Macroptilium atropurpureum (PROSEA)

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Plant Resources of South-East Asia
List of species

Macroptilium atropurpureum (DC.) Urban

Protologue: Symb. Antill. 9: 457 (1928).
Family: Leguminosae
Chromosome number: 2n= 22


Phaseolus atropurpureus DC. (1825).

Vernacular names

  • Siratro, atro, purple bean (En)
  • Thailand: thua-sirato.

Origin and geographic distribution

This species occurs naturally from northern Mexico to Colombia and northern Brazil. The greatest diversity is in Mexico. Being one of the first commercially available tropical legumes, siratro cultivar "Siratro" is now widespread throughout the tropics and subtropics.


Siratro is primarily used as a pasture legume with introduced or natural grasses but could be useful as a cover crop or as a fallow crop in shifting cultivation. Siratro is used for revegetation and stabilization of earthworks, and in road and railway cuttings and embankments. It has shown promise as a leguminous forage crop sown in conjunction with upland rice or after harvesting of lowland rice.


Nitrogen concentrations range from 2-4%, higher than some other tropical legumes such as Stylosanthes spp. Digestibility of young material ranges from 60-70%, but in older material with a lower leaf percentage this may decrease to 45%. It tends to have higher P concentrations than Stylosanthes spp. with levels usually above 0.2%, reflecting its higher P requirement. Sodium concentrations are usually less than 0.02%. It is a palatable legume though cattle prefer to eat grass early in the growing season and siratro later in the growing season. There are 70-90 seeds/g.


A perennial herb with twining stems arising from a taproot of up to 2 cm diameter. Stems close to the ground may develop secondary roots, usually on light textured soils when the soil surface remains continuously moist for some weeks; trailing stems rarely greater than 5 mm diameter but may exceed 5 m in length. Leaf trifoliolate, often 2-3-lobed, dark green and slightly hairy on the upper surface, silvery and very hairy on the lower surface; petiole up to 5 cm long; leaflets ovate to rhomboid, 2.5-8 cm × 2.5-5 cm, often asymmetrically lobed. Inflorescence a raceme; peduncle 10-30 cm long with 6-10 deep purple flowers, about 2 cm long, crowded at the apex; calyx campanulate, 5-lobed; standard orbicular with 2 small basal auricles; wings long stipitate, longer than standard and keel; keel petals apically spiralled, basally adnate to the staminal tube. Pod linear, straight, 4-8 × 4-6 mm, adpressed pubescent, usually with 10 seeds, dehiscing violently when ripe, throwing seed for up to 5 m. Seed oblong-ovoid, 3-4 mm long, light brown, black or mottled, the latter usually with greater hard-seededness.

Growth and development

Siratro sown into a seed-bed grows vigorously and will flower and produce seed in the first year. When broadcast into undisturbed pasture or emerging from seed in undisturbed grassland, growth is slower and seed production is unusual in the first year. The half-life of established plants is 1-4 years. Siratro is a short-day plant. The main flowering period is in the late wet season, but flowering also occurs in the early and mid wet season. The onset of flowering during the wet season partially relates to onset of moisture stress. Under light grazing pressure in higher rainfall areas and on sandy soils, siratro develops stolons. These can live for several years and attain 5 mm diameter. Siratro plants survive dry periods by shedding leaves and then through death of stems to reduce the area of transpiring leaves.

Other botanical information

Only one cultivar of siratro, "Siratro", is presently available. It was developed in Queensland about 1960 from a cross between two Mexican introductions.


Siratro grows best under rainfall regimes ranging from 700-1500 mm, but in Mexico has been collected from sites receiving annual rainfall as low as 250 mm. It is not suited to the wet tropics or to the very hot, dry tropics, nor to elevations above 1600 m in the tropics or to latitudes higher than 30°. Top growth is killed by frost, but the taproots survive frosts in subtropical regions. Growth is optimal at day/night temperatures of 30/25 °C. It grows best on light to medium textured soils and does not persist on poorly drained sites. It requires a soil available P level of at least 10 mg/kg (bicarbonate extraction) or preferably 15 mg/kg. It tolerates a pH(H2O) range of 5-8. There is considerable variation within siratro and some characters, such as an erect growth habit and fewer days to flowering, are related to drier areas of origin.

Propagation and planting

Siratro is established by seed. It is not as hard-seeded as many tropical legumes, but scarification may still be required. Establishment is faster and more reliable when sown into a seed-bed, but it can be established into undisturbed pasture or with minimal cultivation if conditions are favourable. Seeding rates of 2-6 kg/ha are recommended when sown alone. Siratro nodulates freely with native cowpea rhizobia and there are no records of nodulation failure.


In permanent grasslands siratro must be allowed to seed since recruitment of new plants to replace older plants is essential for long-term persistence. Soil seed banks of over 200 seeds/m2are desirable for persistence; levels below 50 seeds/m2may lead to extinction. Siratro responds well to P fertilizer, if required. Liming may be required if soil pH(H2O) falls below 5.0.

Diseases and pests

The main diseases of siratro are leaf blight (Rhizoctonia solani), which is more serious in the wetter end of its zone of adaptation and limits the use of siratro in the wet tropics, and rust (Uromyces appendiculatus). Rust is not a lethal disease but has been shown to reduce the yield and seed set of "Siratro" by 30%. There are different races of the rust pathogen but some lines of siratro have resisted a range of races under experimental conditions. Violet root rot (Rhizoctonia crocorum) has led to death of taproots. It is susceptible to halo blight (Pseudomonas phaseolicola) which, although of minimal consequence to siratro, can act as a source of infection to nearby crops of Phaseolus spp. Seedlings of siratro can be killed by larvae of bean fly (Melanagromyza phaseoli). Siratro is resistant to root-knot nematodes (Meloidogyne javanica).


Siratro is usually harvested by grazing animals, but can be cut for stall feeding, or for hay or silage. Siratro pastures can be continuously or rotationally grazed but cannot withstand sustained heavy grazing. If cut, a rest period of at least 6 weeks and a stubble height of at least 10 cm is suggested.


Under cutting, legume DM yields of 7 t/ha have been measured in pure swards and 5 t/ha in siratro/grass swards, although siratro yield declines markedly with more frequent cutting. DM yields of over 5 t/ha have been reported when used as an intercrop legume. It seldom contributes more than 30% of dry matter in grazed legume/grass pastures. Siratro can yield up to 1 t/ha of seed, although seed yields are usually 100-300 kg/ha. High seed yields can be obtained from hand harvesting seed from plants grown on wire trellises. Good siratro pastures can give the same animal production as grass-only pastures fertilized with 100 kg/ha of N in 700-1000 mm rainfall areas.

Genetic resources

The main germplasm collections are held at ATFGRC (CSIRO, Australia) and CIAT (Colombia).


A plant breeding programme to incorporate rust resistance into siratro, based at CSIRO, Brisbane, is nearing completion. There is scope for further development to select and breed cultivars of siratro for specific regional adaptation.


Although siratro is not as persistent as initially thought, it is still a very useful legume. Its ability to provide high yields of quality material, fix N and improve grass growth and quality, and its fairly wide area of adaptation, mean that further work on the development of new cultivars is justified.


  • Carangal, V.R., Tengco, P.L., Miah, N.I. & Topark Ngarm, A., 1988. Food and forage crop intercropping under rainfed lowland conditions. Proceedings of the Crop-Animal Systems Research Workshop, Serdang, Malaysia. MARDI/IRDC/ARFSN, Serdang, Malaysia. pp. 453-471.
  • Gutteridge, R.C., 1985. Survival and regeneration of four legumes oversown into native grasslands in northeast Thailand. Journal of Applied Ecology 22: 885-894.
  • Hutton, E.M., 1962. Siratro - a tropical legume bred from Phaseolus atropurpureus. Australian Journal of Experimental Agriculture and Animal Husbandry 2: 117-125.
  • Jones, R.J. & Jones, R.M., 1976. The ecology of siratro based pastures. In: Wilson, J.R. (Editor): Plant relations in pastures. CSIRO, Melbourne, Australia. pp. 353-367.
  • Jones, R.M. & Bunch, C.A. , 1988. The effect of stocking rate on the population dynamics of siratro in siratro (Macroptilium atropurpureum)-setaria (Setaria sphacelata) in south-east Queensland. Parts I and II. Australian Journal of Agricultural Research 39: 209-219, 221-234.
  • Oram, R.N., 1990. Register of Australian herbage plant cultivars. CSIRO, Australia. p. 239.
  • Skerman, P.J., Cameron, D.G. & Riveros, F., 1988. Tropical forage legumes. FAO, Rome. pp. 328-337.
  • 't Mannetje, L. & Jones, R.M., 1990. Pasture and animal productivity of buffel grass pastures with siratro, lucerne or nitrogen fertilizer. Tropical Grasslands 24: 269-281.


R.M. Jones & L. 't Mannetje