Crotalaria spectabilis (PROSEA)
- Protologue: Nov. pl. sp.: 341 (1821).
- Family: Leguminosae - Papilionoideae
- Chromosome number: 2n= 16
Crotalaria sericea Retzius (1788), non Burm.f. (1768).
- Showy rattlebox, showy crotalaria (En)
- Thailand: mahing men (northern).
Origin and geographic distribution
C. spectabilis probably originated in tropical Asia, but is now distributed pantropically. It is also cultivated throughout the tropics, including South-East Asia and in the south-eastern United States.
C. spectabilis is used as a green manure and for erosion control in the tropics and in the United States. Its use as fodder has ceased because of its toxicity. It is a spectacular ornamental, which flowers for long periods. A fairly strong fibre is extracted from the stem. In India, plants are used in the treatment of scabies and impetigo.
The seed and other above-ground parts contain the pyrrolizidine alkaloid monocrotaline, which lowers blood pressure and is toxic to farm animals and probably also to root-knot nematodes (Meloidogyne spp.). Because of its bitterness the green material is avoided by farm animals. In Brazil, leaves contained per 100 g dry matter: N 4.0 g, K 1.2 g, Ca 1.1 g, Mg 0.4 g. The weight of 1000 seeds is 15 g.
- Erect, much-branched annual herb, up to 2 m tall. Stem and branches angular, grooved, subglabrous.
- Leaves simple; stipules persistent, obliquely oblong-ovate, acuminate, 3-10 mm × 2-8 mm; petiole 2-8 mm long; blade oblanceolate to obovate, (5-)8-14 cm × 3-8 cm, upper surface glabrous, lower surface appressed pubescent.
- Inflorescence a rather lax, many-flowered raceme, 15-50 cm long; bracts persistent, cordate, 10-20 mm × 5-10 mm, acute or acuminate.
- Pedicel 8-13 mm long; bracteoles lanceolate, 1-2 mm long; calyx campanulate, 11-14 mm long, with 5 unequal lobes which are longer than the tube, glabrous; corolla yellow; standard orbicular, about 20 mm in diameter; wings obovate-oblong, 9-18 mm × 5-9 mm; keel 10-14 mm × 4-9 mm, rounded about the middle, with a short, incurved, twisted beak; stamens 10, monadelphous; anthers dimorphic, with 5 long, basifixed anthers on a short filament alternating with 5 rounded, dorsifixed anthers on a long filament; ovary narrowly oblong, glabrous.
- Fruit a broadly clavate-oblong pod, 4-5 cm × 1.5-2.5 cm, glabrous, brown to dark brown when mature, 20-24-seeded.
- Seed unequal-sided heart-shaped, about 3.5 mm × 4 mm, with the radicular side strongly incurved, brown.
C. spectabilis is grown as an annual with a life cycle of 4-6 months. It develops an extensive root system that may reach to a depth of 120 cm. It nodulates with slow-growing Bradyrhizobium spp. Flowering starts about 2 months after germination.
C. spectabilis is a plant of the tropics and subtropics, requiring an annual rainfall of 900-2800 mm and a mean annual temperature of 12-28 °C. It is drought-tolerant. It occurs in open locations along forest margins and as a weed in cultivated fields, from sea level up to 1500 m altitude, on a wide range of soils, including heavy soils, with a pH range of 4.8-8.0.
C. spectabilis is propagated by seed. A seed rate of 15-20 kg/ha is recommended in the United States, and of 90 kg/ha for broadcasting in Brazil. Seed should not be sown deeper than 5 cm. Germination is rapid and can be complete 5-6 days after sowing. A crop of C. spectabilis is easy to incorporate into the soil, as the lower part of the stem hardly lignifies. It should be ploughed in about 2 months before the following crop is sown or planted. This allows about 70% of the organic matter to decompose and release its nutrients.
Tests on poor sandy soils in Florida found that C. spectabilis had a dry matter production of 10-11 t/ha in 5 months, containing 170 kg nitrogen, slightly lower than Cajanus cajan (L.) Millsp. cv. Norman and Indigofera hirsuta L. The beneficial effect of C. spectabilis on the succeeding crop results not only from the increased nitrogen content of the soil but also from its effect on nematodes. The numbers of root-knot nematodes (Meloidogyne incognita and M. javanica) and of the sting nematode (Belonolaimus longicaudatus) in the soil are greatly reduced by a crop of C. spectabilis. The reduction in root-knot nematodes has been shown to last until the harvest of a subsequent highly susceptible crop of cowpea (Vigna unguiculata (L.) Walp.). Reports on the effect on the lesion nematode (Pratylenchus brachyurus) are contradictory; its numbers increased slightly in Florida, but were reduced in experiments in Nigeria.
When grown for seed, several caterpillars may attack the young pods. They may be very destructive, causing complete failure of the seed crop. Preventive use of insecticides is recommended. The fungus Alternaria cassiae has been tested as control agent for C. spectabilis where it has become weedy. Effective control in the field was achieved by spraying twice with a solution containing spores of the fungus.
Genetic resources and breeding
Germplasm collections of Crotalaria spp. are maintained by the United States Department of Agriculture, including material of C. spectabilis. It is unlikely that any substantial breeding programmes exist.
Its high growth rate and effective control of root-knot and sting nematodes make C. spectabilis a very useful green manure crop, deserving more attention in South-East Asia. Its effect on nematodes and potential as a trap crop for root-knot nematodes should be studied further.
- Everist, S.L., 1974. Poisonous plants of Australia. Angus & Robertson, Sydney, Australia. pp. 415-417.
- McSorley, R., Dickson, D.W., De Brito, J.A., Hewlett, T.E. & Frederick, J.J., 1994. Effects of tropical rotation crops on Meloidogyne arenaria population densities and vegetable yields in microplots. Journal of Nematology 26: 175-181.
- Niyomdham, C., 1978. A revision of the genus Crotalaria Linn. (Papilionaceae) in Thailand. Thai Forest Bulletin (Botany) 11: 105-181.
- Polhill, R.M., 1982. Crotalaria in Africa and Madagascar. Balkema, Rotterdam, the Netherlands. p. 373.
- Reddy, K.C., Soffes, A.R. & Prine, G.M., 1986. Tropical legumes for green manure. 1. Nitrogen production and the effects on succeeding crop yields. 2. Nematode populations and their effects on succeeding crop yields. Agronomy Journal 78: 1-4, 5-10.
- Sabadin, H.C., 1984. Adubação verde [Green manure]. Lavoura arrozeira 37(354): 19, 22-26.
- Suarez-Vasquez, S. & Carillo-Pachon, I.F., 1976. Descomposición biológica de leguminosas y otros materiales de la zona cafetera Colombiano [Biological decomposition of legumes and other plant materials from the Colombian coffee area]. Cenicafé 27: 67-77.