Stylosanthes scabra (PROSEA)
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Introduction |
Stylosanthes scabra Vogel
- Protologue: Linnaea 12: 69 (1838).
- Family: Leguminosae
- Chromosome number: 2n= 40
Vernacular names
- Shrubby stylo (En).
Origin and geographic distribution
Shrubby stylo is widespread in Brazil, but also occurs in Bolivia, Colombia and Venezuela. It is also cultivated in other tropical regions besides South America, e.g. in Australia and India.
Uses
Shrubby stylo is mainly used as a pasture legume for semi-arid to humid tropical regions with an average annual rainfall of 500-2000 mm. In the State of Maharashtra, India, it is cut and fed green to dairy cattle.
Properties
The N and P concentrations of whole plants sampled at flowering averaged 1.7% N (range 1.1-2.2%) and 0.07% P (range 0.07-0.11%). The N and P concentrations and in vitro digestibility (IVD) of leaf and stem fractions generally differ from each other and decline during the growing season. In leaves, the mean N concentration falls from 3% to 1.5%, P concentration from 0.3% to 0.1%, and IVD from 70% to 50%. In stems, the corresponding changes were 1.5% to 0.5% N, 0.3% to 0.03% P and 60% to 30% IVD. Shrubby stylo has a high proportion of stem which increases from 20% in the early growth stage to 75% at the end of the growing season. In grazed pastures, stem contents of 80-96% have been recorded. There are 400-550 hulled seeds, and 600-800 dehulled seeds per g.
Description
A perennial subshrub, erect to suberect to 2 m tall with a strong taproot with small round root nodules and penetrating deeply to depths up to 4 m. Stems with variable branching, covered by short or long hairs with bristles, usually viscose, pale green to dark blue-green, brown or red. Leaves trifoliolate; leaflets elliptical to oblong-laceolate, apex obtuse to mucronate, hairy on both surfaces with 4-7 prominent veins thickening terminally, pale green to dark green or dark blue-green; terminal leaflet 20-33 mm × 4-12 mm, length/breadth ratio 2 to 5; petiole up to 10 mm long, canaliculate above, scabrous with dense short hairs, the rachis 4-5 mm long; stipules obovate, 15-25 mm long including teeth, bristly-hairy. Inflorescence obovoid to ellipsoid, generally longer than wide, 10-30 mm long, several-flowered, each flower surrounded by a bract and 3 bracteoles; bracts unifoliolate, the blade about 4 mm long, the sheath 3.5-6.5 mm long with long teeth; outer bracteole 1, 2-4.5 mm long, bifid and ciliate at apex; axis rudiment 4-5 mm long; inner bracteoles 2, 2-4 mm long, ciliate at apex; calyx tube 3-6.5 mm long, lobes 1.5-3.5 mm long; standard suborbiculate, ca. 7 mm long. Fruit a loment with usually two fertile articles about 2.5 mm broad, reticulately nerved; upper articulation 2-4 mm long, shortly hairy, the lower 2-3 mm long, evenly pilose throughout; beak uncinate, with short hairs, 1-2 mm long, ½ to _ as long as the upper articulation. Seed asymmetrically reniform, small (< 2 mm long), pale to light brown, radical end prominent.
Growth and development
Shrubby stylo appears to have a juvenile phase during which floral initiation does not take place, followed by a short-day flowering response; the critical photoperiod lies between 11.5-12.5 hours, depending on the accession. Seed yields stay at a peak for a long period but vary with the accession and season; 70-90% of the seed may be hard-seeded.
Other botanical information
S. scabra is part of a species complex in which the species cannot be separated satisfactorily. Besides S. scabra , the complex comprises S. tuberculata Blake, S. nervosa Macbr., S. fruticosa (Retzius) Mohlenbrock, and S. suffruticosa Mohlenbrock. If, after a badly needed taxonomic revision, the whole complex is reduced to one taxon, the correct name will be S. fruticosa for reasons of priority.
Although shrubby stylo is very variable, most plants can be readily classified into morphological-agronomic groups that reflect geographical and climatic ecotypes. In Brazil the species is very widespread and two major forms have been recognized. One, often described as S. aff. scabra or S. aff. hamata by some authors, has thin stems with long, narrow leaflets, length/breadth ratio 4 to 5, with dark blue-green stems and leaves, and produces seed all year; this form mainly occurs in the coastal states of Maranhïo, Pernambuco Bahia and Rio de Janeiro. The other major form, which is more readily accepted as S. scabra , has thick woody stems and can be grouped into coastal types and continental types. The coastal types are tall robust subshrubs, leaflets elliptical and a length/breadth ratio of 2.0 to 2.7. The continental types are shorter in growth habit and more densely branched, leaflets lanceolate or elliptical, length/breadth ratio 2.3 to 3.9. The continental types mainly occur in the states of Goiás, Minas Gerais, Matto Grosso, Matto Grosso do Sul, Piauí and São Paulo.
The shrubby stylos from Colombia and Venezuela are morphologically different from the Brazilian material but more closely resemble the continental types than the coastal types of Brazil by their short growth habit and dense branching. The geographical plant groups reflect the climatic and edaphic differences between the regions.
The cultivars released in Australia are: "Seca", an anthracnose-resistant cultivar which is widely adapted in Queensland; "Fitzroy", an early flowering and leafy cultivar which is very susceptible to anthracnose and is no longer used; and "Siran", bred to provide a multigenic resistance to anthracnose.
Ecology
Shrubby stylo is very drought resistant and can survive long dry periods. It is adapted to tropical and subtropical environments with an average annual rainfall of 500-2000 mm and can tolerate light frost to -3 °C; heavy frosts can kill the crowns. It grows on a wide range of soil types, but is particularly well adapted to infertile, moderately acid, sandy surfaced soils with very low P levels. This adaptation is enhanced by its efficiency in extracting less available forms of soil P. Shrubby stylo can grow on heavier textured soils than most other stylos, but is not suited to heavy black basaltic clays.
Propagation and planting
Shrubby stylo is propagated by seed. Hard-seededness can be broken by mechanical scarification to improve germination. Under field conditions, substantial softening of hard seed occurs when the maximum diurnal soil temperatures reach 50-55 °C and is largely restricted to the late dry and pre-wet period in the tropics and to late spring and summer in the subtropics. Seeding rate is 1-2.5 kg/ha with the lower figure for drier regions. In semi-arid environments, the seed can be surface sown onto existing pasture following a burn; in humid environments when sown together with improved grasses, a well prepared seed-bed is desirable. Shrubby stylo nodulates freely with cowpea type rhizobia and does not require inoculation. In drier regions it should be sown with S. hamata (L.) Taub. cultivars "Verano" or "Amiga". In wetter areas it should be sown with other legumes such as Aeschynomene americana L. cultivars "Glenn" or "Lee" and S. guianensis (Aublet) Swartz cultivar "Cook" could be included in the mixture.
Husbandry
Shrubby stylo is used by continuous or rotational grazing or cutting. Because it can grow at very low P levels, it may be necessary to apply 10-20 kg/ha of P as superphosphate or alternatively, directly supplement the grazing animals with P to meet their dietary requirements. Although it is readily eaten by cattle and sheep, it is not very palatable. As with most Stylosanthes species, young growth of shrubby stylo is less palatable than young growth of grasses. This assists in seedling establishment and regeneration of shrubby stylo and enables long-term persistence.
Diseases and pests
Shrubby stylo is susceptible to anthracnose disease caused by the fungi Colletotrichum gloeosporioides and C. dematium ; the cultivars "Seca" and "Siran" are resistant to anthracnose. Blight ( Sclerotium rolfsii ) has been recorded on shrubby stylo in Thailand. Two important stem-borers damage shrubby stylo: Caloptilia sp. in Brazil and Colombia, and Platyomopsis pedicornis in Queensland, Australia. Calotilia sp. is the major pest of Stylosanthes in South America. The larvae tunnel through lower stems, considerably weakening susceptible accessions which usually die within 2 years. Field screening has identified accessions that are resistant to stem-borer. Isolated occurrences of Platyomopsis have been recorded in Queensland but it is not a serious pest; the larvae hollowed the taproot and lower main stem and killed the infested plants. Termites have been recorded on shrubby stylo in Queensland and they can kill older plants.
Harvesting
Shrubby stylo is harvested by grazing animals or it is cut for stall feeding. When mown, care should be taken not to cut the woody stems too low, otherwise regrowth will be adversely affected and some plants will die. In semi-arid environments, forage can be made into hay towards the end of the growing season.
Yield
In pure stands, shrubby stylo can produce DM up to 10 t/ha in areas of high rainfall, and its contribution in mixed pastures can amount to 2-7 t/ha depending on soil fertility and moisture levels. Annual liveweight gains of cattle of 140-160 kg/head have been recorded from shrubby stylo pastures on lightly fertilized poor soils in northern Queensland (Australia).
Seed yields of up to 620 kg/ha have been recorded.
Genetic resources
Germplasm collections are maintained at ATFGRC (CSIRO, Australia) and CIAT (Colombia).
Breeding
Plant breeding programmes are in progress in Queensland, Australia. Cultivar "Siran", a composite of 3 lines, was released in August 1990; it incorporates a multigenic resistance to anthracnose derived from crosses and selections of the anthracnose-resistant accessions Q 10042 × CPI 93116 (Line 1), an early flowering selection of "Seca" (Line 4), and CPI 55860 × Q 10042 (Line 8). Lines 1, 4 and 8 have been named "Jecuipe", "Recife" and "Feira" respectively for plant cultivar rights registration. Current research has shown that partial resistance to anthracnose is stable, heritable and improvable through breeding. Future plant breeding objectives will aim at combining both major gene and partial resistance genes against anthracnose in new cultivars.
Prospects
The extreme drought resistance and wide range of adaptation of shrubby stylo will encourage further plant improvement programmes through plant breeding and the selection of naturally occurring genotypes. Besides resistance to anthracnose, other desirable objectives for plant improvement include increased frost resistance and higher nutritive value.
Literature
- Edye, L.A., Burt, R.L., Nicholson, C.H.L., Williams, R.J. & Williams, W.T., 1974. Classification of the Stylosanthes collection 1928-1969. CSIRO, Australia, Division of Tropical Agronomy Technical Paper No 15. 28 pp.
- Mass, B.L., 1989. Die tropische Weideleguminose Stylosanthes scabra Vog. - Variabilität, Leistungsstand und Möglichkeiten züchterischer Verbesserung [The tropical pasture legume Stylosanthes scabra Vog. - variability, productivity and improvement possibilities through breeding]. Landbauforschung Völkenrode, Sonderheft 97. Institut für Grünland-und Futterpflanzenforschung, Braunschweig, Deutschland. 191 pp.
- Stace, H.M. & Edye, L.A. (Editors), 1984. The biology and agronomy of Stylosanthes. Academic Press, Sydney, Australia. 636 pp.
Authors
L.A. Edye & A. Topark-Ngarm
