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Graminae
Synonyms
Andropogon contortus L.
Common names
Black or bunch spear grass (Australia), tangle head (United States), pill grass
(Hawaii), assegai grass (Zimbabwe).
Description
A caespitosa perennial, the culms erect to 75 cm, branching above; leaf-sheaths keeled,
glabrous. Raceme solitary, 3.5-15 cm long with up to ten pairs of awnless spikelets at the
base and an equal number of pairs above; the fertile sessile spikelets having awns 5-10 cm
long.
Distribution
Throughout the tropics and subtropics.
Season of growth
In Queensland, 60 percent of the yield of dry matter is produced in summer between
January and April (Straw & Bisset, 1955).
Altitude range
Sea-level to 2 000 m in the Himalayas. Less than 300 m in Hawaii.
Rainfall requirements
It occurs naturally in the 500-1 500 mm rainfall regime with a summer maximum, with
20-30 percent variability (Isbell, 1969).
Drought tolerance
It is fairly tolerant of short-term droughts but does not persist in semi-arid areas.
It yields little in New Zealand during dry spells in January and February.
Soil requirements
H. contortus generally thrives best on sandy loams with a pH in the range of 5.0-6.0.
It establishes with difficulty in heavy clay soils.
Ability to spread naturally
Excellent, especially if the country is burnt regularly. The seeds survive the burning
by burying themselves. The awns twist and untwist as moisture changes, enabling the
needle-like seeds to penetrate the soil surface.
Land preparation for establishment
It is rarely sown. The existing natural pastures are utilized.
Vigour of growth and growth rhythm
It produces 90 percent of its growth during the warmer months between mid-October and
mid-April at Rodd's Bay. Its winter contribution is small.
Suitability for hay and silage
Both have been made in India. Hay cut at late vegetative stage contained 5.9 percent
crude protein in the dry matter, and at dough stage 3.5 percent. Immature material made
into silage contained 6.6 percent crude protein in the dry matter (Göhl, 1975).
Cultivars
There are no recognized cultivars, but two varieties are recognized in India (Dabadghao
& Shankarnarayan, 1973).
Main attributes
Its hardiness, perenniality, tolerance of fire and its early palatability. Its ability
to grow on poor soils.
Main deficiencies
Its dominance in burnt areas. Its production of numerous robust awns which shed easily
and cause damage to animals' skin and reduce wool values.
Optimum temperature for growth
In Queensland the summer temperature ranges from 30-33.5°C.
Minimum temperature for growth
At lat. 23°30'S growth ceases at 21°C (Miles, 1949). It makes practically no growth
in winter, irrespective of rainfall, and yields less than 50 kg DM/ha (Straw & Bisset,
1955).t
Frost tolerance
It tolerates frost well, but does not grow during the winter, regardless of frost.
Palatability
It is palatable in the early vegetative stage, but unattractive as it matures.
Chemical analysis and digestibility
Dabadghao and Shankarnarayan (1970) in India found the crude protein content of a
Heteropogon community was 5 percent untreated, and 5.8 percent when treated with nitrogen.
In Queensland the crude protein content, even when very young, does not rise above 10
percent and for the greater part of the growing season it is between 4 and 6 percent,
dropping in winter to 2-3 percent. The digestibility of this protein is also low.
Phosphorus figures as percentage of the dry matter ranged from 0.09-0.15, and calcium
0.23-0.30, indicating that on this soil the phosphorus figures were too low for an
adequate diet for beef cattle (0.15-0.28 percent) and the calcium figures barely adequate
(requirement: 0.15-0.37 percent) (Straw & Bisset, 1955). Göhl (1975) lists analyses
from Zimbabwe, Ghana and India. The fresh material from Ghana showed 9.4 percent crude
protein in the dry matter at four weeks, 6.3 percent at eight weeks, 7.0 percent at 16
weeks and 2.5 percent at 36 weeks. It has low sodium levels (Playne, 1970a).
Natural habitat
Open forest and woodland, grassland.
Tolerance to flooding
It does not tolerate flooding.
Fertilizer requirements
It is not usually fertilized. Weier (1977) showed that H. contortus under natural
conditions has high nitrogenase activity associated with its roots and fixes some of its
own nitrogen. In India, the application of 20 kg N/ha raised production from 3 340 kg to 4
330 kg/ha, while 40 kg N/ha raised it to 5 560 kg/ha (Dabadghao & Shankarnarayan,
1970). Responses up to 1 000 kg N/ha per year were recorded at Marandellas, Zimbabwe. At
Rodd's Bay, Queensland, 't Mannetje (1972) obtained a linear response to increasing
nitrogen, the level of response being linearly related to rainfall in both dry matter and
nitrogen recovery.
Genetics and reproduction
2n=20, 40, 44, 50, 60, 80 (Fedorov, 1974). It is an obligate aposporous apomict. A
higher proportion of flowers are male in inflorescences developed in long days than those
developed in short days, and the most rapidly maturing types at any day length show a
higher proportion of female inflorescences.
Economics
This grass is highly esteemed as a summer fodder grass in India and it can be made into
hay (Bor, 1960). The presence of the awned seed in wool causes "vegetable fault"
and increases processing costs. In addition, the seed pierces the skin and penetrates the
flesh of sheep, resulting in irritation and loss of wool production and downgrading of
carcasses. This grass dominates the beef- raising areas in central coastal Queensland,
from which a large proportion of fat cattle are supplied to the meatworks in autumn. From
autumn to early summer it is unproductive.
Animal production
Generally in Queensland, Australia, the carrying capacity of native spear grass
pastures is one beast to 3.5-4 ha in the southern area and 8-10 ha in the north of the
region. Cattle lose weight in the winter and spring and take four to five years to reach
market weight. On the granitic soils at Narayen, Queensland (lat. 25°60'S, 710 mm rain),
native Heteropogon contortus pastures normally carry 0.27 steers per hectare and produce
about 30 kg/ha per year live-weight gain. The addition of a legume such as Stylosanthes
guianensis (Oxley fine-stem stylo) or S. humilis (Townsville stylo) plus superphosphate at
125 kg/ha per year improved the carrying capacity to 0.8 steers per hectare and
live-weight gain was increased to 100 kg/ ha per year. A fully sown pasture of Biloela
buffer grass (Cenchrus ciliaris) and siratro (Macroptilium atropurpureum) grown in this
soil and fertilized with 125 kg/ha per year superphosphate carried one beast per hectare
and gave live-weight gains of 160 kg/ha per year ('t Mannetje, 1976). At Rodd's Bay,
Queensland (lat. 23°50'S, 813 mm rain), H. contortus dominant pasture stocked at 0.27
beasts per hectare, which is the normal carrying capacity, gave an average live-weight
gain of 84 kg/ha and 10 kg/ha per year over a seven-year period in which five of the seven
years' rainfalls were below average. At a higher stocking rate of 0.62 beasts/ha, the
live-weight gains were 47.3 kg per head and 12 kg/ha. When fertilized with 405 kg
superphosphate and 58 kg potassium chloride and the trace element molybdenum, the
live-weight gains at 0.62 beasts per hectare were 100 kg per head and 25 kg/ha. When
Stylosanthes humilis was included with H. contortus without fertilizer, the legume
improved the figures to a stocking rate of 0.77 beasts per hectare with live-weight gains
of 121.4 kg per head and 60 kg/ha and in two of the years the carrying capacity reached
0.8 beasts per hectare. Moreover, half the steers in the H. contortus! S. humilis
unfertilized pasture, and practically all the steers in the full treatment were marketed
one year earlier (Straw & 't Mannetje, 1970).
Further reading
Shaw & Bisset, 1955; Shaw and 't Mannetje, 1970; Tothill, 1970.
Value for erosion control
It has proved useful in soil erosion control on 20° slopes in India (Misra, Ambasht
& Singh, 1977).
Tolerance to salinity
It cannot tolerate high levels of salinity (Isbell, 1969).
Latitudinal range
In Queensland it occurs mainly between latitudes 19 and 27°S.
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