agricultural production guidelines
m
Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 4.8 1999
BIORESOURCE GROUP 18: MIXED THORNVELD
K G T Camp
KwaZulu-Natal Department of Agriculture
Vegetation
Structure and Compositions
Veld Benchmark
Animal Production
INTRODUCTION
The Mixed Thornveld (BRG 18) is located on the upper margin of the Tugela River Valley Bushveld and in an area on the hills to the north of Wasbank (refer to Figure 1 in Production Guideline 4.1 of the series). While the Mixed Thornveld is essentially a secondary veld invaded by Acacia spp, it is of great agricultural significance and requires separate treatment from the Dry Tall Grassveld.
VEGETATION STRUCTURE AND COMPOSITION
The dominant plant association was a Themeda-Hyparrhenia grassland with sparsely scattered Acacia sieberana trees. However, this grassland has changed in species composition and structure due to poor grazing and burning management which has resulted in the invasion of Acacia karroo and A. nilotica with extensive areas of bushland and bushland thicket.
The grass layer is dominated by Themeda triandra and Hyparrhenia hirta, the latter species being particularly dominant on disturbed areas such as old lands. Other species include Bothriochloa insculpta, Digitaria eriantha, Heteropogon contortus, Setaria sphacelata, Tristachya leucothrix and Elionurus muticus.
On disturbed areas Hyparrhenia hirta increases in relative abundance and the principal grasses in these areas are Eragrostis curvula, E. chloromelas, E. superba, Sporobolus pyramidalis and S. africanus. On severely disturbed areas with a sparse basal cover, grasses found include Cynodon dactylon, Aristida congesta subsp. barbicollis, A. congesta subsp. congesta and Melinis repens.
Woody species in the bushland and bushland thickets include Acacia and broad-leaf species such as Ziziphus mucronata, Rhus rehmanniana, R. pentheri, Ehretia rigida, Cussonia spicata and Acacia caffra.
At low elevations, in a zone transitional to Valley Bushveld, many bushveld species occur. Here, the woody species include Schotia brachypetala, Olea europaea subsp. africana, Euclea racemosa subsp. zuluensis, E. crispa, Acacia robusta, Premna mooiensis, Pappea capensis, Ozoroa paniculosa, Buddleja saligna, Acacia tortilis, Vitex rehmannii and Maytenus cymosus.
A significant change is apparent in the grass layer in the transitional area. The tall, yellow stemmed Themeda triandra shows a marked change in form from the generally blue-leafed variety of T. triandra at higher elevations in adjacent BRGs. Panicum maximum and Sporobolus fimbriatus are dominant under trees and supply much of the palatable fodder in this area. Other grass species include Bothriochloa insculpta and Setaria incrassata, particularly on the black clays, Chloris gayana, Eustachys paspaloides, Cymbopogon plurinodis and Aristida barbicollis.
Bush encroachment into grasslands is considered a major problem facing stock farmers in savanna areas and, at present, no satisfactory solution to the problem is available. In a plant association where trees and a grass layer compete with one another, any utilisation of the grasses which causes them to lose vigour will favour the establishment and growth of woody species (refer to production Guidelines 7.3, 7.4, 7.6 and 7.7 of this series for more details of this process). Fire, while not effectively controlling the growth or density of Acacia species once they have become established, does tend to suppress them, enabling the grass to compete for dominance, but the fuel load must be sufficient for a hot fire. This is seldom the case as very little veld is rested to accumulate a fuel load of sufficient magnitude. As a result the tree density increases and with the introduction of seed into the grasslands by animals, encroachment spreads. With high seed availability and very little change in management, indications are that the rate of spread of woody species into the Dry Tall Grassveld will continue with the ultimate loss of the grasslands of this area as we know them today.
Grassveld in good condition in this Bioresource Group can be expected to have a species composition similar to that presented in Table 1 (bearing in mind that these proportions are estimated, and could change as research provides more information on this vegetation type). Two benchmarks are given, one for upland areas close to the Dry Tall Grassveld and the second for low-lying areas which are transitional to Valley Bushveld vegetation. In addition, Table 1 presents other grass species which occur in this Bioresource Group, but are not necessarily found in the benchmark. The species groupings (i.e. Increaser and Decreaser species), and concepts related to this classification, are explained in detail in Production Guidelines 6.1 to 6.3 in this series.
As with other grassveld types in KwaZulu-Natal, the predominance of redgrass is indicative of veld in good condition. Abundances of other key grass species may be used in conjunction with redgrass to determine veld condition using the Weighted Key Species method (refer to Production Guideline 6.2 in this series for a description of this method). Key species and their weightings proposed foruse in this veld type are presented in Table 2.
Much of the Mixed Thornveld has good potential for animal production. Steers are able to gain 120 to 130 kg during the summer. Veld in moderate condition has a carrying capacity of 3.5 ha/AU. Where bush encroach-ment has occurred, the possibility of introducing browsers in the form of goats can raise the potential for animal production, using the grassland with grazing animals and the woody species with browsers. Management of a high quality is necessary to successfully apply this type of farming. There is a potential for game farming as indicated by the success of the introduction of wild animals into the Weenen Nature Reserve.
VELD MANAGEMENT NORMS AND GENERAL INFORMATION FOR BRG 18: MIXED THORNVELD
BRG 18
Total area (1 000 ha)
Area of veld (1 000 ha)
Potential grazing capacity (ha/AU)
Current grazing capacity (ha/AU)
Grazing season (days)
Grazing cycle (days)
Period of stay (days)
Period of absence (days)
AU grazing days per hectare
Expected intake (kg/AU/annum)
Livestock production potential (kg livemass per ha/annum)
Excellent condition veld (90%)
Moderate condition veld (60%)
Poor condition veld (30%)
212
187
2.1
3.5
300
63
21
42
119
3 000
49
33
16
Rested area
Each camp should be rested once every four years
Winter grazing
Licks are required when grazing rested veld in winter
Animal types
Cattle (high potential); Sheep (low potential); Goats (moderate potential)
Burning
Burn to remove moribund and excess herbage
1 In areas where stocking rates are higher than those given for the potential grazing capacity, the livestock production potential will not be achieved. This is particularly so where continuous grazing is practiced.
2 Current grazing capacity is based on the average condition of veld in the BRG as estimated in 1997.
3 Animal Unit (AU) grazing days per hectare are based on the estimated average CGC of the BRG.
4 Livestock production potential refers to the expected amount of live mass (kg) gain per hectare using a beef weaner production system (see the example of such a system in Production Guideline 2.6).
Table 1. Benchmark species abundances and species not occurring in the benchmark for grassland in the Mixed Thornveld (BRG 18). Two benchmarks are given, one for upland areas close to the Dry Tall Grassveld and the second for low-lying areas which are transitional to Valley Bushveld vegetation.
Scientific name
Common name
Relative abundance (%)
Upland
Transitional
Increaser Ia
Andropogon eucomis
Cymbopogon excavatus
Cymbopogon plurinodis
Digitaria tricholaenoides
Eulalia villosa
Setaria nigrirostris
Trachypogon spicatus
Tristachya leucothrix
Snowflake grass
Common turpentine grass
Bushveld turpentine grass
Purple finger grass
Golden velvetgrass
Large seed setaria
Giant speargrass
Tridentgrass
3
1
1
10
2
2
Decreaser
Bracharia serrata
Digitaria eriantha
Diheteropogon amplectens
Eustachys paspaloides
Melinis nerviglumis
Panicum maximum
Themeda triandra
Velvet signalgrass
Finger grass
Broadleaved bluestem
Fan grass
Bristleleaved redtop
Guinea grass
Redgrass, rooigras
1
1
2
61
2
1
2
70
Increaser IIa
Eragrostis capensis
Harpochloa falx
Heteropogon contortus
Heartseed lovegrass
Catterpillar grass
Speargrass
4
4
4
Increaser IIb
Bothriochloa insculpta
Digitaria monodactyla
Eragrostis chloromelas
Eragrostis curvula
Eragrostis plana
Eragrostis racemosa
Eragrostis superba
Hyparrhenia hirta
Sporobolus africanus
Pinhole grass
One finger grass
Weeping lovegrass
Fan lovegrass
Narrow-heart lovegrass
Sawtooth lovegrass
Common thatchgrass
Ratstail dropseed
1
1
2
3
2
1
4
1
1
Increaser IIc
Aristida congesta subsp. barbicollis
Cynodon dactylon
Microchloa caffra
Forbs
Sedges
Spreading bristlegrass
Couch grass
Pincushion grass
1
1
2
1
4
2
Increaser III
Elionurus muticus
Wire lemongrass
1
1
Table 2. Proposed key grass species and species weightings for grassland in the Mixed Thornveld (BRG 18).
Scientific name
Common name
Species weighting
Cynodon dactylon
Hyparrhenia hirta
Eragrostis chloromelas
Eragrostis curvula
Heteropogon contortus
Themeda triandra
Couch grass
Common thatchgrass
Weeping lovegrass
Speargrass
Redgrass, rooigras
1
2
3
4
6
10