Macroptilium atropurpureum (DC) Urb

Photo1.jpg (1333 bytes)



Phaseolus atropurpureus DC.

Common names

Siratro. The unselected parents are commonly known as atro.


Deep-rooting perennial with trailing pubescent stems which may root anywhere along their length, especially in moist clay soils but rarely in drier sandy soils. Leaves pinnately trifoliate, dark green and slightly hairy on the upper, silvery and very hairy on the lower, surfaces. Lateral leaflets ovate, obtuse, about 4 to 6 cm, often asymmetrically lobed. 

The inflorescence is a raceme; peduncle 10 to 30 cm long, with 6 to 12 flowers crowded at the apex, deep purple with a reddish tinge near the base of the petals. Pod straight, about 7.5 cm long and many-seeded. Pods dehisce violently when ripe. Seeds from light brown to black, flattened ovoid in shape, 4 x 2.5 x 2 mm (Barnard, 1967). Distribution

Siratro was bred by E.M. Hutton (Queensland, Australia) from two ecotypes of M. atropurpureum from Mexico­one from the Vera Cruz waterfront and the other from near Matlopa in San Luis Potosi. M. atropurpureum occurs naturally in Central and South America.

Season of growth

Summer-growing perennial with greatest growth in midsummer to autumn in south-east Queensland.

Altitude range

In Kenya, it grows at elevations up to 1 600 m, but the temperature must be more than 15.5°C (Jones, personal communication).

Rainfall requirements

It requires at least 615 mm and preferably more than 850 mm. It does not thrive in high rainfall regions above 1 800 mm.

Tolerance of flooding

Extremely drought-tolerant by reason of its deep-rooting habit. In summer droughts, large leaves are shed and small leathery leaves produced until conditions are more favourable (Davies and Hutton, 1970). It is not tolerant of flooding.

Soil requirements

Thrives on a wide range of soils, except poorly drained ones (Davies and Hutton, 1970). Will grow in soils ranging from deep sands and loams to light clays (e.g. podzolic, deep latosolic and alluvial soils). Grows over a range of pH from 4.5 to 8.0. Kretschmer (1966) got best growth in Florida from an application of 2.2 tonnes of lime, which lifted the pH from 4.5 to 6.1. It is one of the best of the tropical legumes under moderately saline conditions.

Rhizobium relationships

It nodulates freely with native rhizobia, but seed should be inoculated at sowing with inoculum of the cowpea type. The current Australian strain recommendation is CB 756 (Date, 1969). Van Rensburg (1967) found that siratro had frequent large nodules on the taproots and laterals which were very easily dislodged. Whiteman and Lulham (1970) found that under heavy grazing there is evidence of nodule loss and replacement, as the grazed plants produced larger numbers of smaller nodules. Nodule weight peaked at the end of March (lat. 27°22'S) and then there was a rapid shedding of nodules, with few present during the winter.

Ability to spread naturally

Under favourable conditions, siratro will spread naturally, but it spreads more readily with some preliminary cultivation.

Land preparation for establishment

Establishes best in a well-prepared seed bed, and has been successfully established, though not so rapidly, in roughly prepared ground, in the ashes of a burn from forest debris or from Imperata grassland (Douglas, 1965), and by sod-seeding. In roughly prepared seed beds, the seeding rate should be doubled.

Sowing methods

It is preferably drilled into a well-prepared seed bed, but can be broadcast from the ground or by aerial seeding. If conditions are favourable, it can be oversown into existing pastures. Downes (1966) established it by oversowing into natural grassland in north Queensland. It can be sod-seeded into natural pastures with superphosphate and also oversown into burnt Imperata grassland with superphosphate. The seed is viable after passage through an animal and germinates in dung pats.
Sow at 1.5 to 2.5 cm into prepared seed beds and cover with a harrow or with a roller. Sow from spring to late summer. Whiteman and Lulham (1970) found that the best time to sow for yield in south-eastern Queensland was in December. If it is sown in early summer, it is more likely to escape bean-fly attack at the seedling stage. Van Rensburg (1967) found that sowing distances of 30, 60 and 90 cm made no significant difference to subsequent yield and regrowth in Zambia. Jones and Andrew (1967) found increasing yields by increasing sowing rates from 0.46, 1.84 and 3.38 kg./ha and increasing phosphorus rates from 250 kg./ha to 1 500 kg./ha. Middleton (1970) found seedling density proportional to the sowing rate. Seed is usually sown at 2 to 8 kg./ha.

Number of seeds per kg.

80 000. Percentage of hard seeds in commercial samples is from 40 to 70 percent.

Seed treatment before planting

To break down dormancy: (a) scarify mechanically; (b) treat with concentrated sulphuric acid (sp. gr. 1.8) for 25 minutes, wash and dry (Prodonoff, 1968). Inoculation is not necessary but preferable. Pelleting is not necessary unless to protect rhizobia; pellet with rock phosphate (Norris, 1967). For insect and disease control, treat the seed with 6 cc of dieldrin 15 percent emulsifiable concentrate per kg. seed (Jones, 1965).

Nutrient requirements

Siratro responds in podzolic and solodic soils to molybdenized superphosphate up to 500 kg./ha for maximum yields (Truong, Andrew and Skerman, 1967) but gives a yield increase from dressings of 125 kg./ha. On fertile soils, no fertilizer will be necessary for a number of years. There is need for balanced fertilization; Jones (1967a) showed that high phosphorus plots had 2 percent siratro, while high phosphorus-high potassium plots had 20 percent siratro. An annual application of 125 to 200 kg. superphosphate and 125 kg. potassium chloride should maintain production in this species on sandy soils.

  • Calcium: 

Siratro is not sensitive to calcium deficiency, but on acid soils molybdenum may be unavailable and addition of calcium to raise the pH released molybdenum; thus, it is really a molybdenum effect. Kretschmer (1966) got a direct response to lime up to 2.2 tonnes/ha in Florida, United States. Truong, Andrew and Skerman (1967) found that Ca gave no response, but that 125 and 250 kg./ha of superphosphate increased yield over nil treatment and maximum yield was at 500 kg./ha.

  • Magnesium: 

Siratro is relatively high in magnesium (Andrew and Robins, 1969b), and addition of calcium phosphate to the soil increased magnesium at the expense of potassium.

  • Molybdenum: 

It responded to molybdenum on a solodic soil (Truong, Andrew and Skerman, 1967).

  • Nitrogen: 

At about 100 kg./ha, siratro practically disappeared from a pangola grass mixture grown on a sandy soil in Florida (Brohlman, personal communication). Parbery (1967a) got no response to 100 kg. N/ha in the Kimberleys, northern Australia. Jones (1965) recorded 28 percent, 5.7 percent and a trace of siratro after four years' growth with Nandi setaria to which dressing of nil, 250 kg. and 750 kg./ha of urea had been applied.
Henzell et al. (1968) found that when grown separately, siratro and Rhodes grass each took up an equal amount of nitrogen, but when grown together only one-third was taken up by the siratro and two-thirds by the Rhodes grass.

  • Phosphorus: 

The critical percentage for phosphorus in the plant tops of siratro is 0.24 percent (Andrew and Robins, 1969a). At 250 kg./ha superphosphate, siratro yielded 54 percent of maximum yield at 1 320 kg./ha superphosphate when grown on a gley soil at Samford, Australia. No response to phosphorus was obtained on an alluvial soil derived from basalt.

  • Potassium: 

The critical percentage of potassium in the plant tops was determined by Andrew and Robins (1969c) to be 0.75 percent. It yielded 38 percent, at the nil treatment, of the yield at its maximum response at 185 kg./ ha. In the presence of high phosphorus, potassium may become deficient (Jones, 1966). Jones (1966) increased the percentage of siratro in a mixed pasture with Nandi setaria from 12 to 16 percent with adequate phosphorus to 42 percent with adequate P and 185 kg./ha muriate of potash. Andrew and Pieters (1970a) obtained healthy growth in siratro plants containing 1.30 g/kg. of K in the tops on a dry-matter basis, and 0.42 percent in a plant showing potassium deficiency. Deficiency symptoms commenced as a necrotic spotting on the lower leaves of the plant. This was not preceded by rust-coloured spotting, as in M. lathyroides. The necrotic spots were of pinhead size, irregular in shape and placed interveinally toward the margins of the leaflets. They were mid-brown in colour, surrounded by a pale chlorotic region in an otherwise normal green leaf, visible on both surfaces of the leaflets but with a sunken appearance, particularly on the lower surfaces of the leaflets. With increasing severity, the edges and tips of the leaflets became chlorotic and some of the necrotic spots enlarged and coalesced to give a marginal necrotic effect, particularly near the tip of the leaflets.
In some cases, the interveinal necrotic spotting was not evident, but the symptom commenced as marginal necrosis and interveinal chlorosis of the lower leaves. In severe cases of deficiency, affected leaves abscissed, and this effect progressed toward the younger portion of the plant. In this species, there was a suggestion that affected leaves tended to remain in a "sleeping" position during daylight (Andrew and Pieters, 1970a).

Compatibility with grasses and other legumes

It is compatible with Rhodes grass, buffel, green panic, guinea grasses and setarias. Middleton (1970) found siratro more competitive with Setaria sphacelata than it is with Desmodium intortum. In Florida, Kretschmer (1966) found that it grew better with pangola grass than with Pensacola bahia and Setaria anceps.

Tolerance to herbicides

Bailey (personal communication) found that siratro was one of the most susceptible of the tropical legumes to 2,4-D. It should not be used on this species.

Nitrogen-fixing ability

Siratro fixes a good deal of nitrogen­about 100 to 175 kg./ha/year. Kretschmer (1966) found that introducing siratro at 1.1 kg./ha on a 25-cm grid raised the crude protein level of pangola grass from 4.7 to 7.1 percent in Florida, and at 11 kg./ha of seed the protein content was 11.2 percent for the mixture and 5.8 percent for pangola grass alone. He estimated that siratro contributed 55 to 138 kg. N/ha/year. Jones, Davies and Waite (1967) found that the selections of M. atropurpureum produced as much nitrogen as grass fertilized with 187 kg./ha/year, but produced only as much dry matter as grass fertilized with 100 kg. N/ha. Henzell et al. (1968) found that 43 to 50 percent of the nitrogen in siratro plants at three weeks of age was taken up from the soil, the remainder coming from symbiotic nitrogen fixation, but at 15 weeks only 2 to 4 percent was coming from the soil.

Response to defoliation

Jones (1967a) found that siratro did not persist when cut to a height of 3.75 cm every four weeks. Under grazing by sheep and cutting treatments, Whiteman (1969) found that frequent defoliation by sheep or cutting at 5 cm steadily reduced the yield of siratro, grazing reducing survival more than cutting. Jones (1967a) further found that cutting to 15 cm maintained the vigour of the siratro in a setaria/siratro sward.

Grazing management

Siratro should be lightly grazed at all times. Livestock will eat the runners back toward the crown, which should be protected from overgrazing. The concept that "leaf begets leaf" is valid for siratro, and grazing to 15 cm maintains the stand. In thinning stands, siratro should be shut up to allow it to seed and shatter the seed, so that the stand can be improved by new seedlings. In this way, it will also climb over dominant grass and weeds and suppress them. Stobbs (1969j) found that a rotational grazing system of two weeks' grazing-four weeks' rest maintained the best botanical composition and equalled the weight gain obtained with continuous grazing.ň@

Response to fire

Recovers well, new growth appears from the crown and new seedlings germinate.

Breeding system

Self-pollinated; chromosome number 2n = 22.

Dry-matter and green-matter yields

Roe and Jones (1966) obtained 3 394 kg./ha siratro in a mixture with Nandi setaria, the mixture yielding 12 200 kg./ha at Gympie, Queensland, in 1962/63, and 1 094 kg./ha out of a total of 4 873 kg./ha in 1963/ 64. Van Rensburg (1967) obtained an average dry-matter yield over the two years 1965-66 in Zambia of 7 960 kg./ha. In Florida, siratro yielded 11 610 kg. DM/ha/year when grown with pangola grass (Kretschmer, 1966).

Suitability for hay and silage

Can be made into hay only with difficulty because of the heavy loss by leaf drop, leaving stemmy material and very young shoots. Catchpoole (1970) found that ensilage of siratro without added molasses was never successful, but that the second harvest material in a season was better than the first. Satisfactory silage was made by adding 8 percent molasses with the first cut and 4 percent with the second cut. Two percent molasses markedly improved an otherwise poor silage but was insufficient for a stable product.

Value as a standover or deferred feed

Not very valuable because of leaf shedding. Jones (1967b) found that it lost over 75 percent of its dry matter and over 80 percent of its nitrogen on average over two winter periods (1962-63) .

Feeding value

Quite palatable; it is readily eaten by livestock.

  • Chemical analysis: 

Milford (1967) gave the analyses of mature siratro at the seed-shedding stage, leafy but 20 to 30 percent of the leaf dry. He recorded figures of 35 percent dry matter, 16.8 percent crude protein, 33.4 percent crude fibre, 1.2 percent ether extract, 38.8 percent nitrogen-free extract and 9.8 percent ash.

  • Digestibility: 

The digestibility figures were 50.4 percent for dry matter, 53.4 percent for organic matter, 67.6 percent for protein, 50.9 percent for fibre and 50. 6 percent for the nitrogen-free extract. The intakes of dry matter and digestible dry matter were 37.5 and 18.9 g/kg. live weight/day respectively. Other analyses for siratro are given in Appendix 1. Minson and Milford (1966) showed that the mean energy value of the digestible dry matter for siratro was 4.2 times higher than for pangola grass, while that of the digestible organic matter was 8.2 percent higher.

  • Palatability: 

Although siratro is palatable, Stobbs (1969k) found that cattle grazed Panicum maximum first and allowed the siratro in the mixture to become dominant.


None reported in livestock feeding.

Seed yield

A single vigorous flower flush may produce over 1 000 kg./ha of seed, but seldom is more than 200 kg./ha harvested from a single crop. Direct heading of locked up pasture yields far less than this. Single header harvests of several irrigated flushes have yielded up to 700 kg./ha commercially, and suction harvesting generally recovers 100 to 400 kg./ha.


There is at present only one commercial line of siratro available, though an active breeding programme by the CSIRO Division of Tropical Agronomy (Australia) will soon lead to the release of cultivars for special purposes.


Siratro is attacked by Rhizoctonia solani under very wet conditions (Kretschmer, 1966; Dunsmore and Ong, 1969), and a case of attack by a powdery mildew was recorded in a late-sown crop at Campinas, Brazil. It is relatively resistant to little-leaf, but is attacked occasionally. It is severely attacked by an orange-coloured rust (Uromyces phaseoli) in high rainfall areas of Guatemala (Rodriguez, personal communication).
Jones, Alcorn and Rees (1969) reported death of siratro plants from the attack of violet root caused by Rhizoctonia crocorum. A growth of reddish-brown to purple fungal mycelium occurred over the top 20 cm of taproot, accompanied by decay of internal tissues where the advancing internal margin of the rot sometimes showed a reddish band.

Main attributes

It is productive under a wide range of soils; easy to establish; drought-resistant; combines well with a wide suite of grasses. It is promiscuous in Rhizobium requirement; has high seedling vigour; is very palatable.

Main deficiencies

Low cold tolerance and comparatively low seed yields.


Siratro is performing well in the medium rainfall areas of the tropics such as at Campinas (Brazil), Serere (Uganda), Makulu (Zambia), Ukirigiru (Tanzania), Queensland and Northern Territory (Australia) and Panama. Stobbs (1969h) obtained a mean live-weight gain of 432 kg./ha/year from a Panicum maximum/Macroptilium atropurpureum pasture at Serere, Uganda. The highest percentage of legume was maintained in the sward under continuous grazing at 7 beasts/ha, but weed invasion was also highest. Stobbs concluded that over long periods rotational grazing is necessary to maintain a satisfactory sward. The inclusion of siratro in the mixture considerably extended the quality of the herbage during the dry season.

Main references

Hutton (1962); Jones (1966).

Frost tolerance

Winter frosts cause severe defoliation, but survival is one of the highest among the tropical legumes. Jones (1969) found that it survived a cold winter at Samford in south-eastern Queensland when the terrestrial minimum temperature reached -8.35°C.

Latitudinal limits

About 30°N and S. At latitude 28°S, growth is very slow at elevations higher than 610 m.D

Ability to compete with weeds

Siratro competes well with weeds if allowed to develop to a stage where it can smother them. Douglas (1965) successfully established siratro on burnt blady grass (Imperata cylindrica) country by oversowing it with 370 kg./ha superphosphate containing 0.03 percent molybdenum. By the end of the fifth month, it was overtopping the blady grass.

Maximum germination and quality required for sale

Seventy percent minimum germination with a maximum hard-seed content of 100 percent and a minimum purity of 97.5 percent in Queensland. The seed is tested for germination at 25°C (Prodonoff, 1968).


Colbran (1963) found that siratro was attacked by the root nematode Helicotylenchus dihystera, but was resistant to Meloidogyne javanica and Radopholus similis. He therefore recommended it (Colbran, 1964) in conjunction with green panic as a suitable cover crop for control of nematodes in banana plantations. The bean fly (Melanagromyza phaseoli) will attack seedlings up to three to four weeks of age, but it can be prevented by seed treatment (Jones, 1965). Meloid beetles, which may prevent flowering in the tropics, can be controlled by a DDT spray. The plant is resistant to the Amnemus weevil. In Florida, the bean leaf roller (Urbanus proteus) attacks siratro in late summer and autumn (Kretschmer, 1966).

Toxicity levels and symptoms

Manganese­the toxicity threshold value in the dry matter of the tops was 810 ppm (Andrew and Hegarty, 1969). Siratro only gave 9 percent of its maximum yield in the presence of high manganese concentrations. As manganese content in the solution increased, there was a large reduction in the total uptake of nitrogen by the plant. However, M. atropurpureum had a high concentration of manganese, reaching a maximum of 5 590 ppm.
In tests to ascertain the toxicity symptoms of excess manganese in siratro, Andrew and Pieters (1970b) found that there were two initial symptoms of manganese toxicity. Firstly, young growth was interveinally chlorotic and, secondly, older leaflets showed brown, rust-coloured spots. Leaves slightly affected at emergence retained the interveinal chlorosis effect to maturity and also exhibited increasing numbers of brown spots, which appeared at the extremities of the veinlets and adjacent to the secondary veins, particularly on the underside of the leaflets. In young plants, this was often accentuated on the primary leaves and their petioles. As toxicity increased, emerging shoots were severely chlorotic, the leaflets showing an interveinal effect, but this was not as definite as in M. lathyroides; the main and secondary veins were pale green in colour toward the base of the leaflets but the remainder of the leaflet was almost devoid of chlorophyll. In severely affected plants, the young emerging leaflets had very little chlorophyll, numerous brown areas occurred adjacent to and on the main and secondary veins, and in severe cases puckering of the leaf surface occurred, usually associated with necrosis of the previously mentioned brown areas. This resulted in epinastic curvature of the petiolules and subsequently of the leaf petiole. In the older leaves of the plant, there was an increase in leaflet thickness. Following cessation of growth at the primary shoot, secondary growth was initiated, but this in turn was similarly affected by the toxicity.

Temperature for growth

Optimum, about 26.5 to 30°C with average daily minimum temperatures above 21°C. Growth was poor at a day/night temperature range of 21/16°C and 18/13°C and maximum dry matter was produced at 30/25°C and 27/22°C in a long day.
Ludlow and Wilson (1970) found that siratro gave only 24.5 percent of its yield, had only 43 percent of the growth rate and only 14.3 percent of the leaf area at 20°C as at 30°C. Jones (1967b) and Whiteman and Lulham (1970) found that growth of siratro ceased at 14° ALm l

Vigour of seedling, growth and growth rhythm

Excellent seedling vigour and can establish readily from shattered seed in an existing pasture.
The plant grows vigorously in the hot weather and is most productive in midsummer. Growth rhythm from an irrigated and fertilized pasture mixture of Rhodes grass and siratro, uncut, is shown in Figure 65.

Seed harvesting

Siratro does not seed prolifically in districts where it thrives as pasture. Therefore, although pastures may be harvested for seed, it is better in the long term to seek specific seed producing districts. These should have a very dry and frost-free winter. From one to four crops may be produced each dry season, depending on temperature and rainfall patterns and irrigation use.
Use of insecticide over the flowering period is necessary for heavy yields. Each crop may be harvested as it ripens (if hand harvesting or using a small header); or a single end-of-season header harvest may be taken, followed by suction harvesting. The latter system produces very high yields with minimum labour input, but requires sophisticated management and machinery.
Pods shatter on ripening, and harvest time should be chosen to forestall this. Hand picked pods will shatter during drying. Both hand- and header-harvested seed need drying. Sun drying on tarpaulins is adequate (Hopkinson, personal communication).

Response to photoperiod and light

Flowering occurs in short and long days; best at 24/19°C,27/22°C and 30/25°C. Flowers in 60 to 70 days in south-eastern Queensland and in 57 days in the Kimberley district, Australia (Parbery, 1967a). It will grow reasonably well in the shade, but prefers ample sunlight and has the capacity to climb tall grasses in mixtures.°