Macroptilium atropurpureum. Photo copyright ©2007 by Kristopher Kupsch
Morphological description. Perennial herb with deep, swollen taproot and trailing, climbing and twining stems. Stems at the base of older plants fibrous, >5 mm diameter; younger stems mostly 1-2 mm diameter, pubescent to densely pilose with white hairs; occasionally forming nodal roots under ideal conditions. Leaves trifoliolate; leaflet blades 2-7 x 1.5-5 cm, darker green and finely hairy on the upper surface, grey-green, pubescent on the lower surface, with leaf margins sparsely to densely, finely ciliate. Lateral leaflets ovate to narrowly elliptical, obtuse, mostly with one or two, shallow to deep lobes on the outer margin; terminal leaflet rhomboid, with or without single lobe either side. Inflorescence a raceme comprising 6-12 often paired flowers on a short rachis, peduncle 10-30 cm long. Flower: calyx campanulate-tubular, hairy, tube 8-9 mm long and 3 mm wide, corolla with conspicuous wings 15-17 mm long, deep purple with reddish tinge near base; standard smaller than wings, obovate deltoid at base, reflexed, yellow-green with a purple tint; keel pink, forms a complete spiral. Pods straight, linear-cylindrical, acuminate-beaked, pubescent, 5-10 cm long, 3-5 mm diameter, containing up to 12 (-15) seeds. Pods dehisce violently (shatter) when ripe. Seeds speckled, light brown to black, flattened ovoid , 4 x 2.5 x 2 mm, 75,000/kg.
Scientific name. Macroptilium atropurpureum (DC.) Urb.
- Macroptilium atropurpureum (DC.) Urb. var. atropurpureum
- Macroptilium atropurpureum (DC.) Urb. var. canescens (M.Martens & Galeotti) Hassl. Phaseolus affinis Piper
- Phaseolus atropurpureus DC. [basionym]
- Phaseolus canescens M.Martens & Galeotti
- Phaseolus dysophyllus Benth.
- Phaseolus schiedeanus Schldl.
- Phaseolus vestitus Hook.
Family/tribe. Family: Fabaceae (alt. Leguminosae) subfamily: Faboideae tribe: Phaseoleae subtribe: Phaseolinae. Also placed in: Papilionaceae.
Common names. atro, siratro, purple-bean, purple bush-bean (English); purpurbohne (German); conchito (Spanish).
Native to: North America: USA (Texas), Mexico. Mesoamerica: Belize, El Salvador, Guatemala, Honduras, Nicaragua, Panama. Caribbean: Bahamas, Cuba, Guadeloupe, Hispaniola, Martinique, Puerto Rico, St. Lucia. South America: Brazil, Colombia, Ecuador, French Guiana, Peru.
Naturalised: Widely cultivated and naturalised throughout tropics and subtropics.
Mainly used for permanent and short-term pastures. While best suited to grazing, it can also be used for cut-and-carry or conserved as hay (usually with an associated grass), although the twining habit can make harvesting difficult. It is also used for soil conservation (re-vegetation/stabilisation of earthworks in cuttings and on embankments), and as a cover crop, fallow crop (including after lowland rice), or as a forage crop sown with upland rice. Its value as a protein bank for dry season feeding is reduced by its tendency to drop leaf under very dry conditions.
When used as a short-term ley legume in sorghum crops, the nitrogen benefit can persist for up to 3 crops of sorghum. However, large populations of the twining legume can regenerate from hard seed in the soil, creating difficulties in grain harvest.
Soil requirements. Occurs on a wide range of soils ranging from dark cracking clays, to yellow and red clays, to littoral dunes, red sands and gravels. Soil reaction at collection sites ranges from pH (5.5-) 6.5-8.0 (-8.5). In cultivation, 'Siratro' has been successfully established in soils with pH 5.0 (and even as low as 4.5), and in coral rubble (pH 8.5). It thrives in friable soils, but declines fairly rapidly in hard-setting soils. Tolerant of moderate levels of soil Al and Mn, and better tolerance of salinity than most tropical forage legumes. While preferring well-drained soils of moderate fertility, some collections have been made at poorly drained sites and on infertile soils.
Moisture. Occurs in areas with annual rainfall from (120-) 650-1,500 (-2,500) mm. In cultivation, it is mostly sown in areas with rainfall between about 700 mm and 1,500 mm. Leaf disease can be a problem in higher rainfall environments. It is well adapted to drought, possessing a deep taproot and the ability to minimise evapo-transpiration by virtue of pubescent leaves, and reduction in leaf size and shedding of leaves in response to the onset of dry conditions. It is intolerant of flooding or waterlogging .
Temperature. Found in areas from 30ºN in Mexico to about 22ºS in Brazil, and at altitudes from near sea level in Panama (8º 37'N) to 2,100 m asl in Mexico (28º 30'N) and 2,900 m asl in Ecuador (1º 40'S).
Cultivars grow best in the subtropics (to latitude 30º) and higher altitude tropics with warm season growth, achieving best production with day/night temperatures of 27-30/22-25ºC; growth is poor below 18/13ºC day/night. Leaves are burnt by light frost. More severe frosts kill the plant back to the crowns, but plants recover with the onset of warm, moist conditions.
Light. M. atropurpureum is fairly intolerant of shade, and is best grown in full sunlight. [N.B. This observation is not consistent with the PhD thesis of Heath James Addison, see separate article here.] Siratro seedlings do not establish under the dense shade of a closed canopy . Chemical composition and dry matter digestibility are largely unaffected by shade down to 40% sunlight.
Reproductive development. Flowering is initiated in response to the onset of dry periods and to shortening days. Vegetative growth resumes with the return of moist conditions. Pods shatter when mature. Seeds can rot in pods under wet conditions during seeding.
Defoliation. It prefers light stocking rates and/or rotational grazing. A rotational grazing system of 2 weeks on, 4 weeks rest has given good results in the humid subtropics. Stands deteriorate within a year or two under regular heavy grazing due to loss of bud sites, shorter plant life, reduced seed set, and declining soil seed reserves. Even with judicious management, they tend to become less productive over 5-7 years. While individual plants may persist for 4 years or more, pastures benefit from periodic spelling (every 2-3 years) during seed set to bolster soil seed reserves and reinvigorate plants. Contour ripping with a tyned implement can hasten the rejuvenation of stand over simply resting.
Fire. After fire, plants recover well from crowns or stems, and from new seedlings emerging from soil seed reserves.
Macroptilium atropurpureum. Photo copyright ©2012 by CIAT/ILRI
Guidelines for the establishment and management of sown pastures.
Establishment. Hard seed levels are often high and seed should be scarified before planting. Mechanical scarification can increase germination from 10% to 80%; treatment with concentrated sulphuric acid for 25 minutes can be used on small samples (seed should be washed thoroughly and dried after treatment). Suction-harvested seed is usually sufficiently scarified by the harvesting process. Seed is best sown at 1-2 (-5) kg/ha into a well-prepared seedbed. Seedlings are vigorous and can also establish with minimal cultivation, into herbicide-killed grass , or following a burn. However, the less competition for the developing seedling, the better the establishment. M. atropurpureum has a broad symbiont range for nodulation, and can nodulate effectively on native rhizobia in most soils. If in doubt, seed should be inoculated with 'cowpea' type inoculum such as CB 756 (Australia).
Fertiliser. Additional phosphorus may be required at establishment (20-30 kg/ha), and for maintenance 10-20 kg/ha/yr P on soils with less than about 8 ppm available phosphorus. The need for other nutrients, particularly K and S, should be monitored by soil analysis or foliar deficiency symptoms. Siratro is responsive to molybdenum, requiring 100-200 g/ha Mo at establishment on deficient soils, and 100 g/ha Mo every 4-5 years as maintenance.
Compatibility (with other species). Combines best with erect or tussock grasses, and legumes requiring similar grazing management. It is less compatible with aggressive creeping grasses such as Paspalum notatum and Digitaria eriantha (pangola).
Grasses: Chloris gayana , Cenchrus ciliaris , Panicum maximum , Setaria sphacelata .
Legumes: Chamaecrista rotundifolia , Desmodium intortum , Macrotyloma axillare , Neonotonia wightii , Stylosanthes guianensis , S. hamata , S.scabra, S. seabrana .
Pests and diseases. During periods of high rainfall and high humidity, foliar blight caused by Rhizoctonia solani can reduce leaf yields by 80%. Plants recover with a return to dry conditions. Bean rust (Uromyces appendiculatus var. crassitunicatus) gives rise to many brown pustules on the leaves of susceptible varieties, leading to severe leaf drop and pasture quality decline. 'Aztec' was bred to overcome this weakness in 'Siratro'. False rust (Synchytrium phaseoli) develops during periods of high humidity and can be devastating. 'Siratro' is a known host crop for (Asian) soybean rust (Phakopsora pachyrhizi). Angular leaf spot or grey leaf spot (Phaeoisariopsis (Isariopsis) griseola) can cause moderate to severe leaf loss under cool conditions. Halo blight (Pseudomonas syringae pv. phaseolica), which is carried on, but has little effect on 'Siratro', can infect nearby french beans (Phaseolus vulgaris) causing serious damage to the bean crop. Viruses affecting M. atropurpureum include mung bean yellow mosaic virus and passionfruit woodiness potyvirus. Legume little leaf disease caused by a phytoplasma and spread by leafhoppers (e.g. Orosius orientalis) can kill individual plants but does not cause significant loss. Resistance to the root-knot nematodes, Meloidogyne arenaria, M. hapla, M. incognita, and M. javanica is common in M. atropurpureum , although specific resistances between lines may vary. 'Siratro' is resistant to the burrowing nematode (Radopholus similis) but not spiral nematode (Helicotylenchus dihystera). Bean fly larvae (Melanagromyza phaseolin Diptera: Agromyzidae) burrow in the stems often killing late-planted 'Siratro' seedlings. The bean leaf roller or long-tailed skipper (Urbanus proteus Lepidoptera: Hesperiidae) attacks 'Siratro' in late summer and autumn. Adults of rough brown weevil (Baryopadus (Leptopius) corrugatus Coleoptera: Curculionidae) feed on foliage, and larvae can severely damage roots.
Ability to spread. 'Siratro' is mostly spread by seed, since stolon development is typically very weak. Seed is forcibly ejected from the pod and can be thrown for several metres. It can be spread great distances through water movement and following ingestion by cattle.
Weed potential. It is now considered a weed of ungrazed areas (e.g. roadsides, waste land, domestic gardens) and can also create problems in sugar cane where the tough climbing stems can interfere with mechanical harvesting.
Nutritive value. Nutritive value generally good, although crude protein, mineral content and digestibility can be increased by providing optimal P, S and Mo. CP and IVDMD values of tops varies from about 12% and 45% in older material to 25% and 65% in younger material. Average CP, P and ADF in leaves in the top 15 cm of growth have been measured at 25, 0.26 and 24%, and in stems at 15, 0.22 and 41%, with leaves constituting 76% of the dry matter.
Palatability/acceptability. Readily accepted, although cattle prefer fresh young grass early in the growing season . Siratro is heavily browsed by deer, and quail are attracted to the seed crop.
Toxicity. None recorded.
Dry matter. DM yields are mostly in the range of 5-10 t/ha/yr, although yields are lower under more regular defoliation or grazing.
Animal production. Many grazing trials have been conducted with 'Siratro'. LWG/ha is good at medium stocking rates but yields/ha are moderated because of legume decline and loss at high stocking rates.
In subtropics in Australia, annual LWG improved from 100 kg/hd to 140-200 kg/head when native pasture of Heteropogon contortus was replaced with Cenchrus ciliaris and 'Siratro'. LWG /ha increased from 30 kg/ha/yr to 170 kg at higher stocking rates.
Seed production. Seed can be either hand or machine harvested. For hand harvesting, ripe pods should be picked early in the day because as the day progresses, ripe pods dry out and dehisce violently with little stimulation. For larger scale commercial production, growth flushes are produced through irrigation with flowering occurring as moisture declines. Crops are then fairly synchronous, and can be direct-headed when the majority of pods are ready to shatter. Fallen seed can then be collected with a suction harvester. Seed yields vary greatly from (40-) 100-300 (-1,000) kg/ha.
Herbicide effects. Trifluralin is used for pre-emergence and fluazifop or sethoxydim for post-emergence grass weed control in seed crops. Also tolerant of benfluralin and acifluorfen, but susceptible to bentazone, 2,4-D and 2,4-DB.
- Wide soil adaptation.
- Drought resistant.
- High nutritive value and palatability .
- Good N fixation.
- Needs moderate fertility or added fertiliser.
- Intolerant of poor drainage.
- Declines under grazing.
- Susceptible to leaf disease.
Other comments. Soil seed levels under lightly stocked stands have been measured at 500/m² (67 kg/ha), and 100/m² (13 kg/ha) in heavily stocked stands.
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