grass plants to recover from the damage caused by grazing. When conditions are such that they support fast growth, plants may withstand a higher pressure than they would otherwise.
Continuous grazing will adversely affect perennial grass plants (Skarpe 1992), since it prevents these plants from storing nutrients in their roots; the photosythate is rather used for growth.
The effect of grazing is strongly affected the season (Tainton 1999).
At the begining of the raining season, perennial grass plants translocate rootreserves to the leaves, until the leaves are able to sustain the plant. Excessive grazing at this stage may cause the plant to persist on these rootreserves since the photosynthetic area may remain too small to sustain the plant. In the long-run plants may be seriously weakened.
Towards the end of the growing season the perennial grass plants start translocating photosynthate into the roots. These would again be used by the plant to resprout at the start of the next growing season. Heavy grazing at this stage will reduce the amount of photosynthate that is available for translocation, and the plant will not grow as vigorously at the start of the next season (Tainton 1999).
Tainton (1999) also recommends that veld should be given a rest during flowering in some years, to permit the plants to set seed. Other studies have found that the reproductive effort of grass plants is reduced by grazing in this season.
Apart from the direct effects of grazing on the herbaceous vegetation,
the removal of the above ground biomass also affects the development of
the grass fuel load, and therefore the fire regime of an area.
relating to the development of the herbaceous plant layer
Fuel load development, to see the effect of grazing on fuel load development