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Description
As the four species usually occur together with only minor
variations in ecological environment they will be treated together. They
are perennial, summer-growing tussock grasses whose persistence and extraordinary
resistance to drought and continuous grazing are due largely to the rootstock,
which consists of many short, stout, thick, branched, scaly rhizomes. In
A. Iappacea numerous wiry roots spread outward from the bottom of the rhizome
into the surrounding surface soil horizon and then turn vertically downward,
continuing unbranched through the layer of columnar clay. The roots branch
into fine rootless where soil cracks cease and a gypsum layer begins. Main
tillers arise from the root-stock and axillary tillers develop from axils
on the main tillers (Orr, 1975). A key to the species (Gardner, 1952) is
as follows:
Spikelet or raceme dense, 8-20 mm broad; internodes of the
rachilla less than 1 mm long; spikelets three- to nine-flowered.
1. Lobes of lemma all similar, symmetrically attenuated
into rigid and tough bristle-like points, sometimes hooked; lemmas densely
silky- villous on and around the lateral nerves from the base upwards:
A. squarrosa C.E. Hubb. (bull Mitchell grass) (see Fig. 15.7A).
2. Middle lobe of the lemma alone symmetrically tapering
into a tough bristle-like unhooked point, lateral lobes semi-lanceolate
to semi-ovate; lemmas villous all over the entire portion of the dorsal
surface.
a. Spikelets densely imbricate; lower glume five- to nine-
nerved; glumes much shorter than the lemmas: A. pectinata (Lindl.) F. Muell.
(barley Mitchell grass) (see Fig. 15.7B).
b. Spikelets loosely imbricate and alternate; lower glume one-
to three-nerved; upper glume as long as the spikelet: A. Iappacea (Lindl.)
Domin (curly Mitchell grass) (see Fig. 15.7C).
Spikelets or racemes slender, 2-3 mm broad, the spikelets
distant or loosely imbricate; internodes of the rachilla 2-4 mm long, lower
glume one-nerved; upper glume seven- to 11-nerved, half as long as the
spikelet; spikelets two- to four-flowered: A. elymoides F. Muell. (hoop
or weeping Mitchell grass) (see Fig. 15.7D).
Distribution
They occupy an area of some 450 000 km2, predominantly in western
Queensland with outliers into the Northern Territory extending sparsely
into Western Australia (see Fig. 15.6).
Season of growth
Summer.
Altitude range
300-1 000 m.
Rainfall requirements
The Mitchell grasses reach their best development between the
250 and 550 mm annual rainfall isohyets in regions with pronounced summer
rains. Roe and Allen (1945) decided that the growth of Astrebla spp. depends
on the current season's precipitation rather than on stored soil moisture.
A second rain following germination by about six weeks is necessary to
enable tillers and secondary roots to develop (Everist, 1951). Old plants
respond to light falls of rain by producing axillary tillers from the lower
nodes, heavier rains initiate new basal tillers (Jozwik, Nicholls &
Perry, 1970).
Drought tolerance
The Mitchell grasses have exceptional drought tolerance. They
will "hay off" if no rain falls after maturity and grow again after good
rains in the next summer. The plants do not break up when dry, and while
in that state are still acceptable to stock, but light rain at this time
will cause mould growth and blackening, making them inedible.
Soil requirements
The Mitchell grasses are restricted to grey, brown and red
alkaline cracking clay soils with a minimum of 40 percent clay. The surface
is self- mulching clay to sandy clay loam overlying a massive cracking
clay. Lime is usually present in the upper profile, with gypsum below.
Ability to spread naturally
The Mitchell grasses produce abundant seed in a good season.
In years of exceptional wet season rains, seedlings will appear in abundance
but there is usually some seed emergence in a normal wet season.
Land preparation for establishment
Either prepare a good seed-bed with ploughing and cultivation
or merely broadcast seed over the self-mulching clays and trample it in
with livestock (Breakwell, 1923) during wet weather.
Sowing methods
Sow on a well-prepared seed-bed with suitable drills, or broadcast
into loosely mulched soils.
Sowing depth and cover
Sow no deeper than 1 cm and lightly cover or roll.
Sowing time and rate
Sow in spring at 3.5-4.5 kg/ha.
Tolerance to herbicides
No information is available.
Seedling vigour
Seedling growth is good when heavy rains fall to germinate
the grasses.
Vigour of growth and growth
rhythm
Growth responses are determined by rainfall. If rainfall of
only about 40 mm occurs, fine rootless developed from the main roots stimulate
some culm growth using stored starch. The main root system is stimulated
by 75 mm or more. Once the season breaks dry matter increases fourfold
in one month, 20-fold in two months and 35-fold in three months.
Response to defoliation
The Mitchell grasses stand a good deal of defoliation; but
overstocking removes seedlings, and heavy grazing during active growth
depletes root reserves for recovery.
Grazing management
Mitchell grasses are normally continuously grazed, and rotational
grazing at Cunnamulla (Roe & Allen, 1945) proved no benefit in a long-
term trial. Vagaries of seasons outweigh stocking practices over a series
of years. Normal stocking rates are one sheep per 1.22 hectares. Grazing
should not be heavy during the period of active growth and the grasses
should be allowed to seed freely to recoup the paddock seed supply. Heavy
stocking by horses can destroy a Mitchell grass pasture because they graze
closely and trample and dig up tussocks with their hooves in dry times.
Response to fire
Burning is unnecessary and undesirable in most years, and only
when excessive foliage canopy excludes the inter-tussock herbage is a fire
warranted. Mitchell grass recovers well from occasional accidental fires.
Dry-matter and green-matter
yields
Dry matter yields and rainfall during the growing period have
been recorded as 400 kg/ha (163 mm) at Brunette Downs, Northern Territory,
2 250 kg/ha at Claverton, Wyandra, Queensland.
Suitability for hay and
silage
A good deal of hay has been made over the years in Queensland.
The tussocky nature of the country makes hay-making difficult; the black
clay soils are difficult to negotiate after rain and unless the grasses
are cut early the quality of the hay will deteriorate. It is really low-quality
roughage. In open-air storage, oxidation lowers its nutritive value, but
in its usual habitat, protection of stored hay is often costly (see Plate
19).
Value as a standover or
deferred feed
It is quite valuable as standover feed. Though its nutritive
value has deteriorated as a whole to sub maintenance levels, sheep selectively
graze the more nutritious portions. Light rain falling on mature Mitchell
grasses causes mould growth and blackening, which spoils its value as roughage.
Toxicity
No toxicity has been reported by Everist (1974).
Cultivars
There are no registered cultivars but four species are recognized:
A. Iappacea (curly Mitchell grass); A. pectinata (barley Mitchell grass);
A. elymoides (hoop Mitchell grass); and A. squarrosa (bull Mitchell grass).
Diseases
Of little importance.
Main attributes
Their drought resistance, adaptability to a harsh environment,
and quick response to rainfall in the presence of the grazing animal.
Main deficiencies
Low nutritive value during the winter and lack of growth. Difficulties
with germination of seed.
Optimum temperature for
growth
Mean monthly maximum temperature of 35°C or over. Optimum
tillering in A. pectinata occurs at day/night temperatures of 28/23°C
and growth and leaf production increased with temperature up to 30/25°C
(Jozwik, 1970).
Frost tolerance
It tolerates frosts, as the southern area of its distribution
receives an average of 50 frosts per year, but the vegetative parts are
frosted.
Latitudinal limits
From 18-28°30'S, approximately.
Response to light
Mitchell grasses do not produce well in shade. Scattered trees
(shady downs) with sparse foliage do not interfere with growth.
Ability to compete with
weeds
Their very drought-tolerance allows them to outlast weeds,
and not many common "weeds" are weeds where grazing merino sheep are concerned,
as they do well on numerous edible plants.
Maximum germination and
quality required for sale
35 percent germinable seeds; 75 percent purity (Queensland).
Pests
Of little importance. Kangaroos can be competitive with other
grazing animals in dry times, and on young shoots appearing after fires
have passed through.
Palatability
Mitchell grasses are generally shunned in favour of annual
grass and herb associates during the growing season, but are eaten when
other feed is not available.
Chemical analysis and
digestibility
Crude protein levels recorded vary from a low 3.5 percent in
midwinter, through 5-6 percent for fair quality, 8 percent for good quality,
and 18.4 percent for young three- to four-week-old leafy material; carbohydrates
39-51 percent, crude fibre 26-33 percent (Orr, 1975), ether extract 1.4-
2 percent (dry material), ash 8.8-12 percent (Siebert, Newman & Nelson,
1968).
Natural habitat
Open black and brown clay plains in subtropical areas of Australia
within the annual rainfall isohyets of about 200 and 500 mm.
Tolerance to flooding
The species A. Iappacea and A. pectinata are killed by flooding
but A. elymoides and A. squarrosa are tolerant. A. squarrosa occupies the
lower and wetter drainage lines in Mitchell grass pastures.
Fertilizer requirements
The Mitchell grasses are never fertilized. The potash and phosphorus
status in the soils is usually adequate and the birch effect of nitrogen
release on wetting of the cracking clays helps provide additional nitrogen
for early wet season growth.
Compatibility with other
grasses and legumes
Mitchell grasses form a tussock grassland. When the wet season
arrives the inter-tussock space is occupied by annual grasses and herbs,
e.g. Iseilema spp., Dactyloctenium radulans, Brachyachne convergens, Rhynchosia
minima, Boerhaavia diffusa and a short-term perennial, Dichanthium sericeum,
in very wet years. In the winter rainfall zone Medicago spp. are common
associates (see Plates 20, 21, 22).
Genetics and reproduction
2n=40 (Jozwik, 1969; Fedorov, 1974).
Seed production and harvesting
Mitchell grasses appear to have abundant seed, but "seed" consists
of spikelets with a variable number of caryopses. Some 3 percent of spikelets
have no seed, 18 percent one seed, 18 percent four seeds (Myers, 1942b).
Seed is generally hand picked as demand is low.
Economics
A most important natural tussock grassland occupying some 450
000 km2 across the northern half of Australia.
Animal production
The carrying capacity of the Mitchell grass tussock grassland
is rated at one sheep to one to two hectares in the 600-mm rainfall zone
and one sheep to two hectares in the 300-mm region. Wool production per
hectare from merino sheep ranges from 2.3-3 kg. Cattle carrying capacity
is rated at one per 15 hectares. With beef shorthorn cattle specially selected
for heat tolerance, live-weight gains of 0.7 kg per head per day were obtained
from Mitchell grass pastures at Muttaburra, Queensland, over a whole year,
reaching an average live-weight gain of over 1 kg per head per day during
the winter-spring period (Dowling, 1960).
Further reading
Davidson, 1954; Davies, Scott & Kennedy, 1938; Orr, 1975;
Roe & Allen, 1945.
Dormancy
Post-harvest dormancy exists in all species for a period of
up to 12 months to reach 88 percent germination and maintains this for
at least another year (Myers, 1942b). Germination occurs in a temperature
range of 15-42°C but mainly 22-38°C.
Value for erosion control
Owing to their tussocky nature, Mitchell grasses do not give
very effective control of run-off, but their extensive roots intertwine
to give some protection against soil erosion.
Tolerance to salinity
They are tolerant of mild salinity; many of the soils on which
they grow have a pH approaching 8.5.
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