agricultural production guidelines
m
Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 7.7 1999
BUSH CONTROL
C R Hurt
ARC - Range and Forage Institute, Cedara
Approaches
to Bush Control
Fire
Chemicals
Mechanical Methods
Browsers
Bush Control Management
Programmes
INTRODUCTION
Previous Production Guidelines in this series have examined the relation between the herbaceous and woody components of savanna ecosystems (refer to Production Guidelines 7.2 to 7.6 in this series). It is widely accepted that when bush densities become high enough to effectively depress the growth of grasses, the land manager is faced with a major problem. From an animal production point of view, these areas can generally be considered as a loss to the system until some form of bush control is implemented and grass production is initiated once again. This Production Guideline describes advantages and disadvantages of various methods of implementing a bush control programme on a farm. In addition, it provides recommendations on how to maintain an open savanna situation thereafter. It must be appreciated, however, that veld which is inherently a savanna is unlikely to be successfully transformed into a long-term, stable grassveld. The conditions here are likely to be so favourable for the growth of trees that they will continue to invade, irrespective of the management applied. However, management can assist in the maintenance of an open savanna. In contrast, areas which are inherently open grassveld, but into which trees have invaded because of poor management, may once again be converted into stable open grassveld by appropriate management, provided the soils of the area have not been too badly degraded. It is important that the land manager accept the fact that woody plants are an integral part of savanna communities, and that the land should be managed accordingly.
Various options are available to the land manager for the control of encroaching woody species. These include the use of fire, chemicals, mechanical methods and browsers, and are discussed individually below.
Bush control methods can be divided into two broad approaches, which should be implemented as circumstances dictate.
Woody plants are best eradicated on a selective basis so that desirable species (as defined by the land management objectives) may be encouraged to grow at the expense of less desirable or problem (invasive) species. This approach to bush control, however, can be labour intensive, because plants must be treated individually, and training of the labour force is required in order to recognize undesirable species. Selective methods that may be used to remove woody plants include manual cutting, frilling or ring barking of individual plants, the application of herbicides to individual plants, or a combination of treatments (e.g. manual cutting followed by herbicide treatment to the remaining stump).Selective bush control
Selective control does not refer to the application of soil applied herbicides to individual plants. This form of treatment, in most cases, is not selective because the herbicide is taken up by roots of non-target trees which co-exist with roots of the treated individuals.
Radical bush control
In cases where bush densities are so high that access (i.e. how to get in to kill trees) is virtually impossible, radical control methods may be required. These methods are normally totally non-selective and usually eliminate most woody species present, or at least alter the structure of the community. Examples include the use of bulldozers to physically push the bush out, the use of aerially-applied herbicides, or the adoption of a cut-and-burn programme. These treatments are expensive, and seldom provide a long-term solution to the problem. In certain cases where bush densities are extremely high, however, they may be advocated as the start of an ongoing bush control programme.
Fire is considered to be a natural component of a savanna ecosystem, and most savanna plants have co-evolved with, and thus are adapted to withstand, fire (refer to Production Guidelines 7.5 and 7.6 in this series). Fire is used to manipulate the bush canopy rather than to actually kill individual plants.
Fire per se is a cheap method of controlling woody plants when compared to other methods (e.g. chemicals, mechanical). In addition, the vigour of the herbaceous layer is maintained at the same time, through the removal of moribund grass.Advantages of fire
Disadvantages of fire
The greatest disadvantage of fire in savanna is the
loss of production caused by, and following, a burn. Most savannas are sweetveld
(i.e. the herbaceous layer retains its acceptability and nutritive value after
the plants mature), and thus it can be utilised by animals throughout the year.
Burning this grass material in order to control woody plants therefore
constitutes a waste of forage. In addition, the unreliable nature of the
rainfall presents a risk with regard to the availability of forage during the
spring and early summer following a burn.
Various commercial herbicides and arboricides are available for controlling woody vegetation, and can be divided into selective and non-selective herbicides. Selective herbicides are applied to individuals by either painting the stem base or cut stump, and thus short-term application costs are likely to be high. Non-selective herbicides are applied to the soil in a pellet or granule form, and thus application costs can be relatively lower.
Herbicides have a distinct advantage in that treated individuals are usually killed, and coppicing does not become a problem. Non-selective herbicides can be effectively used as the initial treatment in cases where bush densities are extremely high. Follow-up treatments (herbicide, fire, or mechanical) are, however, essential.Advantages of chemicals
Disadvantages of chemicals
A number of disadvantages are associated with chemical
control methods. Firstly, most herbicides are harmful to humans, animals and the
environment, and many pollute the ground water, and kill trees which were not
earmarked for control. Although certain herbicides are relatively safe if
applied properly, the lack of labour training makes them undesirable. Secondly,
where herbicides are used in a radical bush control approach, low success is
obtained in the long term. Although the herbicides have a relatively long
residual action (i.e. they remain active for a number of years), once they lose
their affectivity, the bush usually recovers quickly. In addition, the high clay
content of the soils of most savanna areas of KwaZulu-Natal causes a decrease in
the affectivity of the herbicides. This necessitates an increase in the
application rates, which, in turn, increases the hazards associated with their
use as well as their application costs.
Relatively little research has been conducted on the effects of herbicides on different woody species in KwaZulu-Natal, and most current recommendations are based on research undertaken in other areas. It might not be safe to extrapolate such research findings to different areas. ‘Selective’ control of individual plants by applying soil-based or pellet herbicides to the base of the stem is not always the answer. Surface roots of trees can extend over large areas (up to approximately half a hectare in the case of large trees), and thus the herbicide can be absorbed by plants which are some distance from the treated individual, and which had not been selected for treatment.
Methods described under this group include the use of machinery (e.g. bulldozers) to remove woody vegetation on a non-selective basis, or the use of brush cutters, axes and bush knives to selectively remove individual plants.
Non-selective mechanical methods have an advantage in situations where high bush densities restrict access. It is absolutely necessary to provide follow-up treatments after the initial clearing. Selective thinning has the advantage that individual species or plants can be removed from the system, and others allowed to remain. Neighbouring individuals are not adversely ed by the treatment, as is the case in certain herbicides (see previous paragraph). The disposal of cut branches and tree trunks can be incorporated into the overall management programme (e.g. sold as firewood or charcoal, or used for brushpacking in eroded areas).Advantages of mechanical methods
Disadvantages of mechanical methods
A major drawback to this method is that the removed
plants are almost immediately replaced by new seedlings and coppice from the
bases and exposed roots of the old plants, necessitating continual follow-up
treatments. The disturbed environment (particularly after bulldozing) is also
ideal for the growth of a variety of undesirable plants and alien invasive
species such as triffid weed (Chromolaena odorata) or tickberry (Lantana
camara). Soil disturbance also creates a temporary unstable environment, and
erosion can be high. The costs involved in mechanical clearing are also high,
particularly in the case of selective removal of plants, which is labour
intensive. A further problem which might be associated with this method is the
disposal of cut material. Very often the treated plants are too small to be used
for firewood or charcoal, and they can pose an access problem or may harm
animals, particularly where the plants have spines such as with sickle bush (Dichrostachys
cinerea) and certain acacias. If the cut material is stacked and burnt, the
sterilized soil patches which result are ideal places for re-invasion by woody
plants or infestation by alien species.
Browsing animals (e.g. goats, eland, kudu, bushbuck, black rhino) and mixed feeders (e.g. elephants, impala and nyala) are an effective natural control of woody vegetation. Goats, in particular, have been used successfully elsewhere in South Africa to control the bush layer. Unfortunately, browsing species of game can seldom be used alone to clear bush because social behaviour of the animals precludes high browsing pressure. They may, however, assist in maintaining savanna communities in a fairly open condition. Furthermore, browsers can be successfully used in conjunction with other bush control methods such as fire or mechanical control, as they are able to utilise the coppice growth associated with these practices.
The obvious advantage of including browsers in the system is that profitability of the enterprise is increased. This is a result of two factors. First, the fact that the browsing animals are able to maintain open communities (and thus high grass production), means that less money needs to be spent on other methods of bush control (e.g. herbicides, mechanical methods). Second, a browser system is managed independently of, and superimposed on, a grazing system. This means that total red meat production per hectare is increased.Advantages of browsers
Disadvantages of browsers
Indigenous browsers are usually not able to control
bush encroachment alone, for the reasons indicated above, although they have a
significant effect on bush structure, particularly in shrublands. Disadvantages
associated with goats are mainly management orientated. Most farmers in the
savanna regions are engaged in large-stock enterprises, and they do not have the
infrastructure necessary for small stock such as goats. Further infrastructural
developments are costly. Stock theft is a major problem associated with goats,
and, in addition, many farmers in KwaZulu-Natal are averse to goat enterprises.
A further problem associated with browsers is that they tend to be selective
feeders, and very often certain woody plants are controlled, while the
unpalatable species are not affected. A good example here is lemoendoring (Maytenus
senegalensis) in the Lowveld. Continued browsing of palatable species by
goats might cause such undesirable species to predominate. The winter period in
deciduous savanna also poses a fodder-flow problem. When the trees lose their
leaves, goats are forced to include more grass in their diet, and thus they
begin to compete with cattle for forage. Disease problems are also associated
particularly with boergoats, and facilities for regular dipping are a necessity.
BUSH CONTROL MANAGEMENT PROGRAMMES
It is obvious that the various methods used to control bush described above have certain merits as well as disadvantages. What is important when initiating a bush control programme, however, is that no single method is likely to work alone in farming situations. An integrated approach needs to be adopted.
It is therefore ultimately up to the farmer to decide which combination of methods to follow in controlling the bush on the farm. It should be remembered, however, that most methods which offer a short-term solution will often turn out to be expensive in the long term. On the other hand, methods based on ecological principles will provide a long-term solution to the problem, provided that the farmer is prepared to persevere, and wait for the results, and to ensure that the applied land-use objectives are realistic.
It is clear that no precise recommendations can be made for any particular system, but rather that each system should be assessed in terms of the management objectives, the existing vegetation and the climatic conditions. An integration of bush control methods is the only viable long-term solution to the problem.