Astrebla spp.

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Graminae

 

Common names

Mitchell grasses.
 
 
 

A - squarrosa
B - pectinata
C - lappacea
D - elymoides

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:
  • A. 
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).
  • B. 
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.