- A Technique for estimating the grazing index values of karoo plants
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A TECHNIQUE FOR ESTIMATING THE GRAZING INDEX VALUES OF KAROO PLANTS
W.v.D. Botha, P.C.V. du Toit, C.D. Blom, H.R. Becker, D.J. Olivier, E.M. Meyer and G.Z.J. Barnard
Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg CP, 5900
At present the Ecological Index Method (ElM) described by Vorster (1981) is used to calculate grazing capacities in the Karoo. Index values are allocated to the Karoo plant species on a group basis (Blom, 1981), which gives rise to a disjunct 10, 7, 4, 1 series. The group classification is based on the ecological importance of the grass species, while the index values accorded to the karoo bush species are based on relative palatability ratings. These index values are used when the veld condition scores are computed. The condition scores are indicative of the state of health of the veld (Tainton, 1981).
Research by Botha (1991) revealed that the index values currently used to calculate the veld condition score may lead to either over- or underestimation of the current grazing capacity.
The construction of a series of values based on different parameters for different plant groups is unacceptable. However, the biggest disadvantage is that the series makes it impossible to distinguish between closely related species with different grazing values. In order to correct these apparent shortcomings, the index values of the species commonly encountered in the Karoo were subjectively adjusted by means of a species-by-species comparison.
The objectives of this article are:
(i) to illustrate the methods employed to assess grazing values, and
(ii) to debate the merits of the methodology.
A number of researchers and extension officers estimated the grazing values of those plant species encountered most frequently when carrying out botanical surveys. In order to ensure that the estimates of the species' scores were comparable, each species was assessed on the basis of the following six parameters:
- The ability of the species to produce forage, i.e. the amount of grazeable dry matter produced per year (low producers score 1; high producers score 10).
- The forage value during the growing season (species with a low feed value score 1, while species with a high feed value score 10).
- The forage value during the dormant season (as for the forage value during the growing season).
- Relative ease with which the species can be grazed, i.e. the presence or absence of spines (very spiny plants score 1, while plants without spines score 10).
- Perenniality (annual plants score 1, while strong perennial plants score 10).
Ability of the plant to protect the soil against surface soil erosion (upright karoo bushes have a low score, while decumbent bushes score fairly high; annual tufted grasses score 1, while tufted perennial grasses, depending on productivity and habit, have an intermediate score due to the erosive channelling effect they have on runoff water, while sod-forming grasses score 10).
The participants scored the species according to their knowledge of a particular species. A total of over 600 species were divided into manageable groups. Species scored during the first round represented those species encountered most often during botanical surveys in the Karoo. During the second and later rounds less well-known species were scored. Each parameter was scored out of 10. Parameter scores were added up and divided by six. The final score lay between zero and 10.
The average of the scores submitted by the participants for each species was determined and one standard deviation from the mean calculated for each score. Values falling outside one standard deviation from the mean were discarded and a new mean was calculated. The resultant mean (score) was vetted and either accepted or rejected. Rejected scores were scrutinised and adjusted on a consensus basis. The scores of related species were taken into consideration when the scores of individual species were adjusted. In the case of undesirable plants, i.e. poisonous and invader plants, a factor was introduced to weigh down the grazing value of the particular species.
Sixty per cent of the scores submitted during the first round were accepted, illustrating a high degree of concurrence between participants. The species scored in the first round represented those encountered most often during botanical surveys in the Karoo. Although less well-known species were scored during the second round, the method was well established and less than 15 % of the scores submitted were unacceptable. In this case species were relatively unknown or presented problems when the different parameters had to be scored.
The disjunct ElM series of 10, 7, 4, 1 changed to a continuous series of grazing values of 0,8 to 10 for the grasses and 0,5 to 7,7 for the karoo bushes. The climax grasses now range in score from 6 to 10, instead of the value of 10 used previously. The value of these adjustments can be seen when ElM computed veld condition scores are compared to veld condition scores computed using the grazing values (Table 1).
These sites (Table 1) were chosen to illustrate the differences between the two methods of scoring veld condition for the purposes of grazing capacity calculation; they do not necessarily represent the best in the area. The calculated grazing capacity figure represents the grazing capacity valid at the time of calculation. It is important that the length of the rainy season/growing season be considered in the final calculation of the grazing capacity. This series of grazing values enables distinctions to be made between species which are closely related, but which have different grazing values as shown in Table 2.
In areas where the botanical composition is dominated by grasses, the ElM apparently overestimates the current grazing capacity, due to the high index value of the climax grasses. Areas dominated by these grasses have high veld condition scores and hence high grazing capacities are calculated. It was found that the scores for these areas lie relatively far above the regression line describing the rainfall-veld condition score relation. In areas dominated by karoo bushes on the other hand, veld condition scores lie close to the regression line describing the rainfall-veld condition score relation.
Fig. 1 illustrates the regression of veld condition score on rainfall arrived at by means of the ElM. It is known that not all the climax grasses are of equal grazing value. When the grazing values of the species are used in the computations of the veld condition score, a "better fit" is obtained when regressing veld condition score on rainfall, as illustrated in Fig. 2.
A list containing the proposed grazing values of a selected number of the common species occurring in the Karoo is presented in Table 3. Nomenclature follows Gibbs Russell, Reid, Van Rooy & Smook (1985), Gibbs
Russel, WeIman, Retief, Immelman, Germishuisen, Pienaar, Van Wyk & Nicholas (1987) and Gibbs Russell, Watson, Koekemoer, Smook, Barker, Anderson & Dallwitz (1990).
Veld condition scores using the grazing values of the species in the computations result in a better fit when regressed on rainfall, than when the ElM scores of the species are used in the computations (see Fig. 1 and 2).
Veld condition scores calculated using the grazing values were compared to veld condition scores calculated using the ElM scores. The veld condition scores and the calculated grazing capacities of four sample sites from three different homogeneous areas are presented in Table 1. The marked influence of the climax grasses on the calculated veld condition score and hence the grazing capacity, is clear. In climax grassveld areas the grazing capacity is high when the ElM scores are used in the calculation of the veld condition score. This led to the assumption that a heavy stocking rate could be applied. When the grazing values are used in the calculation of the veld condition score, the grazing capacity is lower, indicating the necessity for applying a lighter stocking rate. In areas dominated by karoo bushes, using the grazing values in the calculation of the veld condition score yields a higher grazing capacity figure, indicating that a heavier stocking rate can be applied than when the ElM is used. The better fit obtained when regressing veld condition score on rainfall, using the grazing values of the species instead of the ElM scores, agrees closely with regressions of applied stocking rates on rainfall (Botha, 1991).
The adjustments are valuable from a soil conservation point of view. On the one hand, using the new grazing value index method advises a lenient grazing capacity on vulnerable soils prone to erosion. On the other hand, using the grazing values increases profitability, because high potential soils have a higher grazing capacity.
The value of these adjustments can be seen when comparing closely related species, for instance Pentzia species. In terms of Pentzia incana, the closely related Pentzia punctata is not as poor as the value of 1 previously suggested, but it could obviously not be classified in the same category. Likewise, the inclusion of Pentzia sphaerocephala in a higher category than Pentzia incana is justified, but the difference in grazing value is not as large as its index value previously suggested.
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Karoo Agric 5(2) p 10-14