Centrosema brasilianum (L.) Benth.
|Leguminosae||Authors: Michael Peters and Rainer Schultze-Kraft|
Centrosema belongs to the tribe Phaseoleae in the subfamily Papilionoideae, and together with Clitoria, Clitoriopsis and Periandra forms the subtribe Clitoriinae. Williams & Clements (1990) reviewed the taxonomy of the genus and according to morphology classified the Centrosema species into 11 groups, which are considered as possible equivalents to taxonomic 'series' or 'subseries'. According to this classification, C. brasilianum belongs to species group 7, indicating a close relationship with C. angustifolium, C. bifidum and C. tetragonolobum. Chromosome numbers reported are 2n=20 and 2n=22 (Miles et al., 1990).Common names and Cultivars
Common names are not known; a cultivar Oolloo was released in Northern Territory, Australia, in 1997 (Cameron & Lemke, 1997).Description
Centrosema brasilianum is commonly described as a prostrate-trailing to twining, perennial, herbaceous legume. However, in the study by Peters et al. (1998) some erect and semi-erect forms were identified. Similarly the ability to form adventitious roots on trailing stems varies among studies (Schultze-Kraft & Belalcázar, 1988; Cameron & Lemke, 1997; Peters et al., 1998). Leaves are trifoliolate, leaflets elliptical-oblong to lanceolate, sometimes ovate, 3.3-6.6 cm long, 1.5-3.6 cm wide. Flowers usually in racemes of 2-5 flowers, sometimes solitary. Bracteoles glabrous or pubescent, 3-13 mm long, 12-17 mm long and 5-10 mm wide, ovate and flat or cupped. Peduncles in leaf axils, 4-30 mm long. Papilionate flower, usually violet, violet-blue or red-lilac, in rare cases also white or purple. Variation in colour intensity within one accession can occur, suggesting some degree of outcrossing. Time from sowing to flowering < 3-7 months. Pods linear, dehiscent, 70-160 mm long and 4-5 mm wide, containing 8-23 seeds. Seeds light to dark brown, sometimes grey, black or yellow, often with darker stripes or mottled. Seeds cylindrical, 3.4-4.4 mm long, 2.3-3.1 mm wide, Thousand Seed Weight 11-30g (Belalcázar & Schultze-Kraft, 1986; Maass & Torres, 1992; Cameron & Lemke, 1997; Peters et al., 1998, Kramer, 1994). Further morphological description can be found in Barbosa-Fevereiro (1977).
The available world collection of C. brasilianum consists of about 260 accessions (Schultze-Kraft et al., 1989) with major collections presently held at EMBRAPA-CENARGEN (Empresa Brasileira de Pesquisa Agropecuária, Centro Nacional de Recursos Genéticos e Biotecnologia), Brasília, Brazil, and CIAT (Centro Internacional de Agricultura Tropical), Cali, Colombia (Peters et al., 1998).Geographic distribution and adaptation
Basic passport data of accessions can be accessed on the Internet (http://singer.cgiar.org/). C. brasilianum has a wide natural distribution in the tropics of South America, with a latitude range of 23° S 12° N. It is found most frequently in subhumid vegetation zones such as semi-evergreen seasonal forest, deciduous forest and savannah mainly in Northeast Brazil and Venezuela; some accessions were collected in the 'caatinga', the xerophytic vegetation form typical of very dry locations in north-eastern Brazil. It is rarely found in rainforest margin areas. C. brasilianum is a true lowland species, with accessions collected at altitudes of 50 300 m above sea level; few accessions originate from higher altitudes (up to 800 m asl, mostly in Northeast Brazil). Rainfall at the collection sites varied from arid (370 mm annual rainfall, with 10 dry months) to humid environments (2900 mm annual rainfall, no dry season). Soils at collection sites were well drained, acid to very acid (pH 4.1 6.3), of medium to low fertility and of sandy to loamy texture. The natural habitat is mostly scrub, wasteland, open woodland, edges of gallery forest/gallery bush, river banks and along creeks (Schultze-Kraft & Belalcázar, 1988; Schultze-Kraft et al., 1990).Seed treatment before sowing
Germination and establishment of C. brasilianum is, in general, good. Kramer (1994), evaluating a 257-accession collection, reports an average of 40% of field germination, with seed of 84% germinability in the laboratory. Scarification of seeds before sowing is necessary; positive results were obtained with mechanical scarification, hot water treatment, Osram irradiation and sulphuric acid (Mendoza et al., 1990).
Land preparation and sowing
C. brasilianum establishes relatively slowly hence weed control during establishment is beneficial. Once established, it competes well in a sward (Peters et al., 1994; Cameron & Lemke, 1997; Peters et al., 1998). A sowing depth of 5 cm is recommended, with seeding rates of 4-5 kg/ha (Clements et al., 1984; Peters et al., 1994).
As for other Centrosema species, phosphorus (P) is the most limiting nutrient for establishment. Recommendations range from 10 to 40 kg/ha P, depending on soil parameters. In contrast to commercial centro (C. pubescens), C. brasilianum requires only small quantities of magnesium and calcium for establishment (Mendoza et al., 1990).Agronomic performance
C. brasilianum has been evaluated extensively, mostly under cutting, in a wide range of environments in Australia, Africa and South America (e.g., Anning, 1982; Clements et al., 1984; Grof, 1986; Thomas & Grof, 1986; Schultze-Kraft and Clements, 1990). Because of its high productivity and good nutritional value, the species has high potential as a forage plant.
Evaluations under cutting identified C. brasilianum as a promising species for the semi-arid to subhumid environments of Northern Australia, West Africa and South America (Anning, 1982; Clements et al., 1984; Schultze-Kraft & Belalcázar, 1988; Peters et al., 1994; Tarawali, 1994; Peters et al., 1998). Schultze-Kraft & Belalcázar (1988) stress the tolerance to acid soils with high levels of aluminium.
A particular characteristic of C. brasilianum is its high tolerance to drought, with the ability to retain green leaves over extended (> 5-8 months) dry seasons (Clements et al., 1984; Grof, 1986; Peters et al., 1998). The mechanisms for drought tolerance of C. brasilianum were studied by Ludlow et al. (1983), indicating osmotic and stomatal regulation as the main processes to control water loss. However, as the tolerance to leaf water deficits is lower than for other Centrosema spp. and leaf morphology does not indicate adaptation to drought, Clements (1990) attributes the drought tolerance of C. brasilianum to its deep rooting system (tap root).
Persistence under grazing depends on pasture management, with periodic regeneration through seeds seen as the main mechanism for persistence. Such regeneration is favoured by the free seeding habit of this species (Thomas & Grof, 1986; Schultze-Kraft & Belalcázar, 1988). In long-term (11 years) grazing studies in Northern Australia, only a few accessions of C. brasilianum persisted under grazing, among them cv. Oolloo (Cameron & Lemke, 1997). Cameron and Lemke (1997) report positive effects of C. brasilianum on liveweight gains in associations with Setaria sphacelata cv. Kazungula. Clements (1990) reports similar positive effects for the semi-arid tropics of Northern Australia when C. brasilianum was grazed during the dry season for 7 years. Positive effects on liveweight gains of cattle grazing grass-legume accessions containing C. brasilianum are also known from Colombia, as well as an indication of persistence of C. brasilianum in the sward (Lascano et al., 1990).
C. brasilianum nodulates readily with native rhizobia (Clements et al., 1984) though yield could be enhanced through inoculation with specific Bradyrhizobium strains (Sylvester-Bradley et al., 1990; Cadisch et al., 1992). Inoculation with mycorrhizae increased uptake of phosphorus and other nutrients as well as dry matter yields, and reduced the root/stem ratio (Saif, 1986, 1987).Breeding
There has been no breeding work with C. brasilianum as yet. Hybridization attempts with C. tetragonolobum have been successful (Miles et al. 1990).
Diseases and pests
The biggest limitation for C. brasilianum, particularly in humid environments, is its susceptibility to Rhizoctonia Foliar Blight (RFB), a fungal disease caused by a complex of Rhizoctonia solani, binucleate Rhizoctonia sp. (BNR), and R. zeae (Lenné, 1994). In legume-only, small plots, most evaluated accessions were susceptible but a certain degree of resistance has been identified (Schultze-Kraft & Belalcázar, 1988; Lenné et al., 1989). In grazed pastures RFB affects young seedlings, reducing persistence and dry matter production, with yield reductions of up to 50%.
Other diseases include cercospora (Cercospora spp.), anthracnose (Colletotrichum spp.), zonate leaf spot (Cylindrocladium colhounii) and leaf spot (Pseudocercospora bradburyae) (Lenné, 1994; Peters et al., 1998).Seed production
Comprehensive reports on seed production of a wide range of accessions are scarce but Schultze-Kraft & Belalcázar (1988) report good seed yields of the 130 accessions evaluated. For cv. Oollo, seed yields equivalent to 1000 kg/ha have been harvested in trial plots (Cameron & Lemke, 1997). Argel et al. (1990) report seed yields of 1390 kg/ha in Mexico, Ferguson et al. (1990) of up to 1200-1400 kg/ha in South America, with common yields from 200-500 kg/ha (hand-harvested). Seed yields benefit from support systems such as trellises or grass stakes (of elephant grass or King grass).
Forage quality of C. brasilianum is high and compares favourably with other tropical legumes. Schultze-Kraft & Belalcázar (1988) and Peters et al. (1998) report a range of 11.8-19.6% crude protein in large collections evaluated in Colombia and Nigeria (130 and 257 accessions, respectively); age at sampling was 6 and 18 weeks, respectively. Reports on in vitro dry matter digestibility are variable and range from 48-56% (Belalcázar & Schultze-Kraft, 1986) to more than 70% (Lascano et al., 1990). In contrast to many other tropical legumes, C. brasilianum maintains its quality and retains its leaves under drought to a large extent, an important characteristic (Clements et al., 1984; Lascano et al., 1990; Argel et al., 1990; Peters et al., 2000) when searching for forages for dry environments. C. brasilianum is highly palatable (Reyes et al., 1990; Peters et al., 2000).
Main attributes and shortcomings
The biggest limitation of C. brasilianum outside Northern Australia seems to be its susceptibility to RFB. Furthermore, grazing studies are still limited. If these shortcomings can be surmounted , C. brasilianum could become an important species for the dry tropics.Links
Barbosa-Fevereiro V.P. (1977); Peters M. et al. (1998) ; Schultze-Kraft R. and Belalcazar J. (1988) ; Schultze-Kraft R. and Clements R.J. (1990); Schultze-Kraft R. et al. (1990); Schultze-Kraft R.et al.(1997); Williams R.J. and Clements R.J. (1990) Other References