- Understanding defoliation - Essential for sound veld management
|Last update: March 29, 2012 10:19:51 AM|
Understanding defoliation - Essential for sound veld management
How heavily and how often should veld be grazed?
The more severely veld is grazed, the less frequently the plants can tolerate such grazing. In principle, the reverse also applies, i.e. the more lightly veld is grazed during anyone grazing period, the more frequently this can be done.
What remains a controversy, despite research providing usable guidelines, is what is meant by a light or heavy and frequent or infrequent grazing. Another question is how this principle can be applied in practice.
Work on karoo bushes at Grootfontein by C M R du Preez has shown that, when approximately 70 % of the edible material is clipped from irrigated bushes every 120 days, their production is more than halved within 16 months. The quantity of roots per plant also declines accordingly.
In the Western Cape F G J van der Westhuizen showed that under irrigation and optimum growing conditions Osteospermum sinuatum (Bietou) recovers completely in 56 days after a 40 % pruning by weight, but recovers only by about 75 % in the same time period after an 80 % pruning. The same results were observed for the grass Ehrharta calycina (polgras).
Therefore, as a rule of thumb, one can consider 50 % or less to be the recommended level of grazing on karoo plants. If one considers the data obtained on O. sinuatum and E. calycina to be reasonably representative of the response of karoo plants to clipping, then even under optimum growing conditions in the veld, where no irrigation takes place, one would possibly require at least double the 56 day rest period after a 50 % clipping.
Many other karoo plants have a much slower growth rate than O. sinuatum and, therefore, would require a minimum rest between clippings of even more than double 56 days. For instance, Dr Aucamp working with spekboom (Portulacaria afra) in the valley bush veld, found that the plant needed approximately 270 days to recover fully after a 50 % defoliation. This shows that in the Karoo a wide range of rest periods is required to cater for the varying needs of the different species.
It is tempting to disregard the slower growing plants and to arrange one's grazing schedule according to those species which can cope with a short rest period. Unfortunately the solution is not so easily obtained. One needs the slower growing plants as much as the faster growing plants because it is they which provide a more reliable and constant feed supply – which is so essential in any drought-prone area.
By planning the length of rest periods on the requirements of the slower growing plants, one also caters for the faster growing plants. By doing this a more constant feed supply and stable ecology are guaranteed.
A minimum rest period of 120 days together with a moderate grazing level of not more than 50 % is therefore recommended for the Eastern Karoo. During dry periods and in the drier western parts of the Karoo where the rainfall is lower and less reliable, the minimum rest period will be longer than 120 days.
If it were possible to achieve very light grazings in the order of 10 % the minimum rest period could in theory be reduced. However, no matter how short the grazing period, most of the palatable plants are always grazed at least about 30 %, because of the highly selective grazing which takes place on a very heterogeneous veld such as the Karoo.
It is sometimes believed that by grazing at very high stocking densities for very short periods one can reduce this species-selective grazing. Unfortunately this is incorrect. Detailed studies on selective grazing have shown this not to be the case. It has been found that, regardless of the stocking density, the pattern of selection between species remains the same. Increased stocking densities merely speed up the appearance of the selection pattern.
This means that if a reasonably long minimum rest period, determined from the requirements of the slow growing plants, is not maintained, the most palatable as well as the slower growing plants will eventually be killed. This will over time result in a slow degradation of the veld.
Although the very palatable component is small and produces little feed it must never be neglected because its presence is of importance to lead plant succession. The constant elimination of the more palatable species can be likened to the perpetual removal of the front-runners in a non-stop marathon, which would result in a slow decline in the average speed of the race.
During which season should a camp be grazed?
A characteristic of karoo veld is that it consists of a number of components, viz. karoo bushes, grasses, trees, shrubs and "opslag", each of which shows different cycles of growth activity which reach their peaks at different times of the year. Since a plant is more sensitive to grazing during its active growth period, the time of year at which a camp is grazed will favour some components more than others.
This also means that if a camp is grazed at the same time every year some components will be seriously debilitated while others will benefit tremendously. In this regard repeated summer grazing suppresses grasses and promotes karoo bushes. Karoo bushes are, on the other hand, suppressed and grasses promoted by both autumn and spring grazing.
It must always be remembered that grass is more sensitive to seasonal grazing and mismanagement than karoo bushes are. This is because grasses are more seasonal in their growth, growing mainly in the warm seasons, and are generally more sought after by stock.
Therefore, to maintain a mixed veld of karoo bushes and grass, each component must be favoured in turn. This can be done by rotating the time of year at which a camp is grazed through the seasons. This principle must always be incorporated into any grazing system applied on karoo veld.
The easiest and most flexible way in which a seasonal grazing programme can be included in a grazing system is by applying the Group Camp Approach.
Karoo Regional Newsletter Spring 1984