Heteropogon contortus (PROSEA)
Heteropogon contortus (L.) P. Beauv. ex Roemer & Schultes
- Protologue: Syst. Veg. 2: 836 (1817).
- Family: Gramineae
- Chromosome number: 2n= 20, 40, 44, 60, 80
Andropogon contortus L. (1753).
- Speargrass, black or bunch speargrass (En). Tanglehead (Am). Herbe polisson (Fr)
- Indonesia: bejeng-benjeng, merakan (Javanese)
- Philippines: sibat-sibatan (Tagalog)
- Thailand: ya-nuatrusi (central), ya-lem (northern).
Origin and geographic distribution
The natural distribution of speargrass is pantropical/subtropical. It is most likely of Gondwanan origin with a predominant distribution in Australia, the Indian subcontinent and Africa. However, it may be found throughout South-East Asia and the Pacific and in South and Central America in sporadically occurring suitable habitats.
Speargrass is used as part of the naturally occurring savanna pasture resources for domestic livestock production and wildlife. However, pastures containing speargrass are not suitable for wool-producing sheep. The most severe limitation to animal production from speargrass pastures is the wide fluctuation of growth and herbage quality between wet and dry season.
In unimproved pasture its N concentration ranges from 2.5% in very young green material from the early flush of the growing season to as low as 0.3% in dry forage by the end of the dry season. Digestibility varies similarly from 60% down to 40%. The sharply pointed and barbed seeds can be a problem to both humans and animals, particularly woolly sheep.
A tufted perennial, 0.5-1.5 m tall, rather variable in habit with stems erect to geniculate at the base, often branched above, particularly at flowering, flattened towards the base. There is a tendency to being weakly rooted late in the dry season. Stem nodes smooth and hairless. Leaves green or bluish, basal and on the culm; leaf-sheath smooth, compressed, keeled, striate, sometimes with a few hairs near the ligule; ligule a short membranous rim; leaf-blade linear, 3-30 cm × 2-8 mm, folded in the lower part, becoming flat, slightly rough to the touch with a few long hairs, particularly towards the base, apex blunt, almost canoe-shaped. Inflorescence a simple raceme of spikelet pairs arranged in two rows, 3-8 cm long (excluding the awns), the outermost of these are pedicellate and overlapping and enclosing the innermost sessile spikelets; at the base of the raceme the lowermost spikelets are similar and unawned, male or neuter, while the uppermost pairs are of dissimilar bisexual awned sessile and male or neuter unawned pedicellate spikelets; the base of the spikelet has a pronounced callus which is strongly bearded upwards, which at maturity disarticulates obliquely to form a very sharp point, the whole structure forming a most effective barb; the awns, which tangle conspicuously at maturity, are an extension of the inner lemma of the fertile spikelet and 5-12 cm long; they are geniculate, twisted and hygroscopically active, enabling them to bore into the ground. Caryopsis cylindrical, 3.5-4.5 mm long, grooved, whitish.
Speargrass exhibits a high degree of embryo and seed-coat dormancy in freshly ripened seed. This declines over the dry season to a high level of germination at the opening rains of the next wet season, following which the germination falls off rapidly with little or no survival beyond one year. Seeds which are burrowed into the soil are in a favourable environment for germination. Flower initiation is obligately to facultatively short-day, the former being characteristic of the intertropical forms, which flower late in the wet season, and the latter of the subtropical forms which flower early to mid-season. Speargrass is an apomict and this has led to a considerable amount of localized variation, resulting in the proliferation of local species and varieties in the early botanical literature.
An annual form has been reported from India, but this could be a short-lived perennial form growing in a difficult environment. The late-flowering forms have higher yields of leaves and are more responsive to increased soil fertility.
As a typical member of the grass tribe Andropogoneae Dumort., speargrass is a savanna grass in seasonally warm wet and cooler dry climates of moderate rainfall. It is relatively drought-tolerant and intolerant of prolonged waterlogging or high salinity. Its main range of distribution is in areas of 600-1100 mm of annual rainfall with some extension below this (on deep sands of drainage lines) and above this (on well-drained sites). It will grow on a wide range of well to moderately free-draining soil types. Speargrass appears not to have been a climax dominant but it has become so, particularly in Australia, India and Africa, through habitat disturbance such as heavy grazing and burning.
In its naturally occurring state in savanna, speargrass has a relatively short period of growth and adequacy for good livestock nutrition. This causes a large part of the annual biomass production to become unusable and left as dry standing material. Commonly this material is burnt off during or at the end of the dry season so that the new season's growth may come away cleanly. This procedure leads to increasing dominance of speargrass in the pasture, especially in conjunction with grazing. However, speargrass can be reduced to a minor component of pastures when subjected to sustained overgrazing, particularly on poorly fertile soils or where high stocking rates are sustained through supplementary feeding. Up to an 8-fold improvement of livestock production from pastures based on naturally occurring speargrass can be achieved by introducing tropical legumes such as Stylosanthes spp. or Macroptilium atropurpureum (DC.) Urban in the subtropical zones. This improvement comes from both increased carrying capacity and rate of growth of the livestock. However, care must be taken, especially in the more strongly seasonal tropics, not to overgraze speargrass as a result of the increased stocking rates.
The following smut diseases of the inflorescences have been reported: Sorosporium antheseriae, S. caledonicum and Sphacelothica monilifera .
Annual DM yields can range from less than 500 kg/ha in very dry years to 8000 kg/ha in very good years when associated with an effective legume or on fertile soils.
Genetic resources and breeding
It is unlikely that substantial germplasm collections of speargrass are being maintained. There are no breeding programmes. While there is considerable genetic variability for selection, there are two serious obstacles to breeding: it is an obligate apomict and it has a most unpractical seed morphology for easy seed production.
If speargrass is not overgrazed during the growing season, it can perform very well as the basic component of native or improved pastures in the tropical and subtropical savanna zone. It is more likely to be used in extensive commercial beef cattle production than by smallholders.
- Bogdan, A.V., 1977. Tropical pasture and fodder plants. Longman, London. pp. 150-152.
- Bor, N.L., 1960. The grasses of Burma, Ceylon, India and Pakistan. Pergamon Press, Oxford. pp. 163-165.
- Langdon, R.A. & Fullerton, R., 1978. Micotaxon 6: 421-456.
- Lazarides, M., 1980. The tropical grasses of southeast Asia. Phanerogamarum Monographiae Tomus XII, J. Cramer, Vaduz. pp. 46-47.
- Skerman, P.J. & Riveros, F., 1990. Tropical grasses. FAO, Rome. pp. 446-449.
- Tothill, J.C. & Hacker, J.B., 1983. The grasses of southern Queensland. University of Queensland Press, Brisbane. pp. 258-260.
- Tothill, J.C., Nix, H.A., Stanton, J.P. & Russell, M.J., 1985. In: Tothill, J.C. & Mott, J.J. (Editors): The ecology and management of the world's savannas. Australian Academy of Science, Canberra. pp. 125-144.