Mitchell grass alluvial plain pastures in the Pilbara, Western Australia

Pasture potential

In good condition these pastures are of very high pastoral value. Mitchell grasses are the most palatable native grasses and are generally heavily grazed. Other grasses, herbaceous perennials and annuals associated with the Mitchell grasses also provide good quality forage for much of the year. Mitchell grass alluvial plain pasture in good condition can be grazed year–long (at appropriate stocking rates) by all classes of stock.

Suggested levels of use (per annum)

• Good condition: 15 hectares per cattle unit (ha/cu)
• Fair condition: 30 ha/cu
• Poor condition: 45 ha/cu

Managing Mitchell grass alluvial plain pastures in the Pilbara

Grazing management

Mitchell grasses seed readily and the seedlings appear regularly but most do not survive. Grazing animals often trample or pull seedlings out of the crumbly-surfaced soil. The adult tussocks suffer the same fate but to a lesser degree. If this continues year after year without spelling, a population will eventually disappear, especially in poor seasons.

Mitchell grass pastures in fair or poor condition require resting from grazing for at least a year, if not two. Degraded Mitchell grass pastures that are protected from grazing generally recover fairly rapidly, if sufficient plants remain to supply a seed source.

Fire management

Mitchell grass pastures have traditionally been protected from fire. However, burning can encourage more even utilisation in large paddocks, rejuvenate rank pasture, and encourage a temporary increase in the legume and annual component, which may improve nutritional quality. We recommend a minimum of 5 years between fires. Burn late in the year, for example immediately after the first storms, when the fire danger index is moderate.

Pasture condition

The condition of these pastures is assessed by observing the degree of ground cover (% basal cover) afforded by the grass tussocks, the vigour of the tussocks, age structure of the stand, the proportion of undesirables in the stand and the condition of the soil surface.

Traverse data (47 observations) during the Pilbara rangeland survey 1995-97 recorded:

Pasture condition: very good 13%, good 21%, fair 19%, poor 17%, very poor 30%.
Soil erosion: nil 92%, slight 0%, minor 2%, moderate 2%, severe 2%, extreme 2%.

The evidence is that the condition of Mitchell grass alluvial plain pastures is highly variable, and that substantial parts are seriously degraded. Where there has been a long grazing history, and Mitchell grass occurred in small areas as part of a mosaic with other less palatable pasture types, it has been removed by preferential grazing. However, where it occurs over larger areas distant from water, this pasture type can still be found in good condition. Soil erosion is uncommon due to flat topography and heavy textured soils, but is a problem in some localised areas.

Very good–good condition

In good condition (Figure 1) the basal cover of Mitchell grass tussocks will be in the range of about 3 to 6% and there should also be some young age classes in the stand. There should also be a range of useful perennial herbs such as Mardie clover (Rhyncosia minima) and sensitive plant (Neptunia dimorphantha).

Figure 1 An open gilgaied plain on the Brockman land system supporting a fairly dense tussock grassland of barley Mitchell grass (Astrebla pectinata). Basal cover is about 5% and condition is good. Note that there are no shrub layers.

Fair condition

In fair condition (Figure 2) this pasture type has a reduced population of Mitchell grasses and basal cover of the tussocks varies from about 1 to 3%. The populations of other palatable perennials also decrease while those of intermediate or undesirable plants may increase marginally.

Figure 2 A moderate stand of barley Mitchell grass with basal cover of about 2% and the development of a bare patch in the middle distance. Condition is fair.

Poor–very poor condition

In poor condition (Figures 3, 4) the population of Mitchell grass has largely disappeared with only isolated plants remaining. The population of herbaceous perennials has largely disappeared and there can be an increase in unpalatable perennials and annuals.

Figure 3 A gilgaied plain with a very sparse stand (foreground) of barley Mitchell grass (basal cover of less than 0.5%). Note the mimosa bush (Acacia farnesiana) in the middle distance that is increasing due to lack of competition from perennial grass and fire. The site condition is poor.

Figure 4 Mitchell Grass Alluvial Plain Pasture in poor condition on the Brockman land system. The soil is not eroded and shows the typical crumbly gilgaied surface of a Mitchell grass site, but the grass is present only as isolated dead butts. The site will still produce abundant annual herbage in season but has no durability in dry times.

Vegetation structure and composition

This pasture type occurs as tussock grasslands on open treeless plains that may extend for 2 or 3 km, or as considerably smaller patches within stony plains and associated with Roebourne Plains grass pasture.

The tussock grasses consist mostly of barley Mitchell grass (Astrebla pectinata) and weeping Mitchell grass (Astrebla elymoides) with a few other perennial grasses such as Roebourne Plains grass (Eragrostis xerophila), ribbon grass (Chrysopogon fallax) and neverfail (Eragrostis setifolia), and numerous herbs and forbs in season. Curly Mitchell grass (Astrebla lappacea) is found on the Hamersley Plain, the only location where it is known in Western Australia.

Shrubs are usually absent or isolated but occasionally there is a scattered to moderately close layer of snakewood (Acacia xiphophylla) with a projected foliage cover (PFC) of 10 to 25%.

Occurrence

This pasture is occasionally found in the alluvial valleys of the Fortescue River and its tributaries on such land systems as Brockman, Coolibah, Elimunna, Fortescue, Hooley and Kanjenjie. It occurs on level plains with gilgai microrelief and deep cracking clay soils with crumbly self-mulching surfaces. Surface mantles are absent or few to common (2–20%) as pebbles of ironstone and other rocks.

Associated plants

Other resources

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.