Macroptilium atropurpureum (DC.) Urb. var. atropurpureum, Macroptilium atropurpureum (DC.) Urb. var. canescens (M.Martens & Galeotti) Hassl., Phaseolus affinis Piper, Phaseolus atropurpureus DC., Phaseolus canescens M.Martens & Galeotti, Phaseolus dysophyllus Benth., Phaseolus schiedeanus Schldl., Phaseolus vestitus Hook.
Siratro (Macroptilium atropurpureum (DC.) Urb.) is a perennial tropical legume valuable for pasture.
Siratro is trailing, climbing or twining, depending on the species it is grown with (Ecocrop, 2012). Siratro has a deep swollen taproot, up to 2 cm in diameter from which stems arise. The stems close to the ground may root from the nodes under favourable conditions (moist clay soils) (FAO, 2012). The stems are hairy, may reach 5 m long and are rarely broader than 5 mm in diameter (Jones et al., 1992). The leaves are pinnately trifoliate, dark green, hairier on the lower face than on the upper one. Leaflets are ovate, 2-7 cm long x 1.5-5 cm broad. The inflorescence is a raceme bearing 6-12 paired, deep purple flowers (Real et al., 2006). The fruits are 5 to 10 cm long linear pods that contain between 5 and 10 small seeds (3-4 mm long), which are oblong, ovoid, and mottled (light brown to black). Pods dehisce violently, throwing the seeds as far as 5 m (Jones et al., 1992).
Siratro was the first tropical pasture species improved by breeding (Shaw et al., 1977). Two notable cultivars of Macroptilium atropurpureum were bred in Australia: cv. Siratro, released in the 1960s, was selected for its root-knot nematode resistance but lost popularity because of rust sensitivity. Cv. Aztec, released in 1994, was selected for higher yield (increased leaf production) and rust resistance (Morris, 2010; Real et al., 2006). Siratro is a productive and palatable forage legume. It is mainly sown in mixed legume-grass pastures. It can be cut for cut-and-carry systems and for hay, though its twining habit makes harvesting difficult. Siratro is also used as a fallow crop (after lowland rice) and as a ley legume able to provide nitrogen for up to 3 crops of sorghum. It is a valuable cover crop, used for erosion control and for stabilization and re-vegetation operations on roadsides, earthworks and embankments (Cook et al., 2005; Jones et al., 1992). It is used as living mulch in banana plantations (Espindola et al., 2006). However, it should be noted that the commercial usage of siratro in Australia has largely declined: it appears that field trials overestimated its persistence over long periods and its resistance to grazing. Siratro is still used to some extent in Australia, but failed to deliver its initial promise (Jones, 2014).
Siratro originated from Central and South America, and from the Caribbean Islands. It is now widely cultivated and naturalized throughout the tropics and subtropics (Cook et al., 2005). Siratro is one of the major tropical legumes. It grows best in summer and early autumn in moist tropical and subtropical areas, but can grow all year round in the tropics. Siratro can be found within 30° N and 22° S, from sea level up to an altitude of 1600 m in Kenya, and up to 2900 m in the warmer tropical areas (Ecuador). It grows best in full sunlight in areas where day temperatures are about 26-30°C with minimum temperatures above 16°C. It stops growing below 16°C. Siratro is frost sensitive: the leaves and tops are burnt but the plant regrows from the taproot, which has been reported to survive temperatures as low as -8.5°C (Jones et al., 1992). Siratro does better in areas with annual rainfall ranging from 700 to 1500 mm. Higher rainfall is deleterious to siratro because it may favour leaf diseases. Flooding and waterlogging conditions are harmful to siratro. Siratro tolerates dry conditions because of its deep taproot and its ability to reduce evapotranspiration, either by virtue of pubescent leaves or by shedding its leaves. Siratro grows on a wide range of soils, from deep sandy to light clayey soils, provided they are well-drained. It tolerates a wide range of soil pH (4.5-8) and is one of the most tolerant tropical legumes to Al, Mn and NaCl. Siratro is a N-fixing legume and does not require added N fertilizer. It responds positively to additions of P and K fertilizer in sandy soils (FAO, 2012; Cook et al., 2005; Jones et al., 1992).
Siratro is mainly sown in mixed stands with erect or tussock-forming grasses such as Chloris gayana, Cenchrus ciliaris, Panicum maximum, Setaria sphacelata, or with legumes that require similar growing conditions such as Chamaecrista rotundifolia, Desmodium intortum, Macrotyloma axillare, Neonotonia wightii and Stylosanthes species (Stylosanthes guianensis, Stylosanthes hamata, Stylosanthes scabra, Stylosanthes seabrana). Though stoloniferous, siratro mainly spreads by seeds. Under favourable conditions, a stand can self-regenerate. It can naturally regrow from seeds after fire. Seedbed preparation and seed scarification can facilitate establishment. Over-sowing siratro in existing pastures is possible (Real et al., 2006). Siratro can be sown in March in southern Florida, December-April in Australia and practically at any time in frost-free areas (Ecocrop, 2012).
Siratro is sensitive to overgrazing, it is thus recommended to adjust stocking rates or to use a light grazing system to prevent its rapid disappearance (Real et al., 2006). Once well established, siratro can be moderately or rotationally grazed. A grazing height of 10-15 cm is best to encourage regrowth while lower cutting heights hinder fodder yields. A good rotational grazing management would combine a 2-week grazing period and a 4 to 6-week rest period (Cook et al., 2005; Jones et al., 1992). Thinning stands may help siratro sheding its seeds and then climbing over the grasses (FAO, 2012). Under good stand management, siratro sown in pure stand or in mixture can yield 5-12 t DM/ha and remains productive for 5 to 7 years (Ecocrop, 2012; Cook et al., 2005).
Siratro can be harvested for hay but heavy leaf losses occur during drying. Ensiling may be a more suitable mode of preservation, provided that it is mixed with molasses (8% at first cut, 4% at second cut) to ensure that fermentation occurs, resulting in a stable product. Though it can be used as deferred feed during winter, overwintered siratro has a poor nutritive value (FAO, 2012).
Well established siratro competes well with weeds. In fertilized soils, siratro sown after burning Imperata cylindrica was reported to overtop the grass within 5 months (FAO, 2012). This competing ability also makes it a potential weed in ungrazed areas such as roadsides and degraded lands. It is considered a weed in sugarcane crops since the climbing stems tend to hinder mechanical harvesting (Cook et al., 2005).
Siratro readily nodulates and fixes N, from 55 to 175 kg N/ha/year (FAO, 2012; Cook et al., 2005). Siratro can be used as a cover crop though it does not spread as quickly as large-seeded legumes (Zegda et al., 1998). Siratro is used as living mulch in banana plantations, where it increases banana yields (Espindola et al., 2006).
Siratro has a moderate protein content compared to other tropical legume forages: about 16% DM (in the 11-24% range). Leaves contain more protein (up to 27% DM) than the stems, so the composition of the whole plant depends on the leaf:stem ratio. The protein content of the whole plant was found to decrease only slightly between 4 and 10 weeks (19 to 16% DM) (Mero et al., 1997b). In another experiment, protein content slowly decreased between 8 and 14 weeks (28 to 25% DM) and then dropped at 20 weeks (19%). Siratro is relatively rich in fibre (NDF about 50% DM, ranging from 40 to 75%), with highly variable values due to various proportions of leaves and stems in the foliage. The NDF and ADF contents were found to increase up to 14 weeks and then to slightly decrease (Mupangwa et al., 2003b). As with other legume grasses, siratro contains relatively high levels of calcium (about 1% DM) though the range is important (0.5-2.2% DM). Phosphorus content is low, typically less than 0.2% DM. Siratro may contain high levels of silica (1.2% DM), particularly in the leaves (2.3 vs. 0.9% DM in the stems) (Mero et al., 1998c).
Siratro does not contain major antinutritional factors. It was reported to contain condensed tannins (1.2 % DM) (Mupangwa et al., 2000a).
Siratro is used as fresh forage and hay in various ruminant production systems, typically to supply protein in pasture and roughage-based diets of low nutritive value. Unlike other legume forages, siratro has not been reported as causing bloat (Walker, 1977).
The OM digestibility for fresh siratro forage and for siratro hay is relatively low, in the 51-59% range, which is probably due to the high fibre content. The OM digestibilities of siratro at eight weeks (regrowth, Mero et al., 1998c) and 20 weeks (1st cycle, Mupangwa et al., 2000a) were 54% and 59% respectively. In sacco DM degradability for 8 week-old regrowth hay was 54-58% (the rapidly degradable fraction was 38%) (Mupangwa et al., 1997). In sacco DM degradability changed only slightly from 8 weeks (1st cycle) to 22 weeks (regrowth), from 60 to 62% respectively (Bulo et al., 1994). There is a large difference in nutritive value between the leaves and the stems: in sacco OM degradability (k=0.06) was 65 and 43% for leaves and stems respectively (Mero et al., 1997a).
Siratro protein is quite digestible. Apparent N in sacco digestibility of siratro hay at 20 weeks was 81% in the rumen (48 h incubation) and 95% for the total tract (Mupangwa et al., 2003a). Nitrogen retention in sheep fed only siratro was negative (-8.7 g/d), which may have been caused by an insufficient amount of readily available energy. Rapidly degradable N decreased from 66% at 8 weeks to 37% at 20 weeks, while slowly degradable N increased from 26 to 49% during the same period (Mupangwa et al., 2003c).
In a palatability experiment in Australia, cows chose freely autumn siratro over summer siratro, probably because of the higher protein content and digestibility of the former. Growing heifers grazing Setaria sphacelata-siratro pasture selected more siratro when its yield and proportion in sward was high: siratro accounted for 62-73% of the intake whereas its proportion in the pasture was 41-51% (Stobbs, 1977). In a pasture with Dichanthium aristatum (75%), Desmodium incanum (14%) and siratro (4%), lactating cows with calves consumed almost all the siratro during the dry and wet season (Manteaux et al., 1991).
In Tanzania, when offered as sole forage to male sheep (30-31 kg LW), OM intake of siratro was 48-52 g/kg LW0.75 after 8 weeks regrowth, lower than for other legumes (Neonotonia wightii, Stylosanthes scabra and Macrotyloma axillare). Increasing feeding level up to 3 to 4% LW allowed the selection of a more leafy diet with a significant improvement in OM intake (Mero et al., 1998c). In Zimbabwe, also in 31 kg sheep, OM intake was 43.3 g/kg LW0.75 after 20 weeks growth, comparable to the OM intakes for Lablab purpureus and Stylosanthes guianensis (Mupangwa et al., 2000a).
The effects of feeding siratro as a supplementary source of protein have been studied in dairy cattle, beef cattle, sheep and goats, with generally positive results for both milk yield and growth when compared to roughage-only diets. The following table presents examples of trials involving siratro.
Pig can graze tropical pastures containing siratro and grasses. Siratro has only medium palatability to pigs, who prefer butterfly pea (Clitoria ternatea) (Mora et al., 2005). Compared to alfalfa hay, feeding siratro hay at 10% of the diet to pigs resulted in slightly lower average daily gains (680 vs. 690 g/d) and feed efficiency (4.3 vs. 4.2 feed:gain ratio) (Lima et al., 1973).
Literature regarding siratro utilization in rabbit feeding is very scarce. In a study on forage cultivation in Australia, siratro was spontaneously grazed by wild rabbits to a less extent than Stylosanthes hamata (Hall, 1988). In Mozambique, a blend of various fresh forages (including siratro) distributed ad libitum to rabbits resulted in far better performance than fresh leucaena (Leucaena leucocephala). The inclusion of siratro in the forage blend for this study on reproductive performance implies that it was considered as a common and safe forage for rabbit feeding in Mozambique (Muir et al., 1995).
Real, D. ; Moore, G., 2006. Siratro, atro (Macroptilium atropurpureum). In: Moore, G., Sanford, P. & Wiley, T., Perennial pastures for Western Australia. Department of Agriculture and Food Western Australia, Bulletin 4690, Perth.