Bush Encroachment and the Role of Browse in Cattle Production : An ecological perspective from a bush encroached grazing system, Olifants Drift, Kgatleng District, Southeast Botswana

Sammanfattning: The increase in density and cover of woody plant species is coincident with the introduction of cattle in southern Africa, and it is a wide spread phenomenon in the savannas of the world. The causes are little understood, but can only be speculated to be either anthropogenic or natural or a combination of both. In southern Africa this process is termed bush encroachment, which is seen as land degradation and detrimental for cattle production. The context is almost entirely one of increases in the woody layer reducing the amount of graze available, and therefore having an adverse impact on cattle carrying capacity. Indeed, the increase in bush encroachment has major implications for assessments of the sustainability of cattle production and viable livestock management policies.In view of the above, this thesis was designed to investigate the cattle-woody vegetation interactions so as to help understand the significance of browse (woody vegetation) to cattle production in a traditional (communal) grazing system at Olifants Drift, Botswana, where management in the form of supplementary feeding is minimal. Included also within the objectives of this thesis, was the investigation and development of suitable techniques, combining remote sensing and conventional survey methods, for the estimation of green browse biomass over extensive bush encroached cattle pastures. The study area is communal land, which has a history of several centuries of cattle grazing and is recognised as being heavily bush encroached in Botswana. It also contains one of the highest cattle densities in the country.The results empirically confirmed that cattle density was partially responsible for the bush encroachment process. Despite this relationship, the bush encroaching woody plant species contributed significantly towards cattle diets. Contrary to the general view, browsing of green material was highest during the wet season and decreased with drier seasons. This implies that bush encroachment may not necessarily form part of range degradation and that the importance of grasses has been over-emphasised. Browse is nutritionally superior to grasses, a characteristic that is maintained through seasons. Grasses are only nutritious at the beginning of the growing season. The entirely negative connotations that are attached to bush encroached savanna, in terms of their potential to carry domestic stork (especially cattle), should therefore be redressed.The quantification of green browse biomass in this bush encroached grazing pasture for carrying capacity purposes can be achieved by the use of colour infrared photography and relationships involving the mid infrared bands (TM5 and TM7) from the Landsat Thematic Mapper. Potential green browse biomass from the spatially extensive 0.3-1.5 m browse strata in the Olifants Drift is best estimated using the linear regression equation: Browse biomass (0.3-1.5 m) = 163807.9-2946.6'(TM5-TM7).In conclusion, more research focusing upon the dynamics and importance of this much neglected food supply over different regions of southern Africa is required. And if possible the incorporation of some browse species into 'carrying capacity' estimates for some regions, like the Olifants Drift, should be undertaken for sustainable development purposes. High levels of tannins, toxins and other aromatic substances in woody plant species are considered some of the factors that hinder the utilisation of such species by cattle. But yet to be answered is the question why some woody plant species are still highly utilised despite the high levels of these substances. Consideration should be given to the use of the mid infrareds bands in developing more accurate assessments of vegetation cover and biomass in the applications of remote sensing data sets in semi-arid bush encroached savannas, over the conventional indexes such as the NDVI.