- Stocking rate vs defloliation of karoo bushes
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STOCKING RATE vs. DEFOLIATION OF KAROO BUSHES
P.C.V. du Toit
Grootfontein Agricultural Development Institute, Middelburg CP, 5900
The Ecological Index Method of grazing capacity calculation described by Vorster (1982) is commonly used in the Karoo to estimate grazing capacity from floristic composition. The contribution made by each species to the estimated grazing capacity is based on the ecological significance of the grasses and the palatability of the karoo-bush species. Perenniality and seral position of the grasses, i.e. climax, subclimax or pioneer, play a role in the determination of its ecological value. The subjective rating of palatability i.e. palatable, less palatable and unpalatable, determines the "ecological" value of the karoo bushes.
It is known that this so-called ecological value does not necessarily reflect the grazing value of a plant species or its contribution to the grazing capacity. Therefore, the use of this technique of grazing capacity estimation leads to over- and underestimation of the current grazing capacity.
Studies currently undertaken by Du Toit (1992, unpublished) aim to quantify the grazing value of various plant species. For this reason, a number of species, representative of pioneer to climax grasses and unpalatable to palatable karoo bushes, are harvested every 3 months. The plant material is harvested in four reasonably homogeneous farming areas. The harvested plant material is separated into potentially grazeable and non-grazeable fractions, prior to further analysis. By definition, all the material thicker than 2 mm is regarded as non-grazeable. This definition is based on the work of Botha (1981) and Botha, Erasmus & Theron (1990). A field trial was undertaken in order to test the hypothesis.
The purpose of this paper is to:
- Test the hypothesis that sheep select material thinner than 2 mm, and,
- to determine to what degree this hypothesis can be used as an indicator of optimum stocking rates.
The trial was carried out at the Carnarvon Research Station, situated in the Arid Karoo (Acocks, 1988). Two less palatable karoo-bush species were selected for monitoring. They were Pentzia spinescens (doringkaroo) and Rosenia humilis (blouperdekaroo). The less palatable species, Eriocephalus ericoides (gewone kapokbossie) was also considered for measurement, but was excluded, due to its uneven distribution over the experimental terrain. The species were selected in an established stocking rate trial, where Afrino sheep graze the veld at four stocking-rates.
Three considerations support the decision to work with only the less palatable species group.
- Firstly, the less palatable species group accounts for more than 70 % of the veld composition and contributes the major portion of the available forage. This group was therefore considered the most suitable to monitor.
- Preliminary investigations indicated that unpalatable species are seldom grazed and it is therefore unlikely that they will respond to stocking rate manipulations.
- It was observed that the diameter of grazed stems of the palatable species, Limeum aethiopicum (koggel- mandervoetkaroo) and Salsola calluna (kortbeenganna) regularly exceeded 5 mm, regardless of stocking rate. This is partly ascribed to their relative palatability on the one hand and their low numbers on the other. They are therefore not suitable for monitoring.
Twenty five plants per species were randomly selected in an arc between 100 and 200 m away from the watering point, in the most recently grazed camp of each stocking rate. Closer to the watering point, all karoo bushes were fairly severely grazed, while further away, it was more time consuming to find recently grazed stems to measure. Karoo bushes that were obviously grazed quite recently, were selected for measurement. The discoloration of the wounds, left at the apex of the stems after grazing, enabled the distinction to be made between "old" grazed-off stems and recently grazed stems. Old stem wounds were greyish, while recent wounds had a straw-coloured to light-greenish tint. Stems that had a straw-coloured apical stem wound were selected for measurement. A maximum of four readings were taken per plant. The readings were taken with sliding vernier callipers. In laterally compressed stems, recourse was taken to arithmetically mean the widest and narrowest measurements, so as to provide a single measurement for that stem.
The trial period ran from the 1990/91 season to the 1992/93 season, with measurements being taken during February of each year. This date corresponds to the peak growth activity calculated for this area using climatic variables (Du Toit, 1990). It was reasoned that, with growth at its optimum and with young, succulent stems, the defoliation:stocking rate relation of the karoo bushes would be described by a quadratic curve. At low stocking rates fairly thin stems are grazed off. An increase in the stocking rate leads to the grazing off of thicker stems. At high stocking rates during good seasons, the abundance of available forage influences the thickness of the stems being grazed and thus the diameter levels off. It would then be possible to calculate an optimum stocking rate from the stocking rate:grazed stem relation.
The measurements of the grazed stems of Pentzia spinescens and Rosenia humilis, for the three seasons 1990/91 to 1992/93, are presented in Table 1.
Mean diameters for Pentzia over all seasons were 1,14 for the lowest and 1,68 mm for the highest stocking rate. The seasonal means over all stocking rates were 1,30, 1,27 and 1,54 mm for Pentzia pinescens.
Mean diameters measured for Rosenia over all seasons were 1,34 for the lowest and 1,89 mm for the highest stocking rate. The seasonal means over all stocking rates were 1,65, 1,44 and 1,72 mm for Rosenia humilis.
The fairly low mean diameter measured for Pentzia spinescens and Rosenia humilis in the high stocking rate treatment (5,5 ha/SSU) during the 1992/93 season was influenced by the high proportion of Eriocephalus ericoides found in that specific camp. When present, Eriocephalus ericoides seemed to be grazed in preference to Pentzia spinescens owing to the more succulent nature of its stems.
These results coincided with the observations made by Vorster & Blom (1982, unpublished), at the Grootfontein Research Station situated in the False Upper Karoo (Acocks, 1988). They found that Merino sheep on veld dominated by Pentzia incana (ankerkaroo), grazed stems with a mean diameter of 1,35 mm, while Dorper sheep grazed stems with a mean diameter of 1,55 mm.
The hypothesis that sheep voluntarily graze stems with a diameter of up to 2 mm must be rejected. The stems of the less palatable species component, the most abundant species group, were seldom grazed when they were 2 mm thick. However, the stems of palatable species, such as Limeum spp. and Salsola spp., were frequently grazed when thicker than 2 mm. Few measurements of the less palatable species exceeded a thickness of 2 mm. At high stocking rates, dry matter becomes limiting and the grazing of stems thicker than 1,5 mm can therefore not be regarded as being voluntary.
During the 1990/91 season, the measured diameters levelled off with an increase in stocking rate. Precipitation during 1990/91, for the 12 months preceding the month during which the measurements were taken, was 261,9 mm (Immelman, 1993, personal communication). This is 36 % higher than the long-term median annual rainfall, which is 192 mm (Anon., 1993). Sufficient forage was available. Because of the succulence of the stems, fairly thick stems were grazed, but without severe grazing being evident. The abundance of succulent forage prevented the grazing of thicker stems, with the increase in stocking rate.
During 1991/92, only 50 % (94,3 mm) and during 1992/93, only 41 % (78,5 mm) of the long-term median precipitation was measured. This resulted in a slow growth rate and low dry matter accumulation.
During both the 1991/92 and 1992/93 seasons; the measured diameters increased curvilinearly with increasing stocking rate.
However, the measurements made during 1991/92 were lower than during 1990/91. The material dried out due to the drought and the overall stem dimensions decreased. The low mean diameters measured in the case of lower stocking rates were influenced by the carry-over effect of dry matter. At the higher stocking rates, less dry matter was usually carried over to the next season and consequently the mean measured diameters increased. The mean diameters measured in the high stocking rate during 1991/92 were much higher than during the 1990/91 season.
In 1992/93 the effect of the second consecutive dry season became evident. Animals grazed very severely in order to satisfy their requirements, especially in the high stocking rate treatments. Grazed stem diameters increased sharply with increasing stocking rate.
Grazed stem diameter as an indicator of stocking Rate
For the purposes of this analysis, the data obtained for Pentzia spinescens and Rosenia humilis were pooled. The influence of stocking rate on the diameter of stems grazed, is illustrated in Fig.1.
The expected stocking rate:grazed stem relation of the karoo bushes was a quadratic curve. As the stocking rate increased, the grazed diameter of the stems increased but levelled off at the higher stocking rates. During the good rainfall season of 1990/91 it was found that, with growth at its optimum and with young, succulent stems in the low stocking rate treatment, fairly thin stems were grazed off. With the increase in stocking rate thicker stems were grazed off. In the high stocking rate, the abundance of available forage influenced the thickness of the stems being grazed. The measured diameter of grazed stems decreased.
The relation during the 1990/91 season was a convex parabola, in 1991/92 the relation was nearly linear and in 1992/93 the relation was a concave parabola. The shift in the slope and direction of the curve is explained by the dry matter availability. Due to the .low rainfall experienced during the last two seasons dry-matter production was very low. Instead of the bulk of available dry matter restricting the thickness of the stems being grazed, the animals had to graze thicker stems in order to satisfy their requirements.
As the dry seasons progressed, the reserve dry matter produced during the favourable seasons ran out. This was especially true in the case of the higher stocking rate treatments. The fact that the higher diameter stems were being grazed, meant that the plants were overgrazed. The bushes diminished in size and they eventually faded when continued high stocking rates were being applied. It was observed that individual plants of Rosenia humilis had been grazed down to such an extent in the high stocking rate treatments, that they could no longer provide sufficient forage. This led to the next lower order, less palatable species being overgrazed. Eventually the less palatable species group lost its ability to supply grazeable dry matter. Their contribution to dry matter production in the system was replaced by the unpalatable species, Eberlanzia ferox (doringvygie). As a result of the change in species composition, animal production declined.
From this stocking rate:grazed stem relation the optimum stocking rate could be calculated.
A regression of mean grazed stem diameters on stocking rate (data of the three seasons were pooled for each stocking rate) yielded a quadratic curve similar to the Jones and SandIand curve (Jones & Sandland, 1974) of gain per ha on stocking rate recalculated from data quoted by Meyer (1992). The optimum stocking rate calculated from the gain per ha:stocking rate relation is 33,36 ha/LSU. The optimum stocking rate calculated from the grazed stem diameter:stocking rate relation is 32,46 ha/LSU(Fig. 2), while the official grazing capacity in. the area is 30 ha/LSU.
It is clear therefore that the grazed stem diameter is a useful parameter in the calculation of the optimum stocking rate.
DISCUSSION AND CONCLUSIONS
The hypothesis that sheep voluntarily graze stems with a diameter of up to 2 mm is rejected. Less palatable species are seldom grazed as thick as 2 mm while grazed stems of palatable species are often thicker than 5 mm. The method employing stem thickness as criterion in the estimation of grazeable dry matter, often used "in the calculation of the grazing capacity of an area, overestimates the forage component. The consequent overestimate of the grazing capacity leads to over utilisation. The criterion of using 2 mm to estimate forage may prove satisfactory where palatable to less palatable shrubs occur in a 50:50 ratio. The overestimate of the less palatable forage component will be offset by the underestimate of the palatable component. Where the less palatable species contribute the major portion to the forage, the mean grazed stem diameter is 1,5 mm. This measurement is 25 % lower than the previous accepted norm. In terms of the overall accumulated dry matter, it may be lower by as much as 50 %. The tremendous impact that the overestimate of the grazing capacity can have on dry-matter removal is obvious.
In order to avoid "hidden" overgrazing, stocking rates should be lighter than the optimum stocking rate calculated from stocking rate trials, for both the animal production per ha and the grazed stem diameter. This recommendation is especially true in the light of the overestimation of available forage. Hidden overgrazing results during drought conditions when little dry matter is carried over between grazing periods and production cannot keep up with removal, because of the low dry matter production. Lenient stocking rates enable animals to satisfy their dry-matter requirements with a relatively low negative impact on the veld, regardless of the prevailing climatic conditions. The present system of stock reduction, in terms of which farmers qualify for drought aid, benefits the veld in that not all the dry matter is removed during grazing, but is carried over to be utilised during periods of low dry-matter production.
During dry periods animals graze more severely than during good rainfall periods in order to satisfy their dry-matter requirements. Plant stems are not as succulent as during good rainfall periods and the measured diameter of the grazed twigs decreases as a result of the drying out of the stems. It is postulated that although the measured diameter of grazed stems decreases, the animals still graze to the same extent. When animals graze stems as thick as, or thicker during dry periods, than stems during good rainfall seasons, the veld is overgrazed.
In the long run the hidden overgrazing results in a reduction in the vigour of the karoo bushes, ultimately leading to unfavourable species composition changes, with a consequent lower grazing capacity.
Measurements of grazed stems of the less palatable karoo shrubs can be used to calculate optimum stocking rates and to monitor applied stocking rates.
Botanical surveys are conducted on the farms of a number of stockfarmers in the Karoo. These studies are , aimed at the refinement of the long-term grazing capacities for the various reasonably homogeneous farming areas of the Karoo. The survey data are currently being studied in order to identify the dominant plant species for these areas. It is the intention that the work reported on in this article be expanded to farms where measurements of the grazed-stem diameters of the dominant species can be supported by known past animal production and stocking rates. The database created in this way, can then be used to monitor applied stocking rates.
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ANON., 1993. Climate data. Unpublished research data. Institute for Soil, Climate and Water, Grootfontein.
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Karoo Agric nr 5 (1) p 17 -21