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Ricebean (Vigna umbellata (Thunb.) Ohwi and Ohashi, previously Phaseolus calcaratus) is a small hairy warm-season annual vine legume with yellow flowers and small edible beans. To date, it is little known, little researched and little exploited. It is regarded as a minor food and fodder crop and is often grown as intercrop or mixed crop with maize (Zea mays), sorghum (Sorghum bicolor) and cowpea (V. unguiculata) as well as a sole crop in uplands on a very limited area. Like the other Asiatic Vigna species, ricebean is a fairly short-lived warm-season annual. Grown mainly as a dried pulse, it is also important as a fodder, a green manure and a vegetable. Ricebean is most widely grown as an intercrop, particularly of maize, throughout Indo-China and extending into southern China, India, Nepal and Bangladesh. In the past it was widely grown as lowland crop on residual soil water after the harvest of long-season rice, but it has been displaced to a great extent where shorter duration rice varieties are grown. Ricebean grows well on a range of soils. It establishes rapidly and has the potential to produce large amounts of nutritious animal fodder and high quality grain.

The English language name is a literal translation of the Chinese language name.

Taxonomy
The cultivated Asiatic Vigna species belong to the sub-genus Ceratotropis, a fairly distinct and homogeneous group, largely restricted to Asia, which has a chromosome number of 2n = 22 (except V. glabrescens, 2n = 44). There are seven cultivated species within the sub-genus, including mung bean or green gram (V. radiata), black gram or urad bean (V. mungo), adzuki bean (V. angularis) and moth bean (V. aconitifolia) as well as a number of wild species. Artificial crosses have been made between V. mungo and V. umbellata to produce improved mung bean varieties (e.g. Singh et al, 2006).

There are three more or less secondary gene pools within the group: ricebean is closer to V. angularis than to the other species, being in the Angulares group (Kaga et al, 1996, Tomooka et al, 2003).

Origin and distribution
Ricebean’s distribution pattern indicates great adaptive polymorphism for diverse environments, with its distribution ranging from humid tropical to sub-tropical, to sub-temperate climate. The presumed centre of domestication is Indo-China. It is thought to be derived from the wild form V. umbellata var gracilis, with which it is cross-fertile, and which is distributed from Southern China through the north of Vietnam, Laos and Thailand into Myanmar and India (Tomooka et al, 1991). Studies of the genetic and eco-geographical relationships among the wild relatives of Vigna species were made by Saravanakumar et al (2001).

Adaptation and agronomy
Rachie & Roberts (1974) classed ricebean as adapted to subhumid regions with 1000 – 1500 mm precipitation, although they noted that other factors than rainfall amount were also involved in adaptation, for example rainfall pattern, moisture distribution, temperatures, cloud cover and relative humidity, soil characteristics, pests and diseases. They noted the importance of human needs in assessing adaptation – for example taste, the need for a particular use, or market price. Average yields were between 200 and 300 kg ha-1, although with the potential for 1200 kg ha-1, the crop would grow on a range of soils, and was resistant to pests and diseases. It would mature in as little as 60 days, and although performing well under humid conditions, was also tolerant to drought (NAS 1979) and high temperatures. It is tolerant to some degree of waterlogging, although the young plants appear to be susceptible (de Carvallho & Veira, 1996). Ricebean is also known to be tolerant to acid soils (Dwivedi, 1996). Shattering is a problem in comparison with other grain legumes, and can be particularly serious under conditions of frequent wetting and drying.

Ricebean is a neglected crop, cultivated on small areas by subsistence farmers in hill areas of Nepal, northern and northeastern India, and parts of southeast Asia. It can be grown in diverse conditions and is well known among farmers for its wide adaptation and production even in marginal lands, drought-prone sloping areas, and flat rainfed tars (unirrigated, ancient alluvial river fans). It is mainly grown between 700 and 1300 m asl, although in home gardens it is found from 200 up to 2000 m. Most of the crop currently grown in Nepal is used as food for humans, with a smaller proportion used for fodder and green manuring.

Generally, ricebean is grown as an intercrop with maize, on rice bunds or on the terrace risers, as a sole crop on the uplands or as a mixed crop with maize in the khet (bunded parcels of lands where transplanted rice is grown) land. Under mixed cropping with maize it is usually broadcast some time between sowing maize and that crop’s first and second earthing up, so ricebean sowing extends from April-May to June.

Ricebean is valuable for its ability to fix nitrogen in depleted soils and in mixed cropping with local varieties of maize, as well as for its beneficial role in preventing soil erosion. The crop receives almost no inputs, and is grown on residual fertility and moisture and in marginal and exhausted soils. Anecdotal evidence indicates that the area and production of ricebean in Nepal is declining due to the introduction of high yielding maize varieties and increasing use of chemical fertilizers, while consumption is decreasing due to increased availability of more preferred pulses in the local markets. No modern plant breeding has been done and only landraces with low yield potential are grown. These have to compete with other summer legumes such as soybeans (Glycine max), black gram, cowpea, common beans (Phaseolus vulgaris) and horse gram (Mactrotyloma uniflorum). Other production constraints that limit the production of ricebean include small and fragmented land holdings and declining productivity.

Morphology
Wild forms are typically fine-stemmed, freely-branching and small-leaved, with a twining habit, photoperiod sensitivity and indeterminate growth (Lawn, 1995). Flowering is asynchronous, and there is a tendency to hard seeds. In many areas, landraces which retain many of these characteristics persist, in particular with regard to daylight sensitivity, growth habit and hard seeds. Seed colour is variable, but commonly red or yellow. The red type is commonly named as 赤小豆 in Chinese, literally meaning 'red small bean'.

Nutitional aspects
Ricebean plays an important role in human, animal and soil health improvement. All varieties seem to be good sources of protein, essential amino acids, essential fatty acids and minerals (Mohan & Janardhanan, 1994), and the dried seeds make an excellent addition to a cereal-based diet.

Human nutrition
Ricebean is most often served as a dal, either soaked overnight and boiled with a few spices, or cooked in a pressure cooker. Apart from various recipes for dal soups and sauces, pulses are also used in a number of other ways, either whole, cooked or roasted, as flour, or ground to make various deep fried dishes or snacks. Some recipes are specific to specific pulses, but many are open to substitution. The consumption of green pods as a vegetable has been recorded but is not widespread. However, this use should be encouraged since the indeterminate growth habit of many varieties is beneficial in providing a steady supply of green pods over long periods of the year.

Animal nutrition
Ricebean is valuable as a high class fodder which is known to increase milk production in livestock.

Medicine
In traditional Chinese medicine, the red type rice beans help to relieve oedema in some illness; they are also used in combination with Angelica sinensis (當歸) to remedy red eyes (possibly uveitis) with erosion of oropharyngeal or urogenital mucosae, which was suspected to be Behçet's disease.

Germplasm collections
The World Vegetable Centre (formerly the Asian Vegetable Research and Development Center) based in Taiwan has 197 accessions of ricebean, including 8 genotypes from Nepal and 24 from India. However, there is little or no passport data (World Vegetable Center, 2007), other than for a Nepalese genotype (given the name Mogimass), collected at 2000 m in Bajura district. The Indian genotypes IC 7588, IC 8229, EC 18771, and IC 7506 are noted as being less sensitive to photoperiod, but no other information is given. In India, the National Bureau of Plant Genetic Resources (NBPGR) contains over 1700 accessions from a variety of Asian countries (NBPGR, 2007). As well as this, there is a collection held at the Indian Institute for Pulses Research, and the NBPGR station at Bhowali, Uttar Pradesh, also maintain a collection of over 300 genotypes (Negi et al, 1996). In Nepal, the Plant Genetic Resources Unit of the NARC maintains a collection of some 300 accessions from various parts of the country.

Potential
So far little has been done to exploit ricebean's potential: there are several features that need attention from breeders before it could be widely adopted. Most varieties are highly photoperiod sensitive, and so when grown in the subtropics are late flowering and show strong vegetative growth. Their twining habit makes them very suitable for use as intercrops with such species as maize, sorghum and possibly some of the minor millet species, which can provide support, but also makes them difficult to harvest. Many of the current varieties are susceptible to shattering, and show high levels of hard seededness. Some crop improvement work has been carried out on ricebean in India, but not in Nepal. However, the use of ricebean as a green manure crop was studied in a series of field experiments in Nepal, and this revealed that it is one of the best legumes for the purpose due to high biomass production over a short period of time, is easy to incorporate into the soil, and decomposes rapidly (reference needed).