Oryza sativa (PROSEA)
Oryza sativa L.
- Protologue: Sp. pl.: 333 (1753).
- Family: Gramineae
- Chromosome number: 2n= 24
- Oryza glutinosa Lour. (1790),
- O. montana Lour. (1790),
- O. praecox Lour. (1790),
- O. aristata Blanco (1837).
- Rice (En)
- Riz (Fr)
- Indonesia: padi (general), pari (Javanese), pare (Sundanese)
- Malaysia: padi
- Papua New Guinea: rais
- Philippines: palay. Burma (Myanmar): sabarbin
- Cambodia: srö:w
- Laos: khauz
- Thailand: khao
- Vietnam: lúa.
There are specific vernacular names for the rice grain, unhulled grain, polished rice, cooked rice (also depending on how it is cooked), left-over rice and even rice stuck to the bottom of the pot. The crop and the unhusked grain are known as paddy.
Origin and geographic distribution
O. sativa evolved along the foothills of the Himalayas and was probably first cultivated in ancient India. Rice has been cultivated for about 9000 years. In Indonesia, Malaysia and the Philippines rice cultivation began some time after 1500 BC. The earliest evidence of cultivated rice was found in the Ban Chian excavation in Thailand, dating cultivated rice at 500-900 BC. Rice is planted throughout the humid tropics and in many subtropical and temperate areas with a frostfree period longer than 130 days.
Rice is the main staple food of 40% of the world population and the main food throughout South-East Asia. The rice grain is cooked by boiling in water or by steaming, and is eaten mostly with pulses, vegetables, fish or meat. It is often the main source of energy. Flour from rice is used for breakfast foods, meat products, baby foods, bread and cake mixes, and cosmetics. The waxy rice flour has superior qualities as a thickening agent for white sauces, puddings and oriental snackfoods. Glutinous rice is used for making sweetmeats. Starch is made from broken rice, and used as laundry starch, in foods, and textile manufacture. Beers, wines and spirits are manufactured from rice.
The husk or hull is used as fuel, bedding, absorbent, building board, and carrier for vitamins, drugs, toxicants, etc. The charred rice hull is used for filtration of impurities in water, medium for hydroponics and manufacture of charcoal briquettes.
The rice bran or meal obtained in pearling and polishing is a valuable livestock and poultry food. It consists of the pericarp, the aleurone layer, the embryo and some of the endosperm. The bran contains 14-17% oil. Crude rice bran oils are used for producing 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. China, India, Japan, Vietnam and Thailand are the main producers of rice-bran oil.
Rice straw is used for animal feed and bedding, but is nutritionally inferior to other cereal straws unless ensiled. It is used for the manufacture of straw boards and pulp for paper, for mushroom growing medium, for the production of organic manure, for mulching crops such as onions, garlic and cucurbits, and only rarely for rope and roof thatch.
Production and international trade
The yearly fluctuation in rice production resulting from government policies, environmental aberrations such as drought and flood, availability of inputs, and other factors is reflected in the international trade. Some importing countries become exporting in certain years (e.g. the Philippines). Other countries where rice is a main staple food are perennial importers (e.g. Malaysia) or exporters (e.g. Thailand).
Probably half the world production is consumed on the farms where it is grown while only 5% enters into international trade. Approximately 3-4% of the rice crop is required annually for planting.
The 1994 world rice area was 146 million ha with a production of 530 million t. Asia accounts for 90% of the world production and area. China, India and Indonesia are the largest producers. Malaysia was one of the largest importers, while Thailand continues to be the world's largest exporter of rice (34% of world trade) followed by the United States, Pakistan, Vietnam and China. Cambodia and Sri Lanka generally import rice although Cambodia used to export rice.
Per capita annual consumption in South-East Asia is about 142 kg. The trend is for per capita intake to decline with rising income.
The grain is the important economic yield component of the rice plant and its endosperm is the final product consumed. The endosperm consists mainly of starch granules embedded in a proteinaceous matrix. The 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 composition of rice depends upon the method and degree of milling, polishing and whether or not it has been parboiled. It is also influenced by genetic and environmental factors to some extent. Analyses of brown (and white) rice give the following composition per 100 g edible portion: water 12 g, protein (6.7-)7.5 g, fat (0.4-)1.9 g, carbohydrates 77.4(-80.4) g, fibre (0.3-)0.9 g and ash (0.5-)1.2 g. Milling and polishing result in a substantial loss of protein, fat, minerals (phosphorus and potassium) and vitamins (thiamin, riboflavin and niacin). Parboiling results in the retention of more minerals, particularly phosphorus, and vitamins.
The 1000-kernel weight of paddy rice is 20-35 g; about 20% of this weight is the husk. The extracted bran is rich in proteins and carbohydrates. The husk is very rich in silica. Rice straw contains approximately: water 7.0%, protein 3.4%, fat 0.9%, carbohydrates 47.8%, fibre 33.4% and ash 7.5%.
- Annual grass, 50-130 cm tall, up to 5 m long in deep-water rices, forming small tufts.
- Roots fibrous, arising from the base of the shoots.
- Stem (culm) erect to ascending, glabrous, composed of a series of nodes and internodes, the number depending on cultivar and growing season; each node with a single leaf, and sometimes also with a tiller or adventitious roots; internodes usually short at base of plant, progressively increasing towards top.
- Leaves in two ranks; sheaths initially enclosing each other, forming a pseudostem, later enclosing the internodes; ligule triangular to linear-lanceolate, 1-1.5 cm long, often split; auricles often present, falcate, 1-5 mm long, hairy; blade linear, 24-60 cm × 0.6-2.2 cm, glabrous, smooth to scabrous, often with spiny hairs on margin.
- Inflorescence a terminal panicle, 9-40 cm long, with 50-500 spikelets depending on the cultivar.
- Spikelets single, borne on a short pedicel, oblong to lanceolate, 7-11 mm long, about 2-3 times longer than wide, containing a single bisexual flower, with 2 small glumes, a large, 6-10 mm long, boat-shaped lemma sometimes with an awn up to 15 cm long, and likewise palea with very short awn, 6 stamens, a broad ovary, and 2 plumose stigmas.
- Fruit (caryopsis, grain) varying in size, shape and colour, ovoid, ellipsoid or cylindrical, 5-7.5 mm × 2-3.5 mm, often whitish-yellow or brown to fuscous.
Growth and development
Growth and development depend on cultivar and environmental conditions. Seed soaked in water germinates in 24-48 hours. Seed dormancy may last up to 4 months but most cultivars have a short dormancy or none at all. Ten days after germination the plant becomes independent as the seed reserve is exhausted. Tillering begins thereafter, although it may be set back for a week in transplanted seedlings. Roots can grow under low oxygen concentrations. During early growth the roots are positively geotrophic, but by the time of panicle initiation, they are growing horizontally to produce a dense surface mat. The roots are not typically aquatic as they are much branched and have a profusion of root hairs; later, aerenchyma develops in the cortex. The type of root development is largely dependent upon the nature of the soil, the method of cultivation and differences between cultivars. Upland rice cultivars usually have fewer tillers and larger panicles. Floating rices have a long maturation period of 7 months or more. 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 varies greatly among cultivars and depends on the photoperiod sensitivity of the cultivar and the season of planting.
In photoperiodsensitive cultivars, panicle initiation occurs only when daylength is less than the critical. Panicle initiation is delayed by long daylength and occurs several days after maximum tillering stage. Thus, the vegetative stage might range from 7 to more than 120 days. The reproductive stage starts at panicle initiation, and the period from panicle initiation to flowering or anthesis is around 35 days. Rice is mainly selfpollinating, but varying small amounts of cross pollination by wind do occur. Anthesis occurs during the day between 10 a.m. and 2 p.m. depending on temperature and humidity. The higher the temperature and the lower the humidity, the earlier the anthesis. It takes around 7 days to complete the anthesis of all spikelets on a panicle, starting from the top and progressing downwards. The period from flowering to full ripening of all the grains in a clump is usually about 30 days. Low temperature can delay maturity and high temperature accelerates it.
Other botanical information
O. sativa has a polyphyletic origin and developed from hybrids of wild species. There is disagreement about the identity of the wild relatives of rice. Some consider them to be one species (O. rufipogon Griffith), others contend they are two species (O. rufipogon and O. nivara Sharma & Shastry). The evolution of the cultivated rice O. sativa is supposed to extend from perennial forms (O. rufipogon) to annual forms (O. nivara). O. sativa, O. rufipogon and O. nivara form a large species complex together with weedy forms of rice (popularly called "red rice" because of the red endosperm). Cultivars are developed through hybridization, selection, introgression and inter-cultivar recombination. There are believed to be around 100 000 different cultivars and selections of rice. The widespread dispersal of the Asian cultivated forms led to the formation of two major ecogeographical cultivar groups: the indicas, which are mostly from the tropics, and the japonicas, from temperate areas. Traditional indica cultivars are tall, leafy, high 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 rices have short stiff straw, and are less tillering, less leafy, resistant to lodging, insensitive to photoperiod and are early maturing. Modern indicas have similar characteristics to the japonicas. The characteristics of the two cultivar groups have become less distinct because of the interbreeding programmes in recent years. On the basis of chemotaxonomy a third group previously known as javanicas has now been assigned to the tropical japonicas.
Rice may also be classified according to the conditions under which it is grown, namely:
- upland rice (also called hill rice), grown as a rainfed crop, generally low tillering with large tillers and panicles;
- lowland rice (also called swamp rice), grown on irrigated or flooded land;
- deepwater rice (also known as floating rice), grown in areas of deep flooding, up to 5 m or more, in which the rapid growth of the internodes keeps pace with the rising water.
Rice cultivars can also be classified according to the size, shape and texture of the grain, or according to the period needed to mature.
O. glaberrima Steudel cultivars are grown in Africa only.
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. Tolerant cultivars can withstand varying degrees of low temperature. 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 and flower at any latitude, provided temperature is not limiting.
Rice yields are higher when solar radiation during the reproductive and ripening phases is high, so that generally grain yields are higher during the dry season than during the wet season. Low temperature limits the range of the rice crop. The average temperature during the growing season varies from 20-38 °C. Rice is most susceptible to low temperature at the stage of panicle initiation, when temperatures below 15 °C at night can cause spikelet sterility. Low temperature can also result in poor germination or death of seedlings, yellowing of leaves, low tiller number, degeneration of spikelets, high sterility, stunting, and poor panicle exsertion causing low grain yields. Low soil and floodwater temperatures also affect the nutrition, growth and grain yield of rice. Temperatures above 21 °C at flowering are needed for anthesis and pollination.
The chief factor limiting the growth of rice is the water supply. However, the water regime in which rice is growing and the water requirements are variable. Upland rice, grown as a rainfed crop, 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. In SouthEast Asian countries the average water requirement for irrigated rice is 1200 mm per crop or 200 mm of rainfall per month.
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.
Rice is generally grown at sea-level but also in mountainous areas of SouthEast Asian countries. Coldtolerant cultivars are grown up to 1230 m in the Mountain Province of the Philippines and up to 2300 m in the northwestern Himalayas. No direct effect of altitude is evident. Cold-tolerant cultivars do not differ morphologically from other cultivars. However, such cultivars can withstand 12 °C water temperature at seedling stage, 15-17 °C night temperature during panicle initiation and 21 °C day temperature during anthesis.
Rice does best in fertile heavy soils. Rice can be planted in dry soil or puddled soil and grown like an upland crop, or in inundated soils. 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 from almost 0-1%, and nutrient availability from acute deficiencies to surplus. The optimum pH for flooded soil is 6.5-7.0. Because land management depends on soil, climate, water supply, and socioeconomic conditions of the area, there is a considerable range in the pedogenetic and morphological characteristics of ricegrowing soils. Rice is grown primarily in submerged soil, and the physical properties of the soil are relatively unimportant as long as sufficient water is available. Pore spaces are important physical properties as they influence the retention and movement of water and air. The soil pH before and after lowland fields have been flooded is an important determinant of soil fertility and of the management of rice soils. 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 is improved, whereas the concentration of water-soluble Zn and Cu decreases; toxic reduction products such as methane, organic acids and hydrogen sulphide are formed. The chemical composition of the soil varies among regions, countries, and areas. The flooding of rice soils 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. The main biochemical processes in flooded soil are a series of successive oxidationreduction reactions mediated by different types of bacteria. Nitrogen fixation takes place in paddy soils by Azotobacter and bluegreen algae.
Lowland rice and deep-water rice may be subjected to drought or complete submergence. There is varietal tolerance of such adverse conditions.
Propagation and planting
Different systems of growing rice have evolved to suit specific environments and socioeconomic conditions. Rice culture can be classified according to the water source as rainfed, floodfed, or irrigated. Based on land management practices, rice lands can be grouped as: lowland (wetland preparation of fields) or upland (dryland preparation of fields). Using water regime as the criterion, rice lands can be classified as: upland (without standing water), lowland (with 5-50 cm of standing water), or deep-water (with 50-600 cm of standing water). There is a lack of accurate data on the extent of different rice cultures. In Asia, lowland rice culture is the most prevalent system. The upland rice ecosystem accounts for 19% of the total rice area in Asia, but only for 4.5% of the production. Land is prepared either wet or dry and water is generally retained in the field by bunds. Systems of lowland rice cultivation are usually traditional, based on centuries of experience. Most rice is grown on smallholdings, usually of 0.4-2 ha. In Africa and Latin America, upland rice culture is the major system and rice is grown on level and sloping fields that are not bunded.
The rice crop is propagated by seed, which may either be broadcast or drilled directly in the field, or the seedlings may be grown in nurseries and transplanted. Direct seeding is done in dry or puddled soil. In puddled soil the (pregerminated) seeds are broadcast. The water level is kept at 0-5 cm under tropical conditions, but higher in temperate areas. This type of sowing is possible in combination with the use of herbicides, and is becoming an important method of rice culture in Thailand, Malaysia and the Philippines, due to rising labour costs. In dry soil the seeds are sown after land preparation and are then covered lightly with soil by a tooth harrow. Germination occurs after heavy continuous rains. In upland rice cultivation the land is prepared in the dry weather and the rice is broadcast or dibbled in when the rains start. It may be grown in rotation or intercropped with other crops such as cassava, maize, groundnut and other pulse crops. Floating rice is cultivated in areas subject to deep flooding, and the seed is sown either dry or wet.
The three major methods of raising seedlings common in lowland rice cultivation are the dry bed, wet bed and dapog:
- Dry seed-bed: The nursery bed is prepared near the water source before land preparation. The bed is about 1.5 m wide and the seeds are sown at 1 kg per 10 m2 and then covered with a thin layer of soil and watered until saturation for uniform germination. Further watering is applied as needed. The seedlings are ready for transplanting 20-35 days after sowing.
- Wet seed-bed: The raised nursery bed is made in the puddled or wet field, and is about 1.5 m wide. About 400 m2 will accommodate 45 kg of rice, which is sufficient to plant one ha. Seeds are pregerminated and spread on the seed-bed which is kept constantly wet. When seedlings are 2-3 cm tall, continuous shallow irrigation is practised. Water depth is increased to 5 cm as the seedlings grow taller. The seedlings are ready for transplanting 20-35 days after sowing.
- Dapog: Pregerminated seeds are sown on cement or puddled soil covered with banana leaves or plastic sheets. Seeding density is much higher, 60 kg seeds per 40 m2, which is sufficient to plant one ha. The pregerminated seeds are pressed in lightly and continuously watered. The resulting mat is rolled up and taken to the field for transplanting after 11 days. This method is used in some provinces of the Philippines, and has been adopted by other countries.
Although intercropping is practised in upland rice, rice is generally a sole crop under lowland conditions. In many parts of the tropics 2 or even 3 crops of rice can be grown per year, provided water, fertilizer and dayneutral cultivars are available for 1 or 2 of the crops. Near harvest, relay planting is rarely practised.
Land preparation varies, even within the lowland rainfedrice areas:
- Wetland tillage: This method is common in most tropical Asian countries. It consists of soaking the land, which entails water being absorbed until the soil is saturated; ploughing, which is the initial breaking and turning over of the soil, to a depth of 10-20 cm, using a wooden or light iron plough drawn by 1-2 buffaloes or oxen, preferably when there is 7.5-10 cm of 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. The low redox potential of submerged puddled soil helps to conserve water-soluble nutrients, favours accumulation of ammonium, increases biological nitrogen fixation and increases the availability of phosphorus, silicon, iron, and manganese.
- Dryland tillage: The land is prepared in dry weather and the rice is sown just before the rains begin. This method makes it possible to have initial crop growth from early monsoon rains. Less labour is required for seed-bed preparation, land preparation and transplanting, and the soil structure is better for the stand establishment of the following nonrice crop. This method has its disadvantages: weed control is a major problem; percolation losses are high, increasing the risk of drought stress; also, fertilizer requirements are often higher. Bunding and levelling are essential in order to hold the water on the land and maintain it at the required depth. The land is divided into fields by contour bunds; field size and shape vary with the topography. The bunds are usually made of clay, mud and weeds, with controlled openings to allow water in and out.
The agronomy of the rice crop is rather diverse. Rice is mostly hand-transplanted in puddled soil. This is a labour-intensive operation. About a third of the transplanted seedling should be above the water. The spacing is 15-25 cm (160 000-400 000 plants/ha). Transplanting in rows facilitates hand weeding. The operations required after the crop has become established are weeding, application of manure and fertilizers, and the regulation of the water supply. Weeding is not necessary in the first 2 weeks. Weeding up to 40 days after transplanting increases grain yields. Manual weeding is common practice, although rotary weeders are also commonly used in some areas. Chemical weed control, either pre or postemergence, is also becoming popular in the tropics, especially in areas where pregerminated seed is broadcast in puddled soil. Broadcasting of seed will increase with rising cost of labour. The water level in the fields is kept at a height of 5-15 cm to suppress weed growth and to ensure water availability. Weeds are worse in a broadcast crop than in transplanted rice. In many countries the most serious weed of rice is wild red rice (Oryza rufipogon). Other serious weeds include grasses, such as barnyard grass (Echinochloa crus-galli (L.) P. Beauvois), several Cyperaceae and water hyacinth (Eichhornia crassipes (Martius) Solms).
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 where water is guided through irrigation canals or lifted from wells by human or animal power. Continuous flooding at a static 2.5-7.5 cm depth provides the potential to produce optimum rice yields. The fields may be drained temporarily to facilitate weeding and fertilizing. At flowering the water is gradually reduced until the field is almost dry at harvest. Generally speaking 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 20 days before to 10 days after the beginning of flowering.
Fish often occur in paddy fields and help to supplement the rice diet. In some cases they are deliberately introduced and trenches are provided to give deeper water. The most commonly used species are carp (Cyprinus) and Tilapia. Pesticides often prove to be toxic to fish in rice fields.
Fertilizer application is recommended at final harrowing, but farmers generally fertilize later, including topdressing of nitrogen. The amount of fertilizer used depends on cultivar, season, soil, and availability. Modern cultivars produce higher yields with higher nitrogen levels. At high rates of nitrogen the traditional cultivars become too vegetative, tall, and are prone to lodging. A rice crop producing about 3500 kg/ha of grain and an equal quantity of straw removes approximately 56 kg N, 12 kg P and 48 kg K from the soil. 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, which is common on acid, sandy, latosolic (ultisols and oxisols), sulphate and peat soils (histosols). 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.
In Indonesia and elsewhere, phosphorus is often limiting, and must be added to achieve a significant response to nitrogen. Higher nitrogen rates are used during the dry season when solar radiation is higher and increase in grain yields is larger. In many areas of the tropics, the availability of commercial inorganic fertilizer to farmers is still a problem. Generally, only nitrogen fertilizer is topdressed, mostly before or at panicle initiation. Fertilizer is broadcast by hand.
Physiological diseases occur in the rice plant when the uptake of nutrients is disturbed. 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 an excess of harmful 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 seldom used in the tropics although the technology to apply them is available. Sesbania rostrata (Bremek. & Oberm.) Gillet is a promising green manure crop. Organic fertilizer is not commonly applied to rice crops, although it used to be popular in China and Vietnam. Although soil conditions are improved by incorporating such fertilizer, the result is not immediately apparent. The cost and labour involved also discourages its use.
The degree of mechanization varies; in some countries the land preparation, seeding or transplanting, fertilizer application, herbicide application, harvesting, threshing and drying are fully mechanized. Hand tractors are becoming popular, and large deep-water rice areas in Thailand are ploughed with large tractors. It takes 105 hours to plough one hectare with animal traction compared to 35 hours with hand tractors. Threshing machines are the most popular machines in the rice farms of SouthEast Asia.
For various reasons many rice fields are left fallow in the dry season. In areas with suitable climatic and soil conditions for dryseason cultivation, rice may be rotated with crops such as cereals, pulses and vegetables.
Diseases and pests
The most serious diseases of rice are blast (Magnaporthe grisea, anamorph Piricularia grisea), bacterial leaf blight (Xanthomonas campestris pv oryzae), and tungro (virus disease). The blast fungus can infect rice plants at any growth stage. Typical leaf lesions are spindle-shaped (wide in the centre and pointed toward each end). Large lesions usually develop grey centres. Leaves of susceptible cultivars may be killed. At heading, the peduncle may be affected and the whole bent panicle rendered useless. Blast disease is more severe under humid conditions.
Bacterial leaf blight causes yellow to white lesions which begin as water-soaked stripes at the margins of a leaf blade. Lesions may start on one or both edges of a leaf or at any point on injured blades, and advance to cover the entire leaf blade. They may reach the lower end of the leaf sheath in susceptible cultivars. Bacteria invade the vascular system of the rice plant when roots are broken or when leaves are damaged during transplanting. High nitrogen fertilizer rates favour blight epidemics where suspectible cultivars are grown.
Tungro virus is the most important virus disease of rice in tropical Asia. Tungro virus vectors are Nephotettix malayanus, N. nigropictus, N. parvus, N. virescens, and the zigzag leafhopper (Recilia dorsalis). Outbreaks destroy plants in a large area in a short time. Infected plants are stunted and the number of tillers is reduced. Leaf colour changes from green to light-yellow to orange-yellow to brown-yellow, starting from the tips of olders leaves. Young leaves are often mottled or have pale green to white stripes of different lengths running parallel to the veins. Infected plants usually live until maturity but panicles are often small, sterile, and incompletely exserted. Low yields mainly result from fewer filled spikelets per plant.
Chemical control for blast and blight is expensive and is hardly used in the tropics. The insect vectors of the tungro virus can be controlled by insecticides, but this does not prevent the virus disease to occur on susceptible cultivars.
Insects cause extensive damage to the rice crop in the field and to the grain during storage. The brown planthopper (Nilaparvata lugens) can cause rice plants to die by feeding intensely on them. Brown planthoppers can attack susceptible cultivars in large numbers, causing hopperburn. Infested plants turn yellow and die. Hoppers also transmit grassy stunt, ragged stunt, and wilted stunt virus diseases. Different species of stem borers can cause serious damage to the rice crop; the most important species are striped borer (Chilo suppressalis) and yellow borer (Scirpophaga incertulas). Damage results from larvae feeding within the stem, severing the vascular system. Deadheart is the damage to the tiller before flowering. Whitehead is the damage after flowering which causes the entire panicle to dry. The most serious pests of stored rice are the rice weevil (Sitophilus oryzae) and the lesser grainborer (Rhyzopertha dominica). These insects bore into the sound kernels and can completely destroy the whole grain.
The high-yielding cultivars developed in the 1960s caused major shifts in the insect pest complex. Insecticide use has become a part of crop management recommendations in many developing countries. However, the indiscriminate use of insecticides has led to major outbreaks of insect pests such as brown planthopper and green leafhopper because of the destruction of indigenous predators and parasites that had kept pest populations in check. Considerable progress has been made on various methods of pest control. It is important to use all of these measures in developing an integrated pest management (IPM) programme that is sustainable, inexpensive, and environmentally safe. This means using non-pesticidal methods of pest control, and resorting to pesticides only when the pest causes economic loss. The various components of IPM are host plant resistance, cultural methods, biological control and, finally, chemical control when pest damage threatens to exceed the economic injury threshold. Resistant rice cultivars provide an inherent control which does not involve any expense or result in environmental pollution, and are generally compatible with other insect control methods. Cultural methods include sanitation (the destruction of crop residue and of alternative hosts including weeds and of habitats for aestivation), tillage and flooding of fields to destroy insect pests in stubbles, crop rotation, intercropping, timing of planting and harvest to avoid pest infestation, use of trap crops, and proper fertilizer and water management. One of the most important approaches to biological control in the rice ecosystem is conservation of the natural enemy complex. The IPM strategy emphasizes the need-based use of insecticides rather than prophylactic treatment.
In some countries, specific diseases and pests are serious threats to rice growing: ufra (caused by the nematodeDitylenchus angustus) in Vietnam and Bangladesh, gall midge (Orseolia oryzae) in India, Cambodia, Vietnam, Indonesia, Sri Lanka and Thailand.
The most effective control of most diseases and pests of rice is breeding for tolerant or resistant cultivars.
Grains should be harvested before 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. So, delayed harvesting results in a lower percentage recovery of whole grains. The rice plants are cut halfway up the stem and either allowed to dry in the field or bundled for processing in a selected area. Harvesting by hand, the commonest method, is very labour-intensive. In some areas a small knife is used, but the common method is to use a sickle which cuts the heads plus some of the straw. Mechanical harvesters are beginning to be used in many areas of South-East Asia. Drying to 14% moisture is necessary to prevent fungal and bacterial growth and to lessen the generation of heat and the decrease in dry matter caused by respiration.
Grain yields in SouthEast Asia are generally lower than in temperate areas. Average yields in t/ha in 1994 were 4.3 in Indonesia, 3.1 in Malaysia, 3.0 in the Philippines, 2.2 in Thailand, 3.5 in Vietnam, versus 5.8 in Korea, 6.0 in China, and 6.2 in the United States. The average world yield is 3.7 t/ha, higher than the yields in most tropical Asian countries. Yields are generally higher during the dry season than during the wet season and in irrigated than in upland rice. The grain yield of upland rice is around 0.5-2.0 t/ha in Asia but may reach 4 t/ha in Latin America. Upland rice yield in Indonesia is only one third of the yield of irrigated rice. Rainfed lowland rice is also higher yielding than upland rice but may suffer a drastic reduction in years with drought or floods. Although yields in the deep-water rice areas are generally low, they are more stable than in the upland rice areas of SouthEast Asia.
Handling after harvest
Threshing to separate the grain and its enclosing husks from the stalk may be done by hand, by beating the rice stalk bundles on a stone or slatted bamboo platform, or using animals to trample on the panicles, or using threshing machines of various sizes. Threshing machines are becoming popular as there is a need to process the harvest as soon as possible to reduce grain yield losses. Because of the shortage of cemented drying floors, roadsides are often used as drying floors. Winnowing is usually done by shaking and tossing the paddy on a basketwork tray with a narrow rim. The grain falls on the mat and the husk, chaff and dust are carried away by the wind. Handwinnowing machines are also available. After winnowing the paddy 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 grains are mostly stored in sacks after drying. Storage losses are often high because of poor facilities. Moulds, insects, rodents and birds affect both the quantity and quality of the grains. Increase in fat acidity during improper storage reduces the eating quality. Temperature and humidity during storage affect rice quality and these have to be taken into consideration in the proper storage of rice grains. Rice for home consumption is stored unhusked, as it is less susceptible to deterioration. It is then husked in small quantities to supply current domestic needs.
When milling maize or wheat the kernels are broken into small particles, but in rice milling the aim is to avoid breaking the kernels because whole kernels command a higher price. The percentage of head rice (whole kernels) depends on the drying process, cultivar, environment during maturity and the milling machine used. There are different methods of milling. On milling, paddy gives approximately: husk 20%, whole rice 50%, broken rice 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, including the aleurone layer and the germ, after which it is polished to produce white rice. Inevitably some of the grains are broken during husking and milling, giving rise to broken rice. During milling and polishing, some of the protein, fat, minerals and vitamins are removed, reducing the nutritional value but increasing eyeappeal and storability. Much of the vitamin B1(thiamine) may be lost and this may cause beriberi in consumers.
In Bangladesh and India, parboiling is common. This involves soaking, boiling and drying the rice grains before milling. The nutrient value of the kernels is improved with parboiling but the practice is not popular in SouthEast Asia.
Most national programmes have their own collections of rice cultivars. Indonesia has about 1300 accessions, Malaysia 5500 and Thailand 16 000. Most of these accessions are also available at the International Rice Research Institute (IRRI), where the largest collection is found, with about 83 000 accessions which are characterized on the basis of about 80 traits. These traits not only include morphological characteristics but also susceptibility to diseases and pests, and reaction to environmental stresses, mineral deficiencies or toxicities. Other countries have only a working collection but are starting to build up their own germplasm bank consisting of the landraces in their country. Collection of wild rices is being emphasized for possible new sources of important genes.
Potential rice grain yields in the tropics have increased dramatically since the mid 1960s with the introduction of IR8 and similar plant types. The green revolution started with the development of semi-dwarf, stiff-straw plant types which prevented lodging, a serious problem in rice cultivation, and allowed high nitrogen fertilizer doses. The "miracle cultivars" also had upright short leaves which resulted in better light distribution and photosynthesis. Carbohydrate partitioning was also greatly improved with reduction in height. Grain yields reached a plateau in the late 1960s. Subsequent breeding objectives have been to increase disease and pest resistance, early maturity and tolerance of adverse environments. Biotechnology is helping in these objectives and also in the further increase of grain yields.
Improvements in biotechnology opened up new methods (e.g. embryo rescue) of crossing wild relatives of rice and finding new sources of important genes. Resistance to grassy stunt virus was found only in the wild species O. nivara. Fortunately it was compatible with O. sativa. Other wild species with important resistance to diseases and environmental stresses have been found. O. rufipogon is a source of cytoplasmic male sterility and flood tolerance; O. glaberrima, cultivated in Africa, is resistant to green leafhopper; O. barthii A. Chev. to bacterial blight; O. punctata Kotschy ex Steudel, O. officinalis Wallich ex Watt, O. eichingeri A. Peter, O. minuta J.S. Presl to brown planthopper; O. brachyantha A. Chev. & Roehr. to rice whorl maggot; and Porteresia coarctata (Roxb.) Tateoka (synonym: O. coarctata Roxb.) is tolerant of salinity.
Some of the prospects for rice are:
- continued emphasis on stability of yields under tropical conditions;
- greater resistance to diseases and pests; this is more likely to be found in the hardier indica cultivars;
- tolerance of and adaptation to local and environmental stresses;
- better utilization of available nutrients and greater production of endogenous available nitrogen;
- mechanization of rice farming, especially land preparation, direct seeding/transplanting, harvesting, and processing;
- application of integrated pest management (IPM) by more farmers through adequate dissemination of information.
Any new types recommended should be well adapted to the local environment and methods of cultivation, but steps should be taken to improve the latter. This requires adequate research adjusted to the local conditions, a well-functioning extension service, and government support. For instance, hybrid rice is used in China and lately in Vietnam. However, the high cost of seed production and the quality of rice produced have discouraged other countries from pursuing this option of increasing grain yields.
Some of the above topics are actively being researched on. Ricebased cropping systems including the integration of livestock and fish are also being actively pursued and are likely to be an integral part of rice farming in SouthEast Asian countries. Research on saline and sodic soils, which form a vast area for potential expansion of rice production is receiving priority.
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