Soft spinifex plain pastures in the Pilbara, Western Australia

Pasture potential

We refer to 'pastures' throughout, because there is a wide variation of species composition, with the soft spinifexes dominating.

The pasture value and carrying capacity of soft spinifex plain pastures varies from moderate to low, depending on the stage in the burning and recovery cycle. After fire, these pastures have a wide range of palatable short-lived perennial herbs, annuals and young soft spinifex, and has a moderate carrying capacity (about 60 hectares per cattle unit).

Forage value is high with crude protein levels as high as 9% in soft spinifex seedlings. By the third or fourth year after fire, productivity and forage value declines as the spinifex becomes older and less palatable and reasserts dominance in the stand.

Young soft spinifex plain pastures are durable and useful forage in dry times. In a young productive state, they are suitable for carrying stock, including breeders, on a year-long basis.

Suggested levels of use (per annum):

• Good condition: 60 hectares per cattle unit (ha/cu)
• Fair condition: 84 ha/cu
• Poor condition: 120 ha/cu.

Managing soft spinifex plain pastures in the Pilbara

Where possible, extensive areas of soft spinifex should be fenced separately from other more productive pastures. This will allow spinifex pastures to be grazed when there are palatable ephemerals in good seasons after fire, and spelling more valuable pastures at a time when their rejuvenation will be at a maximum.

Fire and spinifex pastures

Soft spinifex plain pastures are of most use to stock when spinifex plants are young and a range of other grasses, herbs and forbs are present. Areas of useful pasture can be maintained by a system of periodic burning of a paddock or sub-paddock scale (using natural firebreaks and cleared lines wherever possible) and rotational use.

Summer burning

In the Pilbara, we recommend summer burning, which encourages grass species rather than shrubs. After burning, defer grazing for 6 to 8 weeks following effective rainfall, to allow spinifex seedlings and other desirable species time to establish and develop. See Pilbara rangeland pastures and fire for more detail.

Winter burning

Shrub invasions can occur after a winter burn. The unpalatable low shrub poverty bush (Acacia translucens) may thrive and establish in thick stands, at the expense of the spinifex. Winter burning appears to promote the vigorous growth of other undesirable and unpalatable plants such as cockroach bush (Senna notabilis).

Recovery after burning

Soft spinifex plants usually regain dominance in a stand within about 5 years. By that time its palatability will be relatively low and there are few other useful species left in the stand. Use rotational burning of these pastures to keep a proportion in the most acceptable stages of growth, with a wide range of plant species available for grazing.

Reducing erosion risk

Exceptions exist to the general recommendation of regular burning. It is not recommended on the Cheerawarra land system, which is inherently highly susceptible to wind erosion. Keep grazing pressure low enough to maintain a ground cover of desirable perennials, such as soft spinifex and buffel grass, to prevent soil erosion.

Pasture condition

Soft spinifex pastures in the Pilbara are mostly in very good or good condition, with occasional areas in poor condition. Condition assessment is based on the composition of the short-lived perennial herbs and the density and vigour of spinifex seedlings that appear in the post-burning phase.

Traverse data (3581 observations) during the Pilbara rangeland survey 1995–97 recorded:

Pasture condition: very good 69%, good 20%, fair 8.5%, poor 2.5%.

Soil erosion: nil 95.5%, slight 1.5%, minor 1.9%, moderate 0.9%, severe 0.2%, extreme 0.1%.

Soil erosion is not usually associated with a decline in condition on these pastures. There are some notable exceptions, especially where the pasture occurs on sandy-surfaced duplex soils, which are inherently susceptible to erosion if vegetative cover is depleted. For example, on alluvial plains of the Paradise land system, minor and moderate erosion in the form of scalding, sheeting and wind piling is common, and severe erosion occurs locally.

About 43.5 km² of the Paradise system (nearly 3%) is severely degraded and eroded (SDE).

As with hard spinifex plain pastures, condition and the effects of grazing are often difficult to assess on soft spinifex plain pasture. The high frequency of fires means that the presence or absence of particular species or suites of species may be more closely linked to season of firing and post-fire stages than to grazing pressure.

Very good–good condition

Long-unburnt dense stands of mature spinifex can be regarded as being in good condition (Figure 1, 2), although it will not be known if the general absence of palatable species is due to direct competition from the spinifex alone or in combination with grazing. Palatability and pastoral value will be low. A marked increase in shrubs after fire is regarded as a decline in condition.

Figure 1 Soft spinifex community in good condition that was burnt a year previously.  There is a diverse assemblage of short-lived perennial herbs and grasses, including spinifex seedlings, many of which are palatable to livestock.  The site is a sandy-surfaced plain on the Urandy land system.

Figure 2 An old, dense community of soft spinifex that has not been burnt for several years. Competition from the spinifex means that there are only a few associated herbaceous perennials.  It can still be regarded as being in good condition but pastoral value and carrying capacity are low compared to Figure 1.

Fair condition

Usually identified by more bare areas and undesirables than expected for the site.

Figure 3 A soft spinifex community in fair condition. There is a much reduced population of palatable short-lived perennial herbs compared to good condition but generally no reduction in soft spinifex.  There has been no increase of undesirable herbs or shrubs.

Poor–very poor condition

Poor condition (Figure 4, 5) can result from long-term overuse, especially on small areas of burnt spinifex among large areas of mature spinifex. Overuse results in the elimination of the edible herbs and grasses, including the spinifex seedlings, and leads to large areas of bare soil with a few isolated shrubs or large spinifex hummocks.

Poor condition is also indicated by dense stands of undesirables such as poverty bush, wax wattle or cockroach bush. Although shrub invasion is not a widespread problem on this pasture type, it is significant in localised areas on some coastal stations.

Figure 5 A soft spinifex community in poor condition. Regrowth from a burn about a year ago consists mostly of the short-lived perennial increasers cockroach bush (Senna notabilis) and Cullen leucochaites.

Figure 5 A dense stand of poverty bush (Acacia translucens) which has taken over in soft spinifex plain pasture on a sandy plain. Condition is poor. Poverty bush is a common minor component in these pastures but can sometimes become a woody weed problem to the exclusion of most other species.  It can be removed by hot fire.

Vegetation structure and composition

Structure and composition are variable and a function of past fire regimes, soil types and prevailing climatic conditions. Pastures may be hummock grasslands, tall or low shrublands and occasionally, low woodlands. In each case the ground storey is dominated by soft spinifex (Triodia pungens or T. epactia) which can have a projected foliar cover (PFC) of up to 50%.

Shrubs and trees are usually very scattered (PFC <10%), or occasionally scattered (PFC 10–20%). On some sandy plains of the Uaroo land system shrubs, such as poverty bush (Acacia translucens) and wax wattle (Acacia ancistrocarpa), can become close or closed (PFC 30% to more than 50%) and there are few associated species.

Hummock grasslands with high PFC are prone to fire. Evidence of past burns is obvious in many areas, where the vegetation has been completely removed or where sub-climax stages exist. Early stages after fire on such systems as Mallina, Macroy, Uaroo, and Urandy often take the form of shrublands, where pioneering perennial shrubs are co-dominant with spinifex seedlings.

Prominent shrubs after fire include fire wattle (Acacia pyrifolia), kanji (A. inaequilatera), poverty bush (A. translucens), wax wattle (A. ancistrocarpa) and pindan wattle (A. tumida).

Other species include the relatively short-lived cockroach bush (Senna notabilis), woolly corchorus (Corchorus walcottii) and Cullen and Indigofera spp. A wide range of small annuals, biennials and herbaceous perennials also occur after fire. About five years after burning, soft spinifex may regain dominance in the stand, as the shrubs either senesce or are crowded out by expanding spinifex hummocks (Suijdendorp 1967).

Pastures gradually resume the form of hummock grassland with scattered or very scattered shrubs. Isolated trees such as corkwood (Hakea suberea), wild walnut (Owenia reticulata) and eucalypts are occasionally present.

A few other perennial grasses occur as isolated clumps or patches within soft spinifex plain pastures. The most important of these are hop-along grass (Paraneurachne muelleri), ribbon grass (Chrysopogon fallax) and buffel grass (Cenchrus ciliaris) on sandy-surfaced soils and Roebourne Plains grass (Eragrostis xerophila) and curly windmill grass (Enteropogon acicularis) on more clayey soils.

Occurrence

Approximate area 44 150 km² (23.0% of total Pilbara area).

Soft spinifex plain pastures are common throughout the Pilbara. They are found on areas that receive extra water such as alluvial fans, alluvial plains and drainage floors and on some stony plains, sandplains and coastal dunes. They are the major pasture of the Bonney, Dune, Mallina, Nita, Paradise and Urandy land systems and an important component on many other systems.

Extensive areas of these pastures occur near the coast on calcareous sandy soils of the Cheerawarra and Dune systems and on the red duplex and gradational soils of the Mallina system. Soft spinifex is often dominant or co-dominant with buffel grass on the alluvial soils of the River system.

These pastures are also common on sandy-surfaced plains, interfluves and drainage floors of such land systems as Macroy and Uaroo. They are common on narrow drainage floor of numerous other systems.

Soils are stony duplexes, red sands or red gradational types.

Associated plants

Other resources

• Suijdendorp, H 1967, A study of the influence of management practices on “spinifex” Triodia pungens grazing, masters thesis, University of Western Australia, Perth.
• van Vreeswyk, AM, Leighton, KA, Payne, AL, & Hennig, P 2004, An inventory and condition survey of the Pilbara region, Western Australia, Department of Agriculture and Food, Western Australia, Perth. Technical Bulletin 92.