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Grazing in the Mitchell Grasslands

Mitchell grass downs

Mitchell grass downs

Mitchell grass downs: some of the best grazing country in the tropical savannas

This region is so named because it is characterised by vast rolling downs of Mitchell grass, growing on relatively fertile cracking clay soils. Properties sizes vary from some of more than 1 million hectares on the Barkly tablelands to less than 15,000 hectares in areas south of Longreach where sheep are grazed. Historical factors explain some of this variation, although higher carrying capacities in the south are in part a result of the longer growing season. Rainfall decreases from east to west, and quite significantly from north to south.

  Land use in the Mitchell grasslands

These boundaries show property sizes across the Mitchell Grasslands: the size varies greatly

Climate

Rainfall decreases from north to south and east to west. In the north 85 per cent of rain falls from December to February. In the south the growing season lengthens to the point where more than 30 per cent of annual rain falls in winter. As total rainfall decreases from around 600 mm to 400 mm, it tends to become less reliable, so that the chance of drought increases. Indeed, based on total rainfall and probability of drought, this is this driest tropical savanna region. Those southern sectors of the Mitchell grass region which have lower and less reliable rainfall benefit from the fact that their rain tends to fall over a longer period of time. Thus the growing season is longer and there is the chance of winter rains which can improve animal health and weight gain. There is also, however, the possibility of frosts in this area.

Rainfall variability and pastures

The greater variability of rainfall in this region can have major impacts on composition of pasture species, especially if there is unseasonably high or low rainfall for a number of consecutive years. For example, a series of wet summers in a southern district will produce a northern type pasture, whereas a drought in the north followed by winter rains will lead to a pasture composition more typical of the south.

In the north rains are confined to summer months, producing summer grasses and forbs. The shorter growing season forces stock to eat Mitchell grass for more of the year, and animal production is lower than in the south where the diet is more varied. Annual grasses such as Flinders (Iseilema sp.) and button grass (Dactyloctenium radulans) may become more predominant under heavy grazing in the north, while in the south white speargrass and feathertop (Aristida sp.) will increase. While Flinders grass provides good feed in the summer and autumn, its nutritive value disappears once the weather turns dry.

Mitchell grass tussock

The Mitchell grass plant itself can live for up to 30 years. The grass grows as scattered tussocks, about 20 cm across, up to half a metre high and with roots well over a metre deep, allowing the plant to access soil moisture deep within the cracking clays. While other grasses and shrubs may grow between the tussocks, there are generally few native trees to be found on Mitchell grass downs country. There are four species of Mitchell grass. The most nutritious are Curly Mitchell (Astrebla lappacea) and hoop Mitchell (Astrebla elymoides) grass, followed by barley Mitchell (Astrebla pectinata). The least nutritious is bull Mitchell grass (Astrebla squarrosa) .

Gidgee country

Between Winton and Blackall the grassland has a light covering of trees including gidgee and coolibah, and is dominated by Mitchell grass and summer forbs including Malvastrum sp., native leek (Bulbine bulbosa) and Psoralea sp. Occasional wet winters will produce winter forbs. The Blackall area is unique because buffel grass has been planted on the clay-loam soils which were cleared of gidgee shrubland in the 60s and 70s. This area of buffel grass is highly productive.

Pasture land communities

The Mitchell grasslands, as the name suggests, is dominated by vast tracts of Mitchell grass, in addition to localised distributions of other midgrass pastures. Bordering much of the region, one finds sandy and skeletal soils which support hummock grasslands. These tend to be associated with midgrass species such as curly spinifex (Plectrachne spp.) and soft spinifex (Triodia pungens).

  Mitchell grassland communities

 This pasture map shows the great swathe of Mitchell grass in yellow and also the prominent areas of Gidgee

To the south of Mount Isa lies a fragmented zone of perennial shortgrass pasture communities, which is generally lacking in 'top feed' or palatable shrubs and trees. These areas are characterised by Acacia woodland/shrubland, especially georgina gidgee (Acacia georginae). Channel pastures grow in the open eucalypt forests of the seasonal riverine plains of the Wills and Diamentina river courses. Chenopod shrubland pastures of northern bluebush have a scattered distribution through the Mitchell grass tussock grasslands in the Northern Territory. In the far north-west of the region, Aristida-Bothriochloa pastures, including Aristida pruinosa (three awn), grow within the dominant open eucalypt forest.