Mitchell grass alluvial plain pastures in the Kimberley, Western Australia

Mitchell grass alluvial plain pastures

These pastures are part of the Kimberley 'black' soils group of pastures, and the perennial Mitchell grasses are the identifier grasses. Use the interactive key to pasture condition to help identify pasture type.

Pastoral value

Productive and resilient under grazing, good condition pastures dominated by Mitchell grass have high pastoral value. As Mitchell grass density declines, pastoral value falls because less productive perennial grasses (those with reduced pasture bulk and resilience) or undesirable species, like feathertop, take over. Red Flinders grass provides good feed for livestock, particularly early in the year, but its bulk depends on the season. In below-average rainfall years, minimal red Flinders grass may be produced.

Occurrence

Mitchell grass alluvial plain pastures are tussock grasslands that occur on level plains with gilgai microrelief. Gilgai (crabhole country) has a markedly undulating surface caused by swelling and shrinking of the clay soil during alternating wet and dry seasons. There are sometimes scattered small trees, such as bauhinias, in these pastures.

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Pasture condition

Good: Desirable Mitchell grasses (barley and/or hoop Mitchell) usually dominate in good pasture condition, often with small amounts of other desirable perennial grasses, such as bundle-bundle and ribbon grass. Mitchell grass tussocks are healthy and evenly spaced (Figures 1 and 2).

Annual grasses, such as red Flinders grass and Kimberley couch, and herbs may occupy the spaces between perennial tussocks early in the year. As the dry season progresses, the groundcover between Mitchell grass tussocks is reduced as annual grasses are grazed.

Figure 1 Mitchell grass alluvial plain pasture in good condition with high utilisation in the Kimberley (photograph taken May 2008).  A There is a dense coverage of desirable Mitchell grass tussocks, grazed down to about 60% utilisation.  B The annual grasses growing between the perennial grass tussocks have been largely removed by grazing. Tussock density is optimal for the site; however, year-in, year-out use at this level would lead to an increase in bare ground and declining pasture condition.

Figure 2 Mitchell grass alluvial plain pasture in good condition with low utilisation in the Kimberley (photograph taken May 2008).  A There is a dense coverage of barley Mitchell grass and other desirable species.  B Red Flinders grass, an intermediate annual grass, is growing vigorously in between the perennial grass tussocks; groundcover is optimal for the site.

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Fair: Density and vigour of desirable Mitchell grasses and other desirable perennial grasses fall as pasture condition declines from good to fair. Bull Mitchell grass is considered an intermediate grass in this pasture type, and it or other intermediate species, such as native millet, may become prominent (Figure 3).

Feathertop, an undesirable perennial threeawn, may increase. Red Flinders grass or less-desirable annual plants may occupy the increased space between perennial grass tussocks.

Figure 3 Mitchell grass alluvial plain pasture in fair condition in the Kimberley (photograph taken May 2008).  A The density of desirable Mitchell grasses is reduced.  B Intermediate perennial grasses, such as this silky browntop, are becoming more prominent.  C There is an increased presence of feathertop, an undesirable perennial grass, and non-grass plants.

Poor: Further deterioration to poor condition results in almost complete loss of the Mitchell grasses and other desirable species, and a significant increase in bare ground. Two things may happen: the pasture may be dominated by non-grass plants (woody and/or weedy species); or the pasture may be completely dominated by feathertop. Both situations are unproductive (Figure 4).

Figure 4 Mitchell grass alluvial plain pasture in poor condition in the Kimberley (photograph taken May 2008).  A Desirable species, such as this Mitchell grass, are not easy to find; they occur in small patches and lack vigour.  B Bare ground is frequent, with some large patches.  C The relative frequency of non­-grass plants has increased.  D The remaining grasses are mostly annual species, such as Kimberley couch; they are patchy and lack vigour.

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Associated plants