Mitchell grass downs
Mitchell grass downs: some of the best grazing
country in the tropical savannas
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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.
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These boundaries show property sizes across the
Mitchell Grasslands: the size varies greatly
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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).
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This pasture map shows the great swathe of
Mitchell grass in yellow and also the prominent areas of Gidgee
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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.
