I.F. Beale
Introduction
Climate
Soil Characteristics
Vegetation
Ecology
Germination and Establishment
Range Condition
Management
Management of the Whole Property as an Enterprise
References
The mulga lands of southwest Queensland are dominated by the woody species Acacia aneura, and have been grazed by domestic animals since about the 1860s. In this time, grazing has gone from a very extensive system with animals dependent on natural waters to relatively well fenced properties that are well supplied with permanent waterpoints. Grazing is now non-migratory with stock spending their entire year within the property boundary. This section deals with some points of management that have been highlighted by experience.
The climate of southwest Queensland is semiarid to arid. Its features are:
· predominantly summer (October to March) rainfall, with an increasing winter component towards the south,
· high summer temperatures,
· increasing frost incidence towards the south,
· high rainfall variability,
· high evaporation rates, and
· frequent drought incidence.
Rainfall and evaporation
Average annual rainfall decreases from 500 mm in the east to less than 180 mm in the west. Rainfall also decreases in a southerly direction, with Blackall receiving 528 mm annually and Cunnamulla 366 mm.
Rainfall is generally, although not always, summer dominant. Evaporation is high and ranges from 2,100 to 3,000 mm annually - four to five times the annual rainfall. Average monthly evaporations vary from 280 mm in December to 75 mm in July (all from a free water surface).
Temperature
Mean temperatures increase with a southeast to northwest trend. The hottest months are January and December with mean maximum and minimum temperatures at Charleville of over 35°C and 21°C respectively. In the coldest month, July, the equivalent temperatures are 19°C and 5°C respectively.
Temperature extremes recorded in the area range from 46°C to -6°C. The bulk of pasture growth occurs in the summer months provided moisture is available. High temperatures affect the eating, drinking and resting behaviour of stock, even when good forage is available. Animal production can therefore fall in excessively hot weather.
Extremely low temperatures limit pasture growth and can also adversely affect animal performance, for example, increased lamb mortality in open areas.
Rainfall effectiveness, plant growth and drought
Rainfall is well below evaporative demand, and moisture deficit is common. Soil moisture is generally inadequate for reliable cropping. This confines rural industry mainly to the grazing of sheep and cattle.
During rainfall, some water enters the soil and some is lost as runoff. If sufficient moisture is retained, vegetative growth results. Under the high evaporative demand the soil dries rapidly and plant growth ceases without further rain. Hence plants in the region are characterised by relatively short periods of growth separated by longer periods of inactivity. Extended periods of low soil moisture and a low pasture growth lead to feed shortages. Droughts are a regular feature of the region, not freak events.
Soil type
The soils are characteristically infertile red earths (Gn 2. 11, Gn 2.12) sandy loams to clay loams. Major features of the soils are severe deficiency of available phosphorus and nitrogen, high levels of iron, manganese and aluminium, an acid profile and a densely packed surface layer (bulk density > 1.4). Even in good conditions, these soils have nutrient levels that are low (Table 7.1.1). Topography is gently undulating, with isolated residuals.
Soil/water considerations and soil erosion
Due to the high soil bulk density (Table 7.1.1), infiltration of water is slow in mulga areas. With rainfall already low, and growth usually limited by moisture availability, loss by runoff exacerbates unfavourable conditions for plant establishment and growth. The amount of runoff and the requirement for ground cover have been addressed by Pressland and Lehane (1980, 1982) and Miles (1990a). As an example, a site with a ground cover of about 35% lost about 80% of the water resulting from a storm. A neighbouring site with 70% cover lost about 15%. Unfortunately, the ready availability of water for stock and the use of mulga as a supplement for livestock allows animals to be maintained on mulga country even in drought, and can result in values of ground cover of much less than 35%.
Table 7.1.1. Properties of a mulga soil from the Charleville Experimental Reserve.
|
Depth |
Bulk density |
pH |
Total P |
Avail. P |
Total N |
|
0-25 |
1.8 |
5.5 |
263 |
8 |
0.05 |
|
2.5-5.0 |
1.6 |
5.4 |
230 |
4 |
0.06 |
|
5.0-10 |
1.6 |
5.5 |
228 |
3 |
0.04 |
|
10-25 |
1.6 |
5.0 |
223 |
3 |
0.04 |
|
25-50 |
1.6 |
4.9 |
171 |
2 |
0.04 |
|
50-100 |
1.6 |
5.2 |
168 |
2 |
0.02 |
|
Depth
|
Exchange cations |
Org. C
|
C:N
|
|||
|
Ca |
Mg |
Na |
K |
|||
|
0-2.5 |
1.7 |
0.3 |
0.05 |
0.36 |
1.3 |
26 |
|
2.5-5.0 |
0.9 |
0.1 |
0.05 |
0.29 |
1.0 |
17 |
|
5.0-10 |
1.6 |
0.2 |
0.05 |
0.31 |
0.9 |
22 |
|
10-25 |
1.6 |
0.2 |
0.05 |
0.29 |
0.8 |
20 |
|
25-50 |
0.9 |
0.1 |
0.05 |
0.19 |
0.5 |
12 |
|
50-100 |
1.2 |
0.2 |
0.05 |
0.16 |
0.4 |
20 |
Nutrients in mulga soils occur in the top 10 cm of soil, with little nutrient cycling to lower soil depths. Loss of this small amount of top soil reduces plant growth potential to about half (Pressland 1985, Pressland and Cowan 1987, Miles 1990b). Loss of ground cover allows accelerated loss of this soil layer.
In contrast, there is a build-up of plant nutrients under the canopy area of some deep-rooted trees (particularly Eucalyptus spp.) which can aid establishment and production of herbaceous species (Ebersohn and Lucas 1965, Christie 1975c).
Native species
Mulga (Acacia aneura) occurs over a wide geographical range in Australia (Nix and Austin 1973) (Figure 7.1.1). In this range, the area around Charleville achieves some of the highest plant densities and yields of mulga. The undisturbed area at the Charleville Experimental Reserve has a density of about 7,500 stems per hectare. Mulga is a surface rooted species. It appears that fire previously kept mulga density low so that plant communities appeared as an open savannah.
There are other trees associated with mulga, for example, poplar box (Eucalyptus populnea) with densities of about 100 per hectare. This is a deep-rooted species. Often there is also a woody shrub population with species such as green turkeybush (Eremophila gilesii), grey turkeybush (E. bowmanii) and false sandalwood (E. mitchellii) common.
The herbaceous ground layer is comparatively poor and is dominated by shallow-rooted species. A range of perennial and annual species may be present depending on season, woody plant density and amount of grazing. While forte species are common after cool season rain, leguminous forte species are rare.
Some of the native grasses are very useful for grazing, for example, mulga Mitchell (Thyridolepis mitchelliana) and mulga oats (Monacather paradoxa). Others (Aristida spp.) provide ground cover but are undesirable from an animal production perspective. In this case, their sharp awned seed heads cause vegetable fault in wool and pelts, reduce income, and can cause problems with animal health. Some species are known to be poisonous to livestock, for example, wild parsnip (Trachymene spp.), Solanum spp. and Euphorbia spp. Others that occur on more fertile drainage lines include fuschias (Eremophila spp.) and ellangowan poison bush (Myoporum desert)).
The potential use of mulga in other regions of the world requires consideration of environmental factors influencing the growth of these species. These include an infertile acid soil, a seasonal rainfall pattern and hot summers with frosts in winter. Similar homoclimates are rare worldwide (Meigs 1955).
Introduced species
Introduced species have not been successful in the mulga region. Extensive testing of introductions at the Charleville Pastoral Laboratory (more than 500 accessions) has not produced species superior to the buffer grass (Cenchrus ciliaris) cultivars previously introduced. The spread of buffer grass in mulga country is limited by soil phosphorus levels (Christie 1975a,b, Christie and Moorby 1975, Silcock et al. 1976) and other factors including slowness to tiller and a long juvenile period before flowering (Silcock and Whalley 1974, Silcock and Williams 1976). Establishment can be enhanced by the use of phosphorus pelleting of the seed (Silcock and Smith 1982), but good management is required to maintain a viable pasture.
Fig. 7.1.1. Distribution of mulga (Acacia aneura) in Australia. (Nix and Austin 1973).
Introduced weed species (both woody and others) are also of some concern, for example, mesquite (Prosopis spp.) at Quilpie and McKinlay and prickly acacia (Acacia nilotica) in the northwestern Mitchell grass region (Section 7.2).
The poor results with introduced pasture species has led to a focus of effort on management of native pasture species. This includes research on establishing a native seed industry, commencing with mulga Mitchell (Thyridolepis mitchelliana) and mulga oats (Monacather paradoxa).
In general as the amount of woody species increases as measured by canopy cover or basal area, the amount of pasture produced decreases (Figure 7.1.2) (Beale 1973, Walker et al. 1972). The same is true of woody shrubs (Carter and Johnston 1986). In this region, grass growth is favoured by summer rains whereas woody species are favoured by winter rains. Thus there is a need in the management of mulga scrubs to balance the production of ground storey pasture against the need for reserves of mulga for drought fodder.
Fig. 7.1.2. The relationship between tree density and herbage yield in mulga and poplar box wood lands.
It has been calculated that, for a mulga property of 12,000 ha, with 710 mulga trees per hectare and 5,000 sheep, that 7,000 ha of mulga could be cleared for ground storey production without depleting drought reserves (Pressland 1975).
Woody shrub species (green and grey turkeybush, hopbush, Cassia) are generally unpalatable. Their increase reduces pasture production without making a meaningful contribution to animal production. There is a need to control these species as a pattern of increase has been observed (Burrows et al. 1985).
Management of the region is also complicated by vegetation responses to climate and management as outlined by Burrows (1980). This is similar to the states and transitions of rangeland vegetation change proposed by Westoby et al. (1989). Miles (1990b) has outlined a likely grazing gradient for mulga (Figure 7.1.3). Some of the transitions are unlikely to reverse without assistance and cost.
Work is in progress to improve management on a property scale by using rainfall use efficiencies for the various land system areas to estimate potential plant growth and thus carrying capacity.
Some germination of pasture species usually occurs after effective rainfall (20 mm in summer, 10 mm in winter). Germination is more prolific in spring and autumn months with midsummer and midwinter being unfavourable. Microhabitats favour seed germination, but these areas may change or migrate with time, for example, accumulations of wind blown sand or surface washed litter.
Generally seed viability increases with seed age up to 3 years, with the effect being more pronounced in grasses. Dormancy is evident in fresh seed but largely disappears after 1 year (Silcock and Williams 1975a,b, Silcock and Smith 1990).
Much of the mulga lands is in a degraded state as shown by Mills et al. (1989). In a survey of some 70 properties (3 million hectares) they found:
· unpalatable woody shrubs on 51 properties (about 132,000 ha),
· potential woody shrub problems on another 17 properties (about 630,000 ha),
· 56 of the properties (79% of the area) had less than the recommended mulga density of 160 trees/ha,
· 54 properties had more than 60% of the soil surface exposed (64% of the area),
· erosion affected 32 properties substantially, was minor on 11 and was negligible on 32,
· perennial grass cover was comparatively low (less than 6% canopy cover) on 30 properties, and
· pasture biomass levels were less than 100 kg/ha on about 80% of the area, indicating low regional productivity.
Pasture utilisation levels
Seasonal pasture production has been used as a basis for grazing management. It has been found that stocking rates set to use about 20% of the end of summer growth over the next year tended to maximise production per animal and to minimise fluctuations in animal numbers (and hence stock trading) and the effects of drought, and thus were financially preferred.
In mulga, in the short term, grazing needs to be managed to maintain ground cover to reduce rainfall runoff and soil erosion. Stock management (particularly in drought with mulga feeding) can work against this. In the longer term, effects of animal behaviour (for example, patch grazing) can change vegetation composition over time.
Control of weeds
While most of the problem species can be controlled relatively easily with chemicals, the cost of this is generally prohibitive. Thus chemical control is generally restricted to special problems and small areas. As property values in the mulga region are low, control methods must be low cost.
The control method that has the most potential is fire, but graziers are generally reluctant to use it. Species vary in their succeptibility to fire. Generally woody species are easier to control as seedlings. As woody species increase in size, there are losses in pasture production and potential fuel load. Thus chances of a control burn decrease. The effect is magnified by heavy stocking rates (Carter and Johnston 1986) (Table 7.1.2).
Table 7.1.2. Percentage of years with total standing dry matter greater than 1000 kg/ha (1889-1984) as affected by canopy cover and utilisation.
|
% Canopy cover
|
Pasture utilisation |
Pasture utilisation |
|
20% |
40% |
|
|
0.0 |
63.8 |
36.2 |
|
7.5 |
36.2 |
14.9 |
|
10.0 |
12.8 |
2.1 |
|
20.0 |
0.0 |
0.0 |
|
50.0 |
0.0 |
0.0 |
Stocking rate
The effects of increasing stocking rate on animal production per head and per area were outlined by Jones and Sandland (1974). Production per area rises to a peak value and then falls. In practice, the best financial returns are at a stocking rate below that at which maximum production is obtained. Indications are that, in semiarid and arid conditions, conservative stocking rates minimise climatic risk and can be financially viable in the long term (Beale et al. 1986, Morrissey and O'Connor 1988).
Stock management in the mulga lands is traditionally by continuous grazing, with animal performance the main criterion of management. Rates have in general been derived by traditional wisdom. While these may be reasonable in concept, there are two problems:
· while the rates are initially reasonable, there may well be changes in vegetation induced by effects of grazing, and· where the resource is in decline, these rates may not be adjusted to match the rate of decline.
It can be argued that both of these problems are evident in the mulga lands. Management to ensure that stocking rates are based on the feed available rather than on unit area, is a start to overcoming these problems.
Grazing time
While continuous grazing can be detrimental in the long term, there also appears to be little place for more formalised rotational systems (Bryant et al. 1989). However, strategic management such as spelling pastures to allow seed set to allow recovery from grazing particularly after drought or to allow fuel build-up for burning will be necessary in mulga country. This is an evolving area of research and management, and will be influenced by work such as that of O'Reagain and Turner (1992) who concluded that:
· stocking rate has a major impact on range condition and animal production,· continuous and rotational grazing differ little in effect on range condition or animal production,
· sheep have a greater potential for range degradation than cattle or goats though this effect can be reduced by stocking in mixtures with cattle,
· separation of vegetation types is important, and
· regular rests for seeding, regrowth and fodder accumulation, are essential.
Effect of stock supplementation
Supplementation with mulga feeding is examined in Section 4.6. Supplementation can increase animal productivity so fewer stock need to be mainlined, but it can also cause stock to be maintained on an area for longer, and promote loss of ground cover. This can lead to increased runoff and soil erosion and long term loss of productivity.
Kangaroos, native and feral animals
Kangaroo numbers have increased with the development of permanent water points for domestic livestock. They now compete with domestic livestock, and are estimated to cost the wool industry up to $A200 million in lost production annually. The effects of the grazing kangaroos can also contribute to land degradation (Miles 1989) (Table 7.1.3).
Mixed animal grazing
Holmes (1986) found that graziers in some parts of the mulga zone run both sheep and cattle. It appears that there is a complementary grazing effect, but there has been no research on optimum ratios. All properties are grazing kangaroos as well.
Monitoring sites
With changes in grazing management systems, there is a need to monitor effects of changes in management. Thus the Mulga Assessment Program has been developed to encourage graziers to monitor botanical and ground cover changes on their own properties. Plant identification manuals have been produced to assist in the monitoring process.
Table 7.1.3. Biomass comparisons of two monitor sites in the mulga lands of southwest Queensland under various exclusion regimes.
|
Site
|
Exclosure type
|
Biomass (kg/ha) |
|
|
Year |
|||
|
1987 |
1988 |
||
|
1
|
Kangaroos and livestock excluded |
521 |
460 |
|
Livestock excluded |
343 |
278 |
|
|
Open |
309 |
269 |
|
|
2
|
Kangaroos and livestock excluded |
313 |
302 |
|
Livestock excluded |
132 |
120 |
|
|
Open |
128 |
108 |
|
In summary, grazing involves a programme of whole property or enterprise management, which takes into account:
· drought and other seasonal conditions,
· use of land system information for fencing, water point distribution and retention of wildlife habitats,
· monitoring of both range and animal condition,
· management of pasture, livestock and wildlife,
· management of finances and marketing, and
· monitoring of potential effects of supplementation on degradation.
Degradation is expensive to treat in extensive low input-low output regions. The best degradation control is not to allow movement from the first state on Figure 7.1.3.
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