|Nitrogen Fixing Trees Highlights (Winrock, 1990-1997, 100 p.)|
Sesbania sesban is a many-branched, soft-wooded tree that grows rapidly and is useful for fodder and green manure. This species has long been used for browse and soil improvement in India and Africa. Recent interest in multipurpose, nitrogen fixing trees has caused it to be collected, studied, and recommended for fodder ''banks" and alley cropping.
Sesbania sesban (L.) Merrill is a tree that grows to 8 m height. This papilionaceous (pea-like flowered) legume bears racemes of 4-20 yellow flowers that may be lightly to heavily streaked with purple. Sesbans have pinnate leaves with 20-50 opposite pinnules on a rachis 3-12 cm long. The leaf rachis and the underside of the leaflets are often pubescent. The pods are usually 10-20 cm long and contain up to 40 seeds that are brown, or dark green mottled with black. The trees usually have one main stem, but they may develop many side branches if they have space. Sesban's many branches often give the tree a shrubby appearance. It tends to have a spreading habit due to its wide branching angle (as wide as 4560°).
Within its genus, sesban is classified in the subgenus Sesbania, and thus is more closely related to the annual sesbanias grown far green manure (such as S. cannabina, others?) than to the - other well known perennial species of the genus, S. grandiflora, which is in the subgenus Agati (Evans 1990). Several varieties of sesban are recognized. The botanical distinctions among sesbanias are often difficult far non-botanists to see, and sometimes sesban is confused with the annual types of sesbania.
Sesban occurs naturally in semiarid to subhumid areas with 5002000 mm of rainfall. It seems to do well under bimodal rainfall distributions, where heavy rains and even flooded conditions are followed by a pmgressively drier season. It grows from sea level to 2000 m elevation, but the upper limit is uncertain. It does not tolerate frost. It is uniquely well adapted periodic waterlogging and flooding. Soil alkalinity and salinity is tolerated to a considerable degree. Some research suggests that certain sesban types may grow well on acidic soils. Sesbans are relatively short-lived, and under intensive browsing or cutting management will not last more than 3-5 years. Their rapid seedling growth is conducive to short-term fallows and to replanting if management should reduce growth vigor.
Sesban is found throughout the tropical and subtropical parts of Africa, Asia, and Australia It is not widely distributed in the Americas. Africa is its center of diversity, and sesban probably originated there; its former name is S. aegyptiaca. From northeastern Africa, S. sesban var. sesban and its variants were spread across southern Asia, possibly by man. Within Africa, S. sesban var. nubica is the type most commonly found, and there arc several sesbanias closely related to sesban, such as S. goetzei and S. cinerascens (Gillett 1963).
Sesban is mostly used as fodder and for soil improvement. its wood is used only to a lesser extent (Evans and Macklin 1990).
The leaves and tender branches of sesban are high in protein (20-25% crude protein) and have high digestibility when consumed by ruminants, such as cattle and goats. Antinutritional factors are suspected to be present in sesban fodder. Feeding sesbania fodders to monogastric animals (such as chickens, rabbits, and pigs) is not recommended.
Reports of feeding sesban to ruminants conflict. Trials in Australia feeding sesban to heifers showed live weight gains, but trials with young goats in Samoa found a lack of weight gain. Until further research provides clear guidelines, caution should be used in feeding ruminants with sesban fodder at more than 10-20 percent of diet.
Sesban establishes quickly and grows rapidly. In Africa it is often allowed to grow scattered throughout annual crop fields for the nitrogen it provides. It has been used in experimental alley cropping systems to provide mulch and greenleaf manure to intercrops. Sesbans can be somewhat shallow rooted, and may compete with adjacent crops.
Sesban's wood is light in weight compared to the woods of Calliandra and Leucaena, but it is often harvested for firewood in Africa and India It has been used in India to make charcoal. The wood is not durable and should not be considered for timber use. The branches have been used as poles in temporary structures such as sheds and mud daub huts.
Because sesban grows so rapidly, it has potential for pulpwood production. Plantings at about 10,000 tree/ha have produced 15-20 tons of woody biomass (dry weight) in one year.
Flowers of sesban are known to be added to stews and omelets in some regions, perhaps mainly as a decorative element.
Various medicinal uses for sesban have been recorded in Africa and Asia (Evans and Rotar 1987, Evans and Macklin 1990). The leaves and flowers are used in medicinal poultices and teas. which are said to have the effect of astringence, or contraction of body tissues. Bark exudates from sesban produce a gum of medium commercial quality.
Culture and Management
Sesban is generally propagated from seed, although it has been rooted from cuttings, and research has revealed that it can be established by tissue culture. Seed scarification usually improves germination. Recommended hot water scarification is a 30second dip in water heated to just below boiling. Seed weights range from 55-80 per gram for S. sesban var. sesban to 80-130 per gram for var. nubica.
Plants grown for fodder production can be placed as close as 30-50 cm apart in rows 1 m apart. Appropriate distances between rows in alley cropping will depend on the variety grown, the ecology of the site, and intensity of management.
Experimental fodder cutting trials have yielded 20 tons/ha dry matter in the first year. However, sesban cannot be managed with the severity that Leucaena tolerates in fodder and wood biomass production systems. If sesban is cut too low (below 50-100 cm) or too frequent (more than 4-6 cuttings per year) death of the plants can result. When cutting sesban it is recommended to leave 10-25% of the foliage on the plants.
In some climates, such as the highlands of Kenya, sesban may have a sparse canopy and weed competition can be a problem. This characteristic makes sesban a good intercrop. Sesban has been grown with the fodder grass Brachiaria mutica in India, and to provide shade to young coffee plants in Kenya In climates where sesban grows more vigorously, weeds are shaded out and companion plants may be adversely affected; this type of growth has been observed in Hawaii and Jamaica (Roshetko et aL 1991).
The rhizobia strains that nodulate sesbanias are somewhat specialized and may not be present where sesbanias have not been grown previously. Test plantings should be done to see if effective rhizobia are present in the soil. If not, use of a rhizobia inoculant at planting will be necessary.
Sesban is not a tree for timber or reforestation in the ordinary sense of forestry or silviculture. Because the range of its ecological adaptability is not yet well known, test plantings should be done before large-scale plantings are planned. Sesban has been observed occasionally to die back under cutting management; fungal infection may be the cause. Leaf-feeding insects sometimes limit production. Seed chalcids can reduce seed recovery.
Evans, Dale O. 1990. What is Sesbania? Botany, taxonomy, plant geography, and natural history of the perennial members of the genus. In: B. Macklin and D. O. Evans (eds), Perennial Sesbania species in agroforestry systems. Nitrogen Fixing Tree Association. p. 5-19.
Evans, D. O., and Macklin, B. (eds). 1990. Perennial sesbania production and use. Nitrogen Fixing Tree Association 41 p.
Evans, D. O., and Rotar, P. P. 1987. Sesbania in agriculture. Westview Press, Boulder, Colorado, U.S.A.
Gillett, J. B. 1963. Sesbania in Africa (excluding Madagescar) end southern Arabia Kew Bulletin 17:91-159.
Roshetko, J. M., Lantagne, D.O., and Gold, M. A. 1991. Direct seeding of fodder tree legumes in Jamaican pastures. Nitrogen Fixing Tree Res. Reports 9:68-70.
Financial support for this NFT Highlight was provided by the Rockefeller Brothers Fund through the Southeast Asia NGO Support Program.
A Publication of the Nitrogen Fixing Tree Association Winrock International 38 Winrock Drive Morrilton AR 72110-9537
NFTA 91-04 July 1991