Oryza sativa (PROTA)
|Geographic coverage Africa|
|Geographic coverage World|
|Cereal / pulse|
|Forage / feed|
Oryza sativa L.
- Protologue: Sp. pl. 1: 333 (1753).
- Family: Poaceae (Gramineae)
- Chromosome number: 2n = 12, 24, 36
- Rice, paddy, Asian rice, Asiatic rice (En).
- Riz, riz asiatique (Fr).
- Arroz (Po).
- Mpunga (Sw).
Origin and geographic distribution
Oryza sativa evolved in Asia, but the exact time and place of its domestication are not known for certain. Remains of rice in China have been dated to 6500 BC; the earliest archaeological evidence from India goes back to 2500 BC. Oryza sativa was brought from Asia into tropical Africa along different routes. Seamen-farmers began sailing from Indonesia to Madagascar probably a few centuries BC and started cultivating Oryza sativa there. Another important contact between Africa and Asia at the dawn of the Christian era was the trade route from Sri Lanka and India via Oman to Somalia and the islands Zanzibar and Kilwa off the coast of Tanzania. Most probably Oryza sativa migrated from Egypt, where it was introduced about 800–900 AD, to West Africa. The final penetration of Oryza sativa into Africa was along the slave trading routes from the East African coast and Zanzibar to DR Congo from about 1500 AD onwards. At the same time Oryza sativa was introduced into Senegal, Guinea Bissau and Sierra Leone by the Portuguese on their return from expeditions to India. Nowadays it is cultivated throughout the humid tropics and in many subtropical and temperate areas with a frost-free period longer than 130 days.
The rice grain is cooked by boiling or steaming, and eaten mostly with pulses, vegetables, fish or meat. Flour from rice is used for breakfast foods, baby foods, bread and cake mixes and cosmetics. Starch made from broken rice is used as laundry starch and in foods, cosmetics and textile manufacture. Beers, wines and spirits are made from rice.
The husk or hull is used as fuel, bedding, absorbent, packing material and as carrier for vitamins and drugs; it is also made into building board. The charred hull is used for filtration of impurities in water, a medium for hydroponics and manufacture of charcoal briquettes.
Rice bran or meal obtained in pearling and polishing is a valuable livestock and poultry feed. Oil is extracted from the bran. Crude rice bran oil is processed into solidified oil, stearic and oleic acids, glycerine and soap. Processed bran oil is used for cooking, antirust and anticorrosive agents, textile and leather finishers, and in medicine.
Rice straw is used for animal feed and bedding, for the manufacture of straw boards and pulp for paper, for the production of compost and mushroom growing medium, for mulching vegetable crops, for making ropes, sacks, mats and hats, for roof thatching, and to make plastering material (mixed with clay mud) for the construction of houses, and for incorporation into the soil or burning on the field as a way to maintain/improve soil fertility.
Several traditional medicinal applications of rice have been reported from tropical Africa: leaf dressings are applied to ulcers and grain decoctions are drunk to treat diarrhoea, as a diuretic and as an emollient. Rice powder is applied against itch in Senegal. In DR Congo a decoction of the roots, leaves and husks is taken against madness and beriberi.
Production and international trade
According to FAO estimates the average annual world production during 1999–2003 was 593 million t paddy (unhusked grain) from 153 million ha. Asia accounts for 90% of the world production and area. During 1999–2003 tropical Africa produced on average 11.9 million t paddy (2% of world production) annually on 7.7 million ha (5% of world area); these data include African rice (Oryza glaberrima Steud.), which occupies less than 20% of the rice area in West Africa. The main producers are Nigeria (3.5 million t from 2.9 million ha), Madagascar (2.6 million t from 1.2 million ha) and Côte d’Ivoire (1.1 million t from 0.5 million ha). The annual world paddy production increased steadily from 241 million t/year in 1961–1965 to 593 million t/year in 1999–2003, and the harvested area from 121 to 153 million ha. In the same period the annual paddy production in tropical Africa increased from 3.6 to 11.9 million t/year, and the harvested area from 2.8 to 7.7 million ha.
Only 5% of the world’s rice production enters into international trade. Thailand is the world’s largest exporter of milled rice (26% of world trade during 1998–2002) followed by Vietnam, India, the United States, China and Pakistan. All countries in tropical Africa are net importers of milled rice and during 1998–2002 an average of 4.8 million t milled rice was imported annually. This means that more than one third of the rice consumption in tropical Africa is satisfied through imports. Main rice importers are Nigeria, Senegal and Côte d’Ivoire. Per capita annual milled rice consumption in tropical Africa varies tremendously between 0.15 kg and 95 kg with an average of about 18 kg for the period 1998–2002. In Madagascar, Sierra Leone and Guinea Bissau it is the main source of energy.
Raw brown rice contains per 100 g edible portion: water 13.9 g, energy 1518 kJ (363 kcal), protein 6.7 g, fat 2.8 g, carbohydrate 81.3 g, dietary fibre 3.8 g, Ca 10 mg, Mg 110 mg, P 310 mg, Fe 1.4 mg, Zn 1.8 mg, thiamin 0.59 mg, riboflavin 0.07 mg, niacin 5.3 mg, vitamin B6 0.56 mg, folate 49 μg, ascorbic acid 0 mg. Raw polished rice contains per 100 g edible portion: water 11.7 g, energy 1536 kJ (367 kcal), protein 6.5 g, fat 1.0 g, carbohydrate 86.8 g, dietary fibre 2.2 g, Ca 4 mg, Mg 13 mg, P 100 mg, Fe 0.5 mg, Zn 1.3 mg, thiamin 0.08 mg, riboflavin 0.02 mg, niacin 1.5 mg, vitamin B6 0.30 mg, folate 20 μg, ascorbic acid 0 mg (Holland, Unwin & Buss, 1988). The essential amino acid composition of raw polished rice per 100 g edible portion is: tryptophan 87 mg, lysine 250 mg, methionine 140 mg, phenylalanine 330 mg, threonine 230 mg, valine 390 mg, leucine 560 mg and isoleucine 260 mg (Paul, Southgate & Russell, 1980). Milling and polishing result in a loss of protein, fat, minerals (phosphorus and potassium) and vitamins (thiamin, riboflavin and niacin). However, these operations improve the storability and reduce the cooking time.
Rice grain endosperm may be waxy (glutinous) or non-waxy (non-glutinous) depending on the content of amylose and amylopectin. The higher the amylopectin content, the more glutinous the product is. The endosperm also contains sugar, fat, crude fibre, vitamins and inorganic matter. The flavour of rice is variable and aromatic rice cultivars are highly appreciated throughout the world. A major component of the flavour is 2-acetyl-1-pyrroline.
Rice bran contains: water 9.9%, gross energy 1940 kJ (463 kcal) per 100 g, crude protein 13.8%, crude fibre 7.8%, ether extract 16.4%. After oil extraction, rice bran contains: water 9.8%, gross energy 1590 kJ (380 kcal) per 100 g, crude protein 14.4%, crude fibre 9.3%, ether extract 3.1%. The husk forms about 20% of the unhusked grain weight, and is very rich in silica. Rice straw contains approximately: water 7.0%, protein 3.4%, fat 0.9%, carbohydrate 47.8%, fibre 33.4% and ash 7.5%. It is nutritionally inferior to other cereal straws unless ensiled.
Rice straw is not particularly suitable for papermaking due to the high silica content (12–18%) and is used for this purpose mainly in countries where wood is scarce, e.g. in India and China. The ultimate fibre cells are (0.4–)1.4(–3.4) mm long and (4–)9(–16) μm wide.
- Annual grass up to 1.8 m tall (up to 5 m long in some floating types), forming small tufts; roots fibrous, arising from the base of the shoots; stem (culm) erect or ascending from a geniculate base, terete, smooth, glabrous.
- Leaves alternate, simple; sheath coarsely striate, tight when young, later somewhat loose, often somewhat spongy, green or sometimes tinged with brown or purple, smooth, glabrous; ligule 1.5–3 cm long, triangular, acute, entire or split, membranous, usually glabrous; auricles often present, falcate, 1–5 mm long, hairy; blade linear, tapering to an acute point, 12–65 cm × 0.5–2 cm, bright green to glaucous, glabrous or puberulous, smooth on the lower surface, slightly rough on the upper surface, midrib usually distinct.
- Inflorescence a terminal panicle up to 50 cm long, erect, curved or drooping, with 50–500 spikelets; branches solitary or clustered, nearly erect to spreading.
- Spikelet solitary, asymmetrically oblong to elliptical-oblong, 7–11 mm × 2.5–3.5 mm, with pedicel up to 4 mm long, 3-flowered but 2 lowest florets reduced to sterile lemmas 2–3 mm long; glumes small; lemma of fertile floret 6–10 mm long, boat-shaped, sometimes awned; palea about as long as lemma; lodicules 2; stamens 6; ovary superior, with 2 plumose stigmas.
- Fruit a caryopsis (grain), ovoid, ellipsoid or cylindrical, 5–7.5 mm × 2–3.5 mm, often whitish yellow or brown to brownish grey.
Other botanical information
Oryza comprises about 20 wild species distributed throughout the tropics and subtropics, and 2 cultivated species, Oryza sativa and Oryza glaberrima. In the most recent classification Oryza has been divided into 3 sections: sect. Padia, sect. Brachyantha and sect. Oryza. Section Oryza is subdivided into 3 series: ser. Latifoliae, ser. Australiensis and ser. Sativae. Oryza sativa is classified in ser. Sativae, together with, among others, Oryza glaberrima, Oryza barthii A.Chev., and Oryza longistaminata A.Chev. & Roehr. Oryza glaberrima cultivars are grown only in Africa. Introgression of characters from Oryza glaberrima, Oryza barthii and Oryza longistaminata may have added new dimensions to the variability of Oryza sativa.
Cultivated rice Oryza sativa is supposed to have evolved from perennial types (Oryza rufipogon Griff.) to annual types (Oryza nivara S.D.Sharma & Shastri, sometimes included in Oryza rufipogon). There is a natural gene flow between these 3 species, and they form a large species complex together with weedy forms of rice (popularly called ‘red rice’ because of their red endosperm). There are 2 major eco-geographical cultivar groups of Oryza sativa: Indica Group, which mainly includes cultivars from the tropics, and Japonica Group, which includes cultivars from temperate/subtropical areas. Traditional cultivars from Indica Group are tall, leafy, strongly tillering, and prone to lodging; they respond poorly to fertilization, particularly to nitrogen, and are sensitive to photoperiod; they are hardy, resistant to disease and tolerate unfavourable growing conditions; they will produce fair yields under conditions of low management. Modern Japonica Group cultivars are small, and are less tillering, less leafy, resistant to lodging, insensitive to photoperiod and are early maturing. The characteristics of the two cultivar groups have become less distinct because of the interbreeding programmes in recent years. Rice may also be classified according to the conditions under which it is grown, according to the size, shape and texture of the grain, or according to the period needed to mature.
Growth and development
Rice seed germinates in 24–48 hours. The optimum temperature for germination is 30–32°C. Most cultivars have a short dormancy or none at all, but in some it may last up to 4 months. Ten days after germination the plant becomes independent as the seed reserve is exhausted. Tillering begins thereafter, although it may be a week later in transplanted seedlings. In modern cultivars with an average maturation period, maximum tillering stage is attained around 45 days after transplanting and coincides with panicle initiation. The duration of the vegetative stage ranges from 7 to more than 120 days. The reproductive stage starts at panicle initiation, and the period from panicle initiation to flowering is around 35 days. Rice is almost 100% self-pollinating, but small amounts of cross pollination by wind do occur. It takes around 7 days to complete the anthesis of all spikelets in a panicle, starting from the top and progressing downwards. The period from flowering to full ripeness of all the grains in a panicle is usually about 30 days. Low temperature can delay maturity and high temperature accelerates it. Floating rice has a long maturation period of 7 months or more. Rice roots can grow under low oxygen concentrations. The roots are not typically aquatic as they are much branched and have a profusion of root hairs; later, spongy tissue (aerenchyma) develops in the cortex.
Rice is grown as far north as 53°N in Moho, northern China and as far south as 35°S in New South Wales, Australia. It grows on dry or flooded soil and at elevations ranging from sea level to at least 2400 m. The average temperature during the growing season varies from 20–38°C. Night temperatures below 15°C can cause spikelet sterility. Temperatures above 21°C at flowering are needed for anthesis and pollination. Upland rice requires an assured rainfall of at least 750 mm over a period of 3–4 months and does not tolerate desiccation. Lowland rice tends to be concentrated in flat lowlands, river basins and deltas. The average water requirement for irrigated rice is 1200 mm per crop or 200 mm of rainfall per month or an equivalent amount from irrigation. Relative humidity within the crop canopy is high, since there is standing water in most rice crops. A low relative humidity above the canopy during the dry season aggravated by strong winds can cause spikelet sterility. Traditional cultivars are generally photoperiod sensitive, and flower when daylengths are short (critical daylength of 12.5–14 hours). Many modern cultivars are photoperiod insensitive.
The soils on which rice grows vary greatly: texture ranges from sand to clay, organic matter content from 1–50%, pH from 3–10, salt content up to 1%, and nutrient availability from acute deficiencies to surplus. Rice does best in fertile heavy soils. The optimum pH for flooded soil is 6.5–7.0. The often sandy texture of soils in tropical Africa is a constraint to productivity due to drought stress, low inherent soil fertility and leaching. Groundwater salinity problems occur in the dry Sahel zone where rice is grown under irrigation. In lowland coastal West Africa rice productivity is affected by saline water intrusion. The majority of mangrove swamp soils along the West African coast are furthermore potential or actual acid sulphate soils. In West Africa iron toxicity in valley bottoms is most severe in areas where the adjacent uplands are strongly leached Ultisols. Lowland rice and deep-water rice may be subjected to both drought or complete submergence. In submerged soil the pH tends to be neutral, i.e. the pH of acid soils increases whereas the pH of calcareous and sodic soils decreases. Ions of Fe, N and S are reduced, the supply and availability of the elements N, P, Si and Mo improve, whereas the concentration of water-soluble Zn and Cu decreases. Toxic reduction products such as methane, organic acids and hydrogen sulphide are formed. The flooding of rice soils also creates a favourable environment for anaerobic microbes and the accompanying biochemical changes. As a result, the decomposition rate of organic matter decreases. However, a thin surface layer generally remains oxidized and sustains aerobic microbes.
Propagation and planting
Rice is propagated by seed. The 1000-seed weight is 20–35 g. The seed may either be broadcast or drilled directly in the field, or seedlings may be grown in nurseries and transplanted. Direct seeding is done in dry or puddled soil. In puddled soil the (pre-germinated) seeds are broadcast. After sowing the water level is kept at 0–5 cm under tropical conditions. In dry soil the seeds are sown just before or after land preparation. In the latter case the seeds are then covered lightly with soil. The seeds are sown just before the rains begin and germination occurs after heavy continuous rains. This method makes it possible to have initial crop growth from early rains.
In tropical Africa various rice-growing systems are distinguished:
- Upland rice, which may be subdivided into dryland rice, whereby moisture supply is entirely dependent on rainfall, and hydromorphic rice where the rooting zone is periodically saturated by a fluctuating water table, in addition to rainfall;
- Lowland rice, including mangrove swamp rice along the coastal regions with tidal intrusion, inland swamp rice on flat or V-shaped valley bottoms with varying degrees of flooding, and rice on bunded fields under rainfed or irrigated conditions;
- Deepwater rice, in which the rapid growth of the internodes keeps pace with the rising water up to 5 m or more, starting from 50 cm of standing water.
In upland rice cultivation the fields are normally cleared through the slash-and-burn practice. Soil preparation is normally minimal. The rice is broadcast or dibbled when the rains start. It is often grown as the first crop in rotation or intercropped with other crops such as cassava, maize, sorghum, cowpea, groundnut and other pulse crops.
In lowland rainfed-rice areas the land is mostly prepared while it is wet and only in rare occasions when it is dry. The wetland tillage method consists of soaking the land until the soil is saturated, ploughing to a depth of 10–20 cm using a plough drawn by oxen/small machines or by using a hand hoe, preferably when there is a little water on the land, and harrowing, during which big clods of soil are broken and puddled with water. The important benefits of puddling include the apparent reduction of moisture loss by percolation, better weed control, and easy transplanting. In lowland rice cultivation seedlings are mostly raised on wet nursery beds and sometimes on dry nursery beds. Wet nursery beds are made in the puddled or wet field. Normally farmers use 50–60 kg of rice seeds to plant one ha. Seeds are pre-germinated and spread on the bed which is kept constantly wet. Dry nursery beds are prepared near the water source before land preparation. The seeds are sown and then covered with a thin layer of soil and watered until saturation for uniform germination. Further watering is applied as needed. In both cases the seedlings are ready for transplanting 20–35 days after sowing. At transplanting heavy tillering cultivars in fertile valley bottoms are wider spaced (30 cm × 30 cm) than slightly tillering cultivars in upper, sandy fields (20 cm × 20 cm). The spacing in irrigated rice is normally 20 cm × 20 cm with 2–4 plants per hill (500, 000–1,000,000 plants/ha). Rice is generally a sole crop under lowland conditions. Near harvest, relay planting is rarely practised. In many parts of the tropics 2 or even 3 crops of rice can be grown per year. There is a lack of accurate data on the extent of different rice systems in tropical Africa. The upland rice ecosystem, including hydromorphic rice, accounts for an estimated 50% of the total rice area in tropical Africa; lowland rice cultivation, including mangrove swamp rice, inland swamp rice and irrigated rice, accounts for 45% of the total rice area; deep-water rice cultivation occupies the remaining 5%. Most rice is grown on smallholdings of 0.5–2 ha.
The agronomy of rice is diverse due to the differences in cultivation systems. Growing of upland rice is usually relatively labour-extensive, but transplanting rice by hand in puddled soil is a labour-intensive operation. Weeding is generally not necessary in the first 2 weeks. Manual weeding is common practice, although chemical weed control is also becoming popular in tropical Africa, especially in irrigated rice areas. Three timely weedings are normally necessary in broadcast rice.
In the cultivation of lowland rice, the land is inundated from the time of planting until the approach of harvest. The water is supplied either by flooding during the rainy season, by growing the crop in naturally swampy land or by controlled irrigation. The water level is kept at a height of 5–15 cm to suppress weed growth and to ensure water availability. Continuous flooding at a static 2.5–7.5 cm depth is best. The fields may be drained temporarily to facilitate weeding and fertilizing. At flowering the water level is gradually reduced until the field is almost dry at harvest. Generally 1.5–2 m of water (rainfall plus irrigation) are required to produce a good crop. The period in which rice is most sensitive to water shortage is from 20 days before to 10 days after the beginning of flowering.
Fertilizer application is limited in rice cultivation in tropical Africa. Only in irrigated rice with controlled water supply and modern cultivars do farmers generally use significant amounts of mineral fertilizers. The amount of fertilizer used is usually 60–120 kg N, 10–20 kg P and 0–30 kg K per ha. Higher nitrogen rates are used during the dry season when solar radiation is higher and increase in grain yield is larger. Generally, nitrogen fertilizer is only topdressed, mostly before or at panicle initiation. Fertilizer is broadcast by hand. The most common mineral deficiencies in rice cultivation are of nitrogen and phosphorus, with potassium and sulphur in limited areas and sometimes zinc and silicon on peaty soils. Deficiency of potassium is often associated with iron toxicity. Upland rice often suffers from sulphur deficiency. Zinc deficiency occurs regularly in rice areas because of the high pH and strong reduction of the soil. Influenced by reduction and poor internal drainage, several toxic elements such as iron, which inhibit the uptake of phosphorus in the plant, may accumulate in the environment of the root. Often a harmful excess of elements such as calcium is accompanied by a lack of other elements such as phosphorus, iron and zinc. Double cropping is inadvisable where physiological diseases occur. Green manure and Azolla are rarely used in tropical Africa. However, the fast growing and actively nitrogen-fixing Sesbania rostrata Bremek. & Oberm. is a promising green manure crop. Nitrogen fixation also takes place in paddy soils by Azotobacter and blue green algae (cyanobacteria). Organic fertilizers such as farmyard manure and compost are not commonly applied to rice crops in tropical Africa. Although soil conditions are normally improved by incorporating organic fertilizers, the result is not immediately apparent. Poor availability, transport problems and the high amount of labour involved also discourage its use.
The degree of mechanization is in general limited in rice cultivation in tropical Africa. Occasionally farmers use tractors or small two-wheel power tillers for land preparation and powered threshing machines during harvest.
For various reasons many rice fields are left fallow in the dry season. In areas with suitable climatic and soil conditions for dry-season cultivation, rice may be rotated with crops such as other cereals, pulses and vegetables.
Diseases and pests
The most common and severe disease of rice in tropical Africa is blast (Pyricularia grisea, synonym: Pyricularia oryzae). Although this disease is often related to drought stress and therefore especially severe in upland and drought-prone areas, it may also be severe elsewhere. Low light intensity, nutritional imbalances (especially K-deficiency), excessive N-supply, and relatively low temperatures (20–28°C) are further factors favouring this disease. The blast fungus can infect rice leaves, nodes and floral parts, particularly the basal part of the panicle. Other important diseases of rice in tropical Africa are bacterial leaf blight (Xanthomonas oryzae pv. oryzae), rice yellow mottle virus (RYMV, only found in Africa), brown spot (Cochliobolus miyabeanus), leaf scald (Microdochium oryzae), sheath blight (Thanatephorus cucumeris), narrow brown leaf spot (Cercospora janseana) and sheath rot caused by Sarocladium oryzae. The use of resistant cultivars, the judicious application of N fertilizer, adjusted planting time, crop rotation and phytosanitary and quarantine measures limit losses from rice diseases. Chemical control for blast and other rice diseases is hardly used in tropical Africa.
Nematodes attack roots and young, unfurled leaves and reduce rice production in certain parts of tropical Africa. Most insect species causing damage to rice in the field and to the grain during storage in tropical Africa are indigenous, and different from those found in Asia. Internal stem feeders such as stem borers, the stalk-eyed fly and gall midge generally cause the most severe damage. The most common species of stem borers in tropical Africa are white stem borer (Maliarpha separatella), pink stem borers (Sesamia spp.) and striped stem borer (Chilo spp.). Damage results from larvae feeding within the stem, severing the vascular system. Dead heart is the damage to the tiller before flowering. White head is the damage after flowering which causes the entire panicle to dry. The damage from the stalk-eyed fly (mainly Diopsis macrophthalma) resembles the dead heart damage from stem borers as it generally attacks the rice plant at the early tillering stage. The feeding of the gall midge maggot (Orseolia oryzivora) stimulates the leaf sheath to grow into a gall and tillers with galls do not bear panicles. Termites and mole crickets attack rice plants especially in rainfed upland rice.
The most serious insect pests of stored rice are the rice weevil (Sitophilus oryzae) and the lesser grain borer (Rhyzopertha dominica). These insects can completely destroy the grain.
Insects can be controlled by chemical, cultural, and biological methods. In tropical Africa farmers use insecticides but at far lower levels than in Asia. It is important to use various crop protection methods in an integrated pest management (IPM) system for rice in tropical Africa that is sustainable, inexpensive, and environmentally safe. It should combine the use of resistant cultivars, cultural methods, biological control and, finally, chemical control when pest damage threatens to exceed the economic injury threshold. Cultural methods include sanitation (the destruction of crop residues, of alternative hosts including weeds and of habitats), tillage and flooding of fields, crop rotation, intercropping, proper timing of planting and harvest, use of trap crops, and proper fertilizer and water management.
Birds eat broadcast seeds, disturb young transplanted seedlings and eat rice grains; losses can be very high. Rodents attack rice at all stages of growth and also stored grain, and losses due to rodents are often serious. Less damage is caused by snails, crabs and shrimps.
Parasitic weeds of the genus Striga may cause serious losses in upland rice, e.g. Striga aspera (Willd.) Benth. and Striga hermonthica (Delile) Benth. in West Africa, and Striga asiatica (L.) Kuntze in the Indian Ocean Islands.
Grain should be harvested before it is fully mature (around 21–24% moisture), usually about 30 days after flowering, or when 90% of the grains are firm and do not have a greenish tint. Wetting and drying cause grain cracking, cracks being formed more readily when the grain is quite hard. Harvesting by hand, the commonest method, is very labour-intensive. In some areas a small knife is used, but in many areas farmers use a sickle to cut the panicles plus some or all of the culms. Mechanical harvesters are very rare in tropical Africa. The harvested rice plants are either allowed to dry in the field or bundled for processing in a selected area.
Average rice yields are 1.4 t/ha in tropical Africa, 4.1 t/ha in Asia and 4.0 t/ha in the world in general. Yields are generally higher during the dry season than during the wet season, and higher in lowland rice than in upland rice. The yield of upland rice varies between 0.5 and 1.5 t/ha in tropical Africa but may reach 4 t/ha in Latin America. Rainfed lowland rice is higher yielding than upland rice but may suffer a drastic reduction in years with drought or floods. In a rainfed bunded lowland rice area in Tanzania yields are 3–4 t/ha in good years, but can drop to 0.5 t/ha in bad years. Yields of irrigated lowland rice in tropical Africa are generally 3–6 t/ha. Yields in the deep-water rice areas are generally low (0.6–1.2 t/ha), but they are more stable than in the upland rice areas of tropical Africa.
Handling after harvest
Threshing is generally done by hand, by beating the bundles on a stone or drum, or by beating the panicles with wooden sticks on a canvas. However, motorized and pedal-driven threshing machines are becoming popular. Winnowing is usually done by shaking and tossing the grain on a basket-work tray with a narrow rim. Sometimes hand-winnowing machines are used. After winnowing, the grain is dried in the sun and is then ready for hulling or transport to the mill. Proper drying of the rice grains is important to prevent germination and rapid loss of quality. Optimum moisture content for storage is 12.5%. Rice grain is mostly stored in sacks after drying. Increase in fat acidity during improper storage reduces the eating quality. Temperature and humidity during storage affect rice quality. Rice for home consumption is stored unhusked, as it is less susceptible to deterioration.
In rice milling the aim is to avoid breaking the kernels because whole kernels command a higher price. There are different methods of milling. On milling, the grain gives approximately: husk 20%, whole kernels 50%, broken kernels 16%, bran and meal 14%. The husked or hulled rice is usually called brown rice, and this is then milled to remove the outer layers, after which it is polished to produce white rice. During milling and polishing some of the protein and much of the fat, minerals and vitamins are removed, reducing the nutritional value but increasing storability and reducing cooking time. Parboiling (soaking, boiling and drying) before milling improves the nutrient value of the grains but it is not common in tropical Africa.
The exploration and collection of germplasm of African wild and cultivated rice species was started in 1959 by Japanese researchers who were attracted by the great diversity. The earliest collections of rice genetic resources in West Africa were built up at research stations at Rokupr, Sierra Leone and Badeggi, Nigeria. Later on the French research institutes ORSTOM and IRAT started collecting rice germplasm from francophone countries and IITA, Ibadan, Nigeria, from mainly anglophone countries. A combination of these germplasm collections with almost 15,000 accessions was then established by WARDA at Bouaké, Côte d’Ivoire. Most of these accessions are also available in the International Rice Germplasm Collection at the International Rice Research Institute (IRRI), Los Baños, the Philippines where the largest Oryza sativa collection is found with more than 86,000 accessions, characterized on the basis of about 80 traits. These traits not only include morphological characters but also susceptibility to diseases and pests, and reaction to environmental stresses, mineral deficiencies or toxicities. Large germplasm collections of Oryza sativa are also held in China (China National Rice Research Institute, Huangzhou, 70,000 accessions) and India (National Bureau of Plant Genetic Resources, New Delhi, 26,000 accessions). Apart from at WARDA, in tropical Africa large collections are present in Nigeria (International Institute of Tropical Agriculture (IITA), Ibadan, 9400 accessions; National Cereals Research Institute, Badeggi, 3500 accessions) and Madagascar (Département de Recherches Agronomiques de la République Malgache, Antananarivo, 2000 accessions). Collection of wild rices is being emphasized for possible new sources of important genes.
Rice grain yields in the tropics have increased dramatically since the mid 1960s with the introduction of ‘IR8’ and other semi-dwarf cultivars, which do not lodge easily and allow high nitrogen fertilizer doses. In tropical Africa these green revolution cultivars are mainly used in irrigated rice with controlled water supply. Genetic improvement of rice in Africa was mainly focused on the upland crop. This has led to the ‘New Rice for Africa’ (‘NERICA’) cultivars, WARDA’s major breakthrough in the early 1990s. ‘NERICA’ cultivars were the result of successful crossing of Oryza glaberrima with Oryza sativa. They combined higher tolerance to deep water, drought, weeds, blast and stalk-eyed fly from Oryza glaberrima with greater grain productivity and retention on the plant from Oryza sativa. ‘NERICA’ cultivars are proving to be popular with farmers, not only because of their growth characteristics, but also for their grain quality and nutritive value. They are further well suited to low-input conditions. Breeding activities of WARDA on lowland cultivars have led to the release of cultivars with improved grain yield, resistance to blast and rice yellow mottle virus and tolerance to drought and iron toxicity. The improved cultivar ‘Sahel 108’, released in 1994 by WARDA, has a short life cycle enabling double-cropping in the irrigated rice systems in the Sahel. Wild Oryza species, such as Oryza barthii, Oryza longistaminata and Oryza punctata are useful sources of resistance to various biotic and abiotic stresses. For instance, resistance to bacterial leaf blight has successfully been transferred from Oryza longistaminata.
Biotechnology techniques used in rice breeding include plant tissue culture, molecular biology and genetic engineering. Two tissue culture techniques, embryo rescue and anther culture, have already made important contributions. Saturated genetic linkage maps based on molecular markers have been developed for rice, using crosses between cultivars of Indica Group and Japonica Group, or between Oryza sativa and Oryza longistaminata. These maps have made possible the identification of QTLs for many useful traits, such as resistance to diseases and tolerance to drought. More than 3000 molecular markers are available now, making rice the best characterized crop. The project for sequencing the complete rice genome has recently been completed. Biotechnology’s most novel contribution will probably be in adding alien genes to the rice gene pool through genetic engineering. One example is ‘Golden Rice’, which is rice enriched with vitamin A. It is, however, still not clear if this genetically modified rice will yield well, not be susceptible to diseases and pests and be palatable. Several insecticidal toxin genes from Bacillus thuringiensis (Bt) have been transferred to rice and plants containing Bt genes have shown substantial resistance to stem borers and leaf folders. Recently, transgenic rice has been obtained conferring resistance to sheath blight. Genetic engineering is a relatively new technology and one of the principal biosafety concerns is the spread of foreign genes by pollen dispersal from transgenic rice to other rice cultivars and wild rice species.
At present, only an estimated 2% of the 200 million ha of wetlands in tropical Africa are used for lowland rice cultivation. Therefore one of the biggest challenges for rice development in tropical Africa is the utilization of the large potential for expansion of lowland rice. The emphasis of genetic improvement should be directed to lowland rice ecosystems, which have a higher production potential than upland rice, for example the breeding of crosses of Oryza sativa and Oryza glaberrima for lowland rice ecosystems. Any new types recommended should be well adapted to the local environment and methods of cultivation. For that matter it is advisable that in the breeding process greater use is made of farmer participatory varietal selection (PVS) and farmer participatory plant breeding (PPB). Breeding activities for tropical Africa should include tolerance of and adaptation to iron toxicity, salinity, alkalinity, acid sulphate soils, and relatively extreme cool and hot temperatures. In tropical Africa there is still much room for increased and integrated use of organic and mineral fertilizers with a higher efficiency and greater use of nitrogen-fixing legumes, bacteria and blue-green algae. The applicability of methods of integrated soil fertility management (ISFM) in a certain locality can be best tested through farmer field schools. An increased use of farmer field schools is also advocated for the adoption of methods of integrated pest management (IPM) by more rice farming households in tropical Africa. Further improvements are expected from mechanization of rice farming, especially regarding land preparation, weeding, harvesting, threshing and further processing.
All these suggestions require research adjusted to the local conditions, a well-functioning extension service, government support, and active participation of farming households. Some of the above topics are already being researched.
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Correct citation of this article
Meertens, H.C.C., 2006. Oryza sativa L. In: Brink, M. & Belay, G. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. Accessed 8 December 2022.
- See the Prota4U database.