Last update: November 17, 2010 01:52:18 PM E-mail Print

 

Reclaiming degraded veld in the Kalahari

 

PCV du Toit

Grootfontein Agricultural Development Institute

Private Bag X 529,

MIDDELBURG.

5900

 


Introduction

Large parts of the northern Cape, from Britstown in the south to Mier in the north and, south-eastern Namibia and Botswana are encroached by Rhigozum trichotomum. This shrub contributes very little to the forage bank and also dries out the soil to the extent that very little herbaceous vegetation, which can be grazed, is able to survive in its immediate vicinity.

The work reported on here was carried out on a farm scale on the farm Gnous in the Gordonia district of the Kalahari Thornveld (Acocks 1988). Herbicide treatments commenced in 1999. Although large areas of the farm have been encroached by Rhigozum, the system has not suffered the severe degradation that can be seen elsewhere in Gordonia and the veld was still in a position to recover. It soon became obvious that reclamation work would have positive effects. Extensive botanical research work therefore started in 2000.

 

Method

The Rhigozum trichotomum shrub is easily controlled, although the control measure is relatively expensive. The registered herbicide for use on this shrub is Molopo 200 GG (Grobler et al. 2000)(previously known under the trade names grazer and graslan). The encroached Rhigozum trichotomum site is treated with a rig towed by a small tractor. The herbicide is spread at the rate of between 300 and 360 g active tebuthiuron per hectare (Moore et al. 1985) and, depending on the rainfall directly afterwards, is soon taken up by the shrub. Depending on the size of the pellets, between 10000 and 30000 pellets are distributed per hectare. Total weight of these pellets applied per hectare is between 1.5 and 1.8 kg. The rig with which the herbicide is distributed treats a swath of 5 metres at a go, placing the pellets at one metre intervals and one metre apart. There are 10000 placements per hectare. The present cost of treatment ranges from R97.00 to R117.00 per hectare. In the absence of rain immediately following application, the herbicide stays active in the soil.

It is very important to not only treat the encroaching bush, but to couple this to a favourable veld management system immediately, based, as in this case, on an indicator species, Centropodia glauca the well known Kalahari-Gha grass. The treated areas were protected from grazing for a period of two years. This follow-up treatment is imperative to effectively control Rhigozum trichotomum in the Kalahari and to be in a position to recoup moneys laid out during the treatment as soon as possible, without degrading the resource again. The effects of these treatments can be clearly seen. Favourable results are obtained when Centropodia glauca is used as an indicator of the intensity of grazing, for instance, this grass was grazed down to a height of approximately 100 mm during the previous round of grazing and, after a rainfall event of only 10 mm, grew out by more than 100 mm within 10 days after the rainfall event.

Line-point and 10 square metre quadrat botanical surveys of 500 points per line were carried out since 2000 (Du Toit 1998a; Du Toit 1998b). Line-point surveys were carried out to determine the botanical species composition and percentage cover and this data were used to compute the current grazing capacities making use of the species’ grazing index values (Du Toit et al. 1995). Quadrat surveys indicated the number of plants per hectare, on the one hand the number of Rhigozum trichotomum shrubs per hectare which encroached the area and on the other hand giving an indication of the aboveground available phytomass produced per hectare contributing to the forage available to grazers.

 

Results

Observations on deep Kalahari sand

The leaves of the shrubs turned yellow almost immediately after the rainfall event, 336 mm of rain was received during 1999/2000 and grasses started to grow around the dying shrubs immediately after the shrubs’ excessive water use were curtailed. Bear in mind that a rainfall event of 25 mm, will wet the soil to a depth of 25 cm, this translates to a total water supply of 250 cubic metres of water per hectare. A Rhigozum trichotomum stand of between 3000 to 9000 plants per hectare, will pump this quantity of water out of the soil within 14 days. Coupled to this tremendous suction ability is an extensive "secondary" shallow root system, which dries out the surface soil, making it impossible for herbaceous vegetation to establish.

A curious phenomenon on the Kalahari sands, is that a definite pioneer phase during the recovery period is missing during the seral regenerating development of the vegetation and the vegetation explodes with an increase in higher seral stage sub-climax grass species, e.g. Eragrostis lehmanniana, as well as the climax grass species Stipagrostis ciliata and S. obtusa. Based on the observations, it is obvious that enough seed of the desirable species were still available in the system.

On the encroached site, the forage available to the grazing animal is made up of single tufts of either Stipagrostis ciliata or Eragrostis lehmanniana which manage to survive the harsh, dry conditions in the system. Total dry matter production on the encroached site measures only 36 kg/ha. Rhigozum trichotomum does not provide much by way of available forage, only between 70 to 100 kg of soft twigs is produced per hectare, a little extra forage is produced during the flowering and fruiting season, which is, however, very short. Two years after treatment, a mean of 2847.5 kg of dry matter was produced per hectare while a mean of 3308 kg of dry matter is produced on good veld.

Within two years after treatment of the shrubs on deep sand, the grazing capacity of the veld improved eightfold, from around 260 ha/LSU in encroached areas to 34 ha/LSU in the area treated two years earlier. This figure, however, is still some 50% lower than the long term grazing capacity of veld which is in a good condition in the area, of which the grazing capacity norm is 22 ha/LSU to 23 ha/LSU determined from botanical surveys.

 

Observations on shallow Kalahari sand overlying calcrete

On shallow sand overlying a thick layer of calcrete, the grazing capacity improved as follows: in the year following treatment the grazing capacity is 54.6 ha/LSU; two years after treatment it is 25.6 ha/LSU and three years after treatment it is 22.4 ha/LSU. But once again, as on the deep sand a pioneer stage was missed and there was a tremendous explosion of higher seral stage sub-climax and climax grasses as soon as the Rhigozum was killed and its excessive water use curtailed.

 

Succession after herbicidal treatment

The complete absence of a definite pioneer stage during the recovery period has already been mentioned, similar phenomena have been described for the area between the Soutpansberg and the Limpopo river (Snyman 1989) and, at Kirkwood in the Eastern Cape (Furstenburg undated). However, in the Gordonia district, the pioneer grass Schmidtia kalihariensis, completely covers the bare areas in between the Rhigozum shrubs during very good or above normal rainfall years, before the shrubs have been killed. Sufficient water is then available in the soil to sustain the growth of both the Schmidtia kalihariensis as well as the Rhigozum. At this stage the Schmidtia simply contributes to the green drought in the area. Schmidtia kalihariensis is a very unpalatable grass on account of its high acid content and during years when it is abundant, does not appreciably contribute to the grazing capacity. Schmidtia kalihariensis is also visible during the recovery period, although in very small numbers and do not seem to play a large role in the build-up of soil nutrients, however, after the above normal rainfall experienced on the farm during the 1999/2000 season when 336 mm was measured, the veld was covered in dense stands of Schmidtia kalihariensis. This cover put down nutrients and formed a light mat under which the seedlings of desirable species could develop. This condition does not normally obtain, in another studied system, the Schmidtia kalihariensis put down such a thick carpet of mulch after a seasonal rainfall of over 500 mm, that the seedlings of desirable species smothered. The long term annual rainfall in this area of the Gordonia district is only 145 mm.

The fact that this system bounced back to an acceptable climax vegetation, is indicative of the nutrient and water cycles being in place to sustain the dense cover of grasses in the absence of the Rhigozum. Sufficient water is now available for seedling development and a high forage production of established grasses.

 

Economic evaluation

The average cost of treatment per hectare is R107.00. The veld prior to treatment can carry about a sheep to 5 hectares. The cost of treating 5 hectares is R535.00. This treated veld is rested for two years to aid recovery. A slaughter lamb carcass taken off this veld after two years of grazing, weighs 16.5 kg on average. From the fourth year after treatment onwards, there will be a steady stream of lambs to be slaughtered every year. The average current price per kg lamb is R19.00. The carcass therefore, sells for about R313.50. Over two years R627.00 is realised. A clear profit of R18.40 per hectare can be realised in the fourth year.

 

Conclusion

The cost of treatment can be written off within two years following normal production. This is not a bad business proposition taking into account that with good veld management, the veld should remain clear from re-infestation by anything up to 10 years. This is one aspect for which no hard data exist at present, the period could be much longer. However, in areas where there are dense grass stands with a known history of up to fifty years in places, the competition afforded the Rhigozum is so high that no seedlings of Rhigozum could become established. With further improvement in the veld, in the species composition and in the grazing capacity following sound veld management, a sheep can be run on three hectares, which means an annual income of R104.50 per hectare. This income should then more than pay for follow-up treatment or further eradication.

 

Literature references

Acocks, J.P.H. 1988. Veld Types of South Africa. Memoirs of the Botanical Survey of South Africa, 57:1-146.

Du Toit, P.C.V., Botha, W. van D., Blom, C.D., Becker, H.R., Olivier, D.J., Meyer, E.M. & Barnard, G.Z.J. 1995. Estimating grazing index values for Karoo plants. Technical Communication 239:1-15.

Du Toit, P.C.V. 1998a. Description of a method for assessing veld condition in the Karoo. African Journal of Range and Forage Science 14(3):90-93.

Du Toit, P.C.V. 1998b. Grazing index method procedures of vegetation surveys. African Journal of Range and Forage Science 14(3):107-110.

Grobler Hermien, Vermeulen, J.B. & Van Zyl, Kathy 2000. A guide to the use of herbicides 2000(17):1-154.

Moore, A., Van Niekerk, J.P. & Knight, W. 1985. The effect of tebuthiuron on the vegetation of the thorn bushveld of the Northern Cape - a preliminary report. Journal of the Grassland Society of southern Africa 2(4):7-10.

 

Unpublished dissertations

Furstenburg, D. Undated. The influence of environmental and animal factors in sustaining production in semi-arid vegetation. Ph.D. thesis still to be submitted to the University of Port Elizabeth.

Snyman, D.D. 1989. Verwantskap tussen veldtoestand, reënval en dierebelading in die mopanieveld. Unpublished MSc Agric thesis, submitted to the University of Pretoria.

 

Fig. 1. This is a view of an encroached site with Rhigozum trichotomum. In the background, Stipagrostis amabilis and Stipagrostis ciliata can be seen. Note the covering of dead and decaying Schmidtia kalihariensis on the soil surface in between the shrubs

 

 

Fig. 2. This figure provides a view of a Rhigozum trichotomum site treated two years previously, with the grasses Eragrostis lehmanniana, Stipagrostis ciliata and Stipagrostis obtusa abundant

 

 

Fig. 3. This figure is a view of what good veld in the area should look like. Centropodia glauca, Stipagrostis ciliata, Stipagrostis obtusa, Eragrostis lehmanniana and Aristida meridionalis are abundant

 

 

Fig. 4. This figure gives an idea of the bare ground surrounding the Rhigozum trichotomum shrub

 

 

Fig. 5. This figure shows the rig with which the herbicide is distributed

 

 

 

Fig. 6. The canopy spread cover of the vegetation on the encroached site (1), on the site treated two years previously (2), veld in a good condition (3)

 

 

 

Fig. 7. The number of plants counted per hectare on the three sites. Sites 1 to 3 as previously

 

 

 

 

Fig. 8. The forage available to the grazing animal, total production was measured, forage available to the grazer will be roughly 50% of the total. Sites 1 to 3 as previously

 

 

 

Fig. 9. The canopy spread cover plus the number of plants per hectare plus the available forage, determines the grazing capacity per hectare. Sites 1 to 3 as previously

 

 

Published

Karoo Agric 5 (1)