- Defoliation frequency with respect to three karoo bush species
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DEFOLIATION FREQUENCY WITH RESPECT TO THREE KAROO BUSH SPECIES
F.G. Hobson and E. Sykes
Pasture Research Section
Maximizing income from karoo veld, from the grazing point of view, depends on the efficiency of utilization and level of herbage production. These two factors encompass almost the whole ideology of grazing management but for this article the scope is restricted to maximizing production. For maximum yield in a given season the harvests of herbage must be as large as possible with the shortest time span between harvests as possible, as governed by the vegetation (Booysen, 1966). To accommodate this, the grazing programme must allow for a rapid recommencement of new growth after grazing, and this rapid growth rate must be maintained right up to the next grazing (Booysen, 1966). The plant requirements for such a response are an adequate supply of labile non-structural carbohydrates in the stems and roots, together with a fair proportion of the original leaf area remaining (Booysen, 1966; Barnes, 1972). This means that regrowth will be fuelled from photosynthate from the residual leaves rather than from the non-structural carbohydrate accumulate; this being used for initiating regrowth after dormant periods e.g. drought. Maximum regrowth rate then occurs with lenient defoliation. Conversely there is widespread agreement that the more severe the defoliation the longer the recovery period needed, because of the slow initial rate of regrowth (Booysen, 1966). The fact that severe defoliation has been demonstrated to cause a tempary cessation of root growth (Crider, 1955, cited by Barnes, 1972) also indicates that lenient defoliations might be advantageous.
Almost all quantitative research in this field to date has been done on grasses, so there is an inherent danger of expecting the same responses from karoo bush vegetation, although the basic principles can most probably still be expected to apply. An important consideration when applying these to karoo bush communities is the governing influence of erratic rainfall. This means that the principles applied to growth and growth rate for maximum production apply only from after rain, when growth commences, to when growth for all intents and purposes ceases. Therefore in trials to study the influence of defoliation on maximizing producing the plants should be actively growing, and parameters from both the above ground and below ground parts of the plant should be measured.
Preliminary experimentation was conducted with various clipping frequencies, at a constant severe defoliation level, on three karoo bush species in order to gain a better understanding of the effects of and responses to severe defoliation, by karoo bushes.
This preliminary experimentation consisted of two trials, one and two, running for periods of four and a half and sixteen months respectively. (trial one: 3.9.69-3.2.70; Trial two 3.9.69-18.1.71). Both trials were carried out in raised beds with concrete floors (Beds were 6,1m long, 1,8m wide and 1,22m deep). Planting sites in the beds were equally spaced and plants randomly allotted to these. The three species involved, Felicia muricata (Bloublommetjie), Pentzia incana (Ankerkaroo) and Eriocephalus ericoides (Kapokbos), were subjected to three treatments.
- Clipping every 14 days. .
- Clipping every two months (60 days).
- A control which was only subject to a final clipping.
Initial defoliation was to a constant level which was determined by the plant size (Fig. 1), and all subsequent defoliations were always back to the original level.
There were five plants per species per treatment; each treatment carried out in a separate bed i.e. three beds were used each containing fifteen plants.
After completion of trial one of the plants were washed free and the following parameters determined for each plant.
Total yield of above ground herbage (including that remaining after final clipping).
Total root mass.
Root volume (Displacement technique).
In trial two only total dry matter production was determined. All results were statistically analysed.
Both trials were commenced once the transplanted plants had become well established. To achieve this, monthly irrigation was applied and continued throughout the trials in order to ensure conditions which favoured active growth.
RESULTS AND DISCUSSION
Average total yield per plant of the above ground herbage for the trials are presented in Fig. 2 (a & b).
Inspection of Fig. 2 indicates, as would be expected, an increase in production with decreased defoliation frequency, even although no statistically significant difference was found between the 14 day and two monthly clippings. Due to the lack of intermediate defoliation frequency treatments it was not possible to establish whether production per plant peaked before the control treatments (single defoliation at the end of the experiment) as is indicated when the data from the two trials are pooled as shown in Fig. 3.
The validity of conclusions from the combined trials as shown in Fig. 3 can be doubted but may be considered as an indication, however, it must be remembered that a vague indication might be more dangerous than knowing nothing at all.
Fig. 1 & 2 clearly show that E. ericoides, under the treatments applied, produces more than F. muricata or P. incana and that this increases, the less frequent defoliations occur. Such a difference can be attributed to the growth habit of E. ericoides which is much more open, and generally capable of attaining greater heights than F. muricata or P. incana. The statistical analysis supports the visual appraisal by giving a highly significant difference in favour of E. ericoides. No significant difference existed between P. incana and F. muricata, possibly due to them having a similar growth habit. One can say that under the conditions of these trials. E: ericoides was able to utilize the longer rest periods better than F. muricata or P. incana.
A healthy root system is indirectly as important as a vigorous above ground appearance of a grazing plant (May, 1960). Hedrick (1958) points out that the consequences of overgrazing occur first in the roots and only at a later stage become visible in the tops. The same could possibly be said for any defoliation regime.
Relationships between the total root weight and root volume against defoliation frequency are presented in Fig. 4 & 5 respectively. No root parameters were measured in trial two; Fig. 4 & 5 being results from trial one only.
Both Fig. 4 & 5 show the same trends, indicating a constant root density for the three species although nothing could be concluded from this fact. The response to decreased defoliation frequency is shown as an increase in root mass and volume. An apparent exception is the decrease in root mass and volume of F. muricata in the 60 day defoliation frequency treatment, however, this could be due to experimental error, as indicated by the standard error (SE). For both root volume and weight no statistical difference was found between the 14 day and 60 day defoliation frequency treatments while both were highly significantly less than the control treatment. The root mass of E. ericoides was found to be significantly greater than F. muricata and highly significantly greater than P. incana; F. muricata being significantly greater than P. incana. The root volumes of E. ericoides and F. muricata were found not to differ significantly but P. incana was found to be significantly less than. F. muricata and highly significantly less than E. ericoides.
In general it can be said that the responses shown by the roots are similar to those shown by the aerial plant parts, although the differences between plant species appear more evident from the roots. The results set out above tend to support the basic principles and trends mentioned in the introduction for grasses. One can therefore expect that the lighter the defoliation the shorter the recovery period required. However, now one can pose the question: What recovery period is required for any particular degree of defoliation? It is only logical to expect this to vary among species (as is evident from Fig. 2, 4 & 5) of which the karoo vegetation has no shortage. This means that the degree of selective grazing, in a sense, determines the regrowth period required, because as selection increases so does the degree of defoliation of the more acceptable species. Hence the answer to maximizing production from the vegetation appears to be in the direction of lenient defoliation with a minimum of selective grazing. The practicability of such principles is however, open to discussion.
BARNES, D.L., 1972. Defoliation effects on perennial grasses - continuing confusion. Proc. Grassld. Soc. Sth. Afr. 7, 138-145.
BOOYSEN, P. DE V, 1966. A physiological approach to research in pasture utilization. Proc. Grassld. Soc. Sth. Afr. 1,
HEDRICK, D. W, 1958. Proper utilization - a problem in evaluating the physiological response of plants to grazing use: a review. J.. Range Mgmt. 11, 34-43.
MAY, L.H., 1960. The utilization of carbohydrate reserves in pasture plants after defoliation. Herb. Abst. 30,239-243.
Karoo Agric 1 (5), 9-11