ix The facts of rice Production Rice farming is the largest single use of land for producing food. Rice is nearly all (90%) produced in Asia. Rice production totaled 696 million tons in 2010. Rice is the single largest food source for the Rice production is one of the most important poor. economic activities on Earth. Rice is the source of one quarter of global per Thousands of varieties of rice are farmed. capita energy. Only 7% of all rice production is Rice is synonymous with food throughout Asia. exported from its country of origin. Rice is the most important food grain in most of the tropical areas of Latin America and the Caribbean, where it supplies more calories in Employment people’s diets than wheat, maize, cassava, or Rice eaters and growers form the bulk of the potato. world’s poor. Toyota means bountiful rice field. Rice is the single most important source of Honda means the main rice field. employment and income for rural people. Rice is grown on some 144 million farms, mostly smaller than 1 hectare. Benefits of rice research Research has provided 75% of the rice varieties now grown. Significance in human culture Research has increased potential yields from 4 Rice farming is about 10,000 years old. to more than 10 tons per hectare per crop. Rice cultivation was once the basis of the social Research has been a major factor in more than order and occupied a major place in Asia’s doubling world rice production from 260 to religions and customs. nearly 700 million tons over the past 50 years. Rice is still sometimes used to pay debts, Research has provided rice plants that grow wages, and rent in some Asian rural areas. faster, enabling 2 or even 3 crops per year; plants that resist various pests and diseases, need less fertilizer, or thrive in saline water; Significance as food and plants with enhanced levels of micronutrients. Rice is the staple food for the largest number of people on Earth. Many more facts on rice production are contained Rice is eaten by nearly half the world’s in the Rice Facts on page 261. population. x Chapter 1 Introduction and setting 1 A brief history of rice farming Early spread of rice From early, perhaps separate, beginnings The origins of rice have long been debated. in different parts of Asia, the process of The plant is of such antiquity that the exact diffusion has carried rice in all directions time and place of its first development and today it is cultivated on every continent will perhaps never be known. It is certain, save Antarctica. In the early Neolithic era, however, that domestication of rice ranks rice was grown in forest clearings under a as one of the most important developments system of shifting cultivation. The crop was in history. Rice has fed more people over a direct seeded, without standing water— longer time than has any other crop. conditions only slightly different from those Pottery shards bearing the imprint of to which wild rice was subject. A similar but both grains and husks of the cultivated rice independent pattern of the incorporation of species Oryza sativa were discovered at wild rice into agricultural systems may well Non Nok Tha in the Korat area of Thailand. have taken place in one or more locations in Plant remains from 10,000 B.C. were Africa at approximately the same time. discovered in Spirit Cave on the Thailand- Puddling the soil—turning it to mud— Myanmar border. and transplanting seedlings were likely In China, extensive archeological refined in China. Both operations became evidence points to the middle Yangtze and integral parts of rice farming and remain upper Huai rivers as the two earliest places widely practiced to this day. Puddling of O. sativa cultivation in the country. Rice breaks down the internal structure of soils, and farming implements dating back at least making them much less subject to water loss 8,000 years have been found. Cultivation through percolation. In this respect, it can be spread down these rivers over the following thought of as a way to extend the utility of a 2,000 years. limited water supply. Transplanting is the planting of 1- to 6-week-old seedlings in puddled soil with standing water. Under these conditions, the rice plants have an important head start over a wide range of competing weeds, which leads to higher yields. Transplanting, like puddling, provides farmers with the ability to better accommodate the rice crop to a finite and fickle water supply by shortening the field duration (since seedlings are grown separately and at higher density) and adjusting the planting calendar. With the development of puddling and transplanting, rice became truly domesticated. In China, the history of rice in river valleys and low-lying areas is longer than its history as a dryland crop. In Southeast Asia, however, rice originally was produced under dryland conditions in the uplands, and only recently came to occupy the vast river deltas. Migrant people from southern China or perhaps northern Vietnam carried the traditions of wetland rice cultivation to the Philippines during the second millennium B.C., and Deutero-Malays carried the Transplanting rice in the Philippines. practice to Indonesia about 1500 B.C. From 2 Rice almanac China or the Korean peninsula, the crop was Valley. The introduction into California introduced to Japan no later than 100 B.C. corresponded almost exactly with the timing Movement to western India and south to of the first successful crop in Australia’s Sri Lanka was also accomplished very early. New South Wales. Rice was a major crop in Sri Lanka as early as 1000 B.C. The crop may well have been Present rice-growing areas introduced to Greece and the neighboring Rice is produced in a wide range of areas of the Mediterranean by returning locations and under a variety of climatic members of Alexander the Great’s expedition conditions, from the wettest areas in the to India around 344-324 B.C. From a center world to the driest deserts. It is produced in Greece and Sicily, rice spread gradually along Myanmar’s Arakan Coast, where throughout southern Europe and to a few the growing season records an average of locations in northern Africa. more than 5,100 mm of rainfall, and at Al Hasa Oasis in Saudi Arabia, where annual Rice in the New World rainfall is less than 100 mm. Temperatures, As a result of Europe’s great Age of too, vary greatly. In the Upper Sind in Exploration, new lands to the west became Pakistan, the rice season averages 33 °C; available for exploitation. Rice cultivation in Otaru, Japan, the mean temperature for was introduced to the New World by early the growing season is 17 °C. The crop is European settlers. The Portuguese carried produced at sea level on coastal plains and it to Brazil and the Spanish introduced its in delta regions throughout Asia, and to a cultivation to several locations in Central and height of 2,600 m on the slopes of Nepal’s South America. The first record for North mountains. Rice is also grown under an America dates from 1685, when the crop was extremely broad range of solar radiation, produced on the coastal lowlands and islands ranging from 25% of potential during the of what is now South Carolina. The crop may main rice season in portions of Myanmar, well have been carried to that area by slaves Thailand, and India’s Assam State to brought from the African continent. Early in approximately 95% of potential in southern the 18th century, rice spread to what is now Egypt and Sudan. Louisiana, but not until the 20th century Rice occupies an extraordinarily was it produced in California’s Sacramento high portion of the total planted area in As long as there is enough water, Australia's highly efficient rice industry achieves some of the highest yields in the world. Introduction and Setting 3 South, Southeast, and East Asia. This area is subject to an alternating wet and dry Awn seasonal cycle and also contains many of the Palea Lemma world’s major rivers, each with its own vast delta. Here, enormous areas of flat, low- lying agricultural land are flooded annually Pericarp Tegmentum during and immediately following the rainy Aleurone layer Starchy (caryopsis) Brown rice season. Only two major food crops, rice endosperm and taro, adapt readily to production under these conditions of saturated soil and high Scutellum Embryo Epiblast temperatures. Plumule Radicle The highest rice yields have Rachilla traditionally been obtained from plantings in high-latitude areas that have long Sterile lemmas daylength and where intensive farming techniques are practiced, or in low-latitude Fig. 1.1. Cross-section of the rice grain. areas that have high solar radiation and cool nights. Southwestern Australia, northern Seeds California, southern Brazil, Uruguay, and The rice grain, commonly called a seed, the Nile Delta provide the best examples. consists of the true fruit or brown rice In some areas, such as South Asia, the (caryopsis) and the hull, which encloses the crop is produced on miniscule plots using brown rice. Brown rice consists mainly of enormous amounts of human labor. At the embryo and endosperm. The surface other locations, such as in Australia and the contains several thin layers of differentiated United States, it is raised on huge holdings tissues that enclose the embryo and with a maximum of technology and large endosperm (Fig. 1.1). expenditures of energy from fossil fuels. The palea, lemmas, and rachilla The contrasts in the geographic, economic, constitute the hull of indica rice. In japonica and social conditions under which rice is rice, however, the hull usually includes produced are truly remarkable. rudimentary glumes and perhaps a portion of the pedicel. A single grain weighs 10–45 milligrams at 0% moisture content. Grain The rice plant length, width, and thickness vary widely Morphology among varieties. Hull weight averages about Cultivated rice is generally considered 20% of total grain weight. a semiaquatic annual grass, although in the tropics it can survive as a perennial, Seedlings producing new tillers from nodes after Germination and seedling development harvest (ratooning). At maturity, the rice start when seed dormancy has been broken plant has a main stem and several tillers. and the seed absorbs adequate water and Each productive tiller bears a terminal is exposed to a temperature ranging from flowering head or panicle. Plant height 10 to 40 °C. The physiological definition varies by variety and environmental of germination is usually the time when conditions, ranging from approximately 0.4 the radicle or coleoptile (embryonic shoot) meter (m) to more than 5 m in some floating emerges from the ruptured seed coat. Under rice. The morphology of rice is divided into aerated conditions, the seminal root is the the vegetative phase (including germination, first to emerge through the coleorhizae seedling, and tillering stages) and the from the embryo, and this is followed by reproductive phase (including panicle the coleoptile. Under anaerobic conditions, initiation and heading stages). however, the coleoptile is the first to emerge, with the roots developing when the coleoptile has reached the aerated regions of the environment. 4 Rice almanac the auricles. Coarse hairs cover the surface of the auricles. Immediately above the Second leaf auricles is a thin, upright membrane called (first complete leaf) the ligule. The tillering stage starts as soon as the Primary leaf seedling is self-supporting and generally (first seedling leaf) finishes at panicle initiation. Tillering Coleoptile usually begins with the emergence of the Nodal roots first tiller when seedlings have five leaves. (or adventitious) This first tiller develops between the main Mesocotyl stem and the second leaf from the base of the plant. Subsequently, when the sixth leaf Mesocotyl emerges, the second tiller develops between roots the main stem and the third leaf from the Seminal root base. Rootlets Tillers growing from the main stem are called primary tillers. These may generate secondary tillers, which may in turn generate tertiary tillers. These are produced in a synchronous manner. Although the tillers remain attached to the plant, at later stages they are independent because they Fig. 1.2. Parts of a young seedling germinated produce their own roots. Varieties and races in the dark. of rice differ in tillering ability. Numerous environmental factors also affect tillering If the seed develops in the dark as when such as spacing, light, nutrient supply, and seeds are sown beneath the soil surface, cultural practices. a short stem (mesocotyl) develops, which The rice root system consists of two lifts the crown of the plant to just below the major types: crown roots (including mat soil surface (Fig. 1.2). After the coleoptile roots) and nodal roots (Fig. 1.3). In fact, emerges, it splits and the primary leaf both these roots develop from nodes, but develops. crown roots develop from nodes below the soil surface. Roots that develop from nodes Tillering plants Each stem of rice is made up of a series of nodes and internodes (Fig. 1.3). The Leaf blade internodes vary in length depending on Ligule variety and environmental conditions, but Auricle Leaf sheath generally increase from the lower to the Leaf sheath upper part of the stem. Each upper node Node Tiller bears a leaf and a bud, which can grow into Internode a tiller. The number of nodes varies from 13 Nodal roots to 16, with only the upper 4 or 5 separated by long internodes. Under rapid increases in water level, some deepwater rice varieties can also increase the lower internode lengths by more than 30 centimeters (cm) Mat roots Crown roots each. The leaf blade is attached at the node Ordinary roots by the leaf sheath, which encircles the stem. Where the leaf blade and the leaf sheath meet is a pair of clawlike appendages, called Fig. 1.3. Parts of the rice stem and tillers. Introduction and Setting 5 Flag leaf blade Awn Spikelet (floret) Secondary Paleal apiculus branch Axis Anther Filament Primary branch Palea Lemma Stigma Style Base Ovary Rachilla Sterile lemmas Rudimentary glumes Pedicel Fig. 1.4. Rice panicle and spikelets. above the soil surface usually are referred to Since rice has only one fully developed as nodal roots. Nodal roots are often found floret (flower) per spikelet, these terms are in rice cultivars growing at water depths often used interchangeably. The flower is above 80 cm. Most rice varieties reach a enclosed in the lemma and palea, which maximum depth of 1 m or more in soft may be either awned or awnless. The flower upland soils. In flooded soils, however, rice consists of the pistil and stamens, and the roots seldom exceed a depth of 40 cm. That components of the pistil are the stigmas, is largely a consequence of limited oxygen styles, and ovary. diffusion through the gas spaces of roots (aerenchyma) to supply the growing root Growth tips. The growth duration of the rice plant is 3–6 months, depending on the variety and the Panicle and spikelets environment under which it is grown. The major structures of the panicle are the During this time, rice completes two distinct base, axis, primary and secondary branches, growth phases: vegetative and reproductive. pedicel, rudimentary glumes, and spikelets. The vegetative phase is subdivided into The panicle axis extends from the panicle germination, early seedling growth, base to the apex; it has 8–10 nodes at 2- and tillering; the reproductive phase is to 4-cm intervals, from which primary subdivided into the time before and after branches develop. Secondary branches heading, that is, panicle exsertion. The develop from the primary branches. Pedicels time after heading is better known as the develop from the nodes of the primary ripening period (Fig. 1.5). and secondary branches; the spikelets are Potential grain yield is primarily positioned above them (Fig. 1.4). determined before heading. Ultimate yield, 6 Rice almanac Amount of growth increases plant height), a decline in tiller number, emergence of the flag leaf (the last Tiller number Plant height leaf), booting, heading, and flowering of the Panicle spikelets. Panicle initiation is the stage about number Ineffective tillers 25 days before heading when the panicle has grown to about 1 mm long and can be Grain weight recognized visually or under magnification following stem dissection. Spikelet anthesis (or flowering) begins with panicle exsertion (heading) or on the 0 30 60 90 120 following day. Consequently, heading is Days after germination considered a synonym for anthesis in rice. It takes 10–14 days for a rice crop to complete Panicle primordia initiation heading because there is variation in End of effective tillering Maximum tiller number panicle exsertion among tillers of the same Heading/anthesis plant and among plants in the same field. Seedling growth Active tillering Agronomically, heading is usually defined Germination Emergence Yelow-ripe Maturity as the time when 50% of the panicles have Booting Dough Milky exserted. Vegetative Reproductive Ripening Anthesis normally occurs from 1000 h to 1300 h in tropical environments and Fig. 1.5. Schematic growth of a 120-day rice variety fertilization is completed within 6 hours. in the tropics. Very few spikelets have anthesis in the afternoon, usually when the temperature is low. Within the same plant, it takes which is based on the amount of starch that 7–10 days for all the panicles to complete fills the spikelets, is largely determined anthesis; the spikelets themselves complete after heading. Hence, agronomically, it anthesis within 5 days. is convenient to regard the life history of rice in terms of three growth phases: Ripening phase vegetative, reproductive, and ripening. A Ripening follows fertilization and can be 120-day variety, when planted in a tropical subdivided into milky, dough, yellow- environment, spends about 60 days in the ripe, and maturity stages. These terms vegetative phase, 30 days in the reproductive are primarily based on the texture and phase, and 30 days in the ripening phase. color of the growing grains. The length of ripening varies among varieties from about Vegetative phase 15 to 40 days. Ripening is also affected The vegetative phase is characterized by temperature, with a range from about by active tillering, a gradual increase in 30 days in the tropics to 65 days in cool plant height, and leaf emergence at regular temperate regions, such as Hokkaido, Japan; intervals. Tillers that do not bear panicles and Yanco, Australia. are called ineffective tillers. The number of ineffective tillers is a closely examined Genetic diversity trait in plant breeding since it is undesirable in irrigated varieties, but is sometimes an Two rice species are important cereals advantage in rainfed lowland varieties in for human nutrition: Oryza sativa, grown which productive tillers or panicles may be worldwide, and O. glaberrima, grown in lost because of unfavorable conditions. parts of West Africa. These two cultigens— species known only by cultivated plants— Reproductive phase belong to a genus that includes about 25 The reproductive growth phase is other species, although the taxonomy is still characterized by culm elongation (which a matter of research and debate. Introduction and Setting 7 Oryza is thought to have originated Among the wild relatives of O. sativa, about 14 million years ago in Malesia.1 the perennial O. rufipogon is widely Since then, it has evolved, diversified, and distributed over South and Southeast Asia, dispersed, and wild Oryza species are now southeastern China, and Oceania; the distributed throughout the tropics. Their morphologically similar O. glumaepatula genomes can be classified into 11 groups is found in South America, usually in labeled AA to LL, and most of the species deepwater swamps. A closely related annual can be grouped into four complexes of wild form, O. nivara, is found in the Deccan closely related species in two major sections Plateau and Indo-Gangetic Plains of India of the genus (Table 1.1). Just two species, and in many parts of Southeast Asia. The both diploids, have no close relatives and are habitats of O. nivara are ditches, water placed in their own sections of the genus: O. holes, and edges of ponds. Morphologically australiensis and O. brachyantha. similar to (and sometimes indistinguishable Species of the O. meyeriana complex from) O. nivara are the very widely are genetically most different from the distributed weedy forms of O. sativa, cultigens; they are diploid perennials which represent numerous different hybrids found in dry hillside forests. Species of between O. sativa and its two wild relatives. the O. ridleyi complex are tetraploids Throughout South and Southeast Asia, these inhabiting lowland swamp forests. These spontaneous hybrids are found in canals and two complexes, together with the tetraploid ponds adjacent to rice fields and in the rice species O. schlechteri and O. coarctata, fields themselves. form the most primitive section of the genus, The primary center of diversity for O. with a geographical distribution ranging glaberrima is in the swampy basin of the from South Asia through Malesia to New upper Niger River. Two secondary centers Caledonia. are to the southwest near the Guinean The O. officinalis complex consists coast. O. glaberrima varieties can be of diploid and tetraploid species found divided into two ecotypes: deepwater and throughout the tropics. All the species in this upland. In West Africa, O. glaberrima is a complex are perennials found in seasonal dominant crop grown in the flooded areas wetlands; some are rhizomatous and others of the Niger and Sokoto River basins. It form runners. They also differ in the habitats is broadcast on hoed fields. On shallowly where they are found. Some occur in full flooded land, a rainfed wetland crop is sun in grasslands, others in partial to full directly sown by either broadcasting or shade in forests. Variation exists within dibbling, or transplanted. About 45% of the these species as shown by the responses of land planted to rice in Africa belongs to the different populations to pests and diseases. upland (dryland) culture, largely under bush The O. sativa complex consists of the fallow or after the ground has been hoed. wild and weedy relatives of the two rice Some African farmers still use axes, hoes, cultigens as well as the cultigens themselves. and bush knives in land preparation. In All are diploids and are found throughout hydromorphic soils, O. glaberrima behaves the tropics. The wild relatives of O. like a self-perpetuating weed. In wetland glaberrima in Africa consist of the perennial fields planted to O. sativa, O. glaberrima rhizomatous species O. longistaminata, has become a weed. which grows throughout sub-Saharan Africa Ecological diversification in O. and Madagascar, and the annual O. barthii, sativa, which involved hybridization- which extends from West Africa to East and differentiation-selection cycles, was southern Central Africa. The annual and enhanced when ancestral forms of the weedy relatives of O. glaberrima are found cultigen were carried by farmers and traders primarily in West Africa. to higher latitudes, higher elevations, dryland sites, seasonal deepwater areas, and tidal swamps. Within broad geographic 1 A biogeographic region encompassing the Philippines, New regions, two major ecogeographic races Guinea, Borneo, the Indonesian islands, and the Malay Peninsula and archipelago. or variety groups were differentiated as a 8 Rice almanac Table 1.1. Classification and distribution of species in the genus Oryza. Taxa Genome Distribution Comments/alternative classification Section Oryza Series Oryza: sativa species complex O. sativa AA Worldwide O. glaberrima AA West Africa O. nivara AA Tropical Asia Annual ecotype of O. rufipogon O. rufipogon AA Tropical Asia to northern Australia O. meridionalis AA Northern Australia O. barthii AA Africa O. longistaminata AA Africa O. glumaepatula AA South America South American O. rufipogon; O. glumaepatula Series Latifoliae: officinalis species complex O. minuta BBCC Philippines, Papua New Guinea O. officinalis CC Tropical Asia to Papua New Guinea O. rhizomatis CC Sri Lanka O. malampuzhaensis CCDD India Tetraploid race of O. officinalis O. punctata BB Africa O. schweinfurthiana BBCC Africa Tetraploid race of O. punctata O. eichingeri CC West, Central, and East Africa, Sri Lanka The only species found in both Africa and Asia O. alta CCDD Central and South America O. grandiglumis CCDD South America O. latifolia CCDD Central and South America Section Australiensis O. australiensis EE Australia Member of officinalis complex Section Brachyantha O. brachyantha FF Africa Section Padia Basal or primitive section of Oryza O. schlechteri HHKK Indonesia and Papua New Guinea O. coarctata KKLL South Asia to Myanmar Series Ridleyanae: ridleyi species complex O. longiglumis HHJJ Indonesia and Papua New Guinea O. ridleyi HHJJ Southeast Asia to Papua New Guinea Series Meyerianae: meyeriana species complex granulata species complex O. granulata GG South and Southeast Asia Variety of O. meyeriana O. meyeriana GG South and Southeast Asia O. neocaledonica GG New Caledonia Introduction and Setting 9 result of isolation and selection: (1) indica, features, especially grain size, shape, adapted to the tropics; and (2) japonica, color, and endosperm properties. The adapted to the temperate regions and complex groups of cultivars now known tropical uplands. Recent DNA studies have are categorized on the basis of hydrologic- identified five subgroups within these two edaphic-cultural-seasonal regimes as well major groups. Indica is divided into indica as genetic differentiation. Within the last proper, and aus, a group of diverse varieties 2,000 years, dispersal and cultivation of from northeastern India and Bangladesh, the cultivars in new habitats have further named for the aus growing season, and accelerated the diversification process. which have been found to contain a number Today, thousands of rice varieties are of stress tolerance genes that are absent grown in more than 100 countries. The full from other variety groups. The Basmati spectrum of germplasm in the genus Oryza or aromatic group of varieties, mainly consists of the following: from northwestern India and Pakistan, • Wild Oryza species, which occur is an offshoot of the japonica variety throughout the tropics, and related group, which is further subdivided into genera, which occur worldwide in temperate and tropical japonica. both temperate and tropical regions. The combined forces of natural and • Natural hybrids between the cultigen human selection; diverse climates, seasons, and wild relatives, and primitive and soils; and varied cultural practices cultivars of the cultigen in areas of (dryland preparation and direct seeding vs rice diversity. puddling of the soil and transplanting) led • Commercial types, obsolete varieties, to the tremendous ecological diversity now minor varieties, and special-purpose found in Asian cultivars. Selections made to types in the centers of cultivation. suit cultural preferences and socioreligious • Pure-line or inbred selections of traditions added diversity to morphological farmers’ varieties, elite varieties of hybrid origin, F1 hybrids, breeding materials, mutants, polyploids, aneuploids, intergeneric and interspecific hybrids, composites, and cytoplasmic sources from breeding programs. The diversity of Asian, African, and wild rices has given breeders a wealth of genetic material to draw on for breeding improved cultivars. Rice as human food Rice, wheat, and maize are the three leading food crops in the world; together they directly supply more than 42% of all calories consumed by the entire human population. Wheat is the leader in area harvested each year with 225 million hectares (ha) in 2009, followed by maize and rice, both with 159 million ha. Human consumption in 2009 accounted for 78% of total production for rice, compared with 64% for wheat and 14% for maize. Although rice farming is important In the Philippines, a large plate of rice to particular regions in some developed dominates the dinner table. 10 Rice almanac In southwestern Bangladesh, rice is part of a balanced diet with fish, vegetables, and fruit. countries, it is of greatest importance in about 60%, but this is due to increased low- and lower-middle-income countries, biofuel demand, not increased human where it accounts for 19% of total crop area consumption. For wheat, about one-fifth of harvested. In upper-middle- and high- production is typically used as animal feed. income countries, it accounts for just 2% Of the remaining four-fifths, a large share of total crop area harvested. There are now is consumed in developed countries. In the some 144 million rice farms in the world, case of rice, very little is used for feed, and the vast majority in developing countries. rice consumption is relatively low in Europe The numbers of households farming the and the United States. other two most widely grown crops in the Even though rice is the dominant food world, wheat and maize, are likely to be crop for low- and lower-middle-income much lower because a large proportion countries, Table 1.2 still understates its of the wheat and maize area is in upper- importance to the poor because much of middle-income and developed countries, the wheat consumption in low- and lower- where farm sizes are larger. In 2008, 94% of middle-income countries is restricted to the total rice area was in low- and lower-middle- upper parts of the income distribution. Table income countries compared with just 52% 1.3 shows the proportions of rice and wheat for maize and 41% for wheat. consumption by the poorest and richest 20% Of the three major crops, rice is by of the population in a few large low-income far the most important in terms of human countries. These data show that, although consumption in low- and lower-middle- rice consumption is spread across income income countries. Maize has always been classes relatively equally, the poorest people primarily a feed crop for animals—feed use actually consume relatively little wheat— has historically accounted for about two- most of the wheat consumption is by people thirds of total consumption. This proportion in the upper part of the income distribution has declined slightly in recent years to (who are not below the poverty line). The Introduction and Setting 11 Table 1.2. World food picture, 2009. Human population (million) 6,815.8 Land use, 2009 (million ha) Total land area 13,003.5 Arable land 1,381.2 Permanent crops 152.1 Permanent meadows and 3,355.7 pastures Forest area 4,038.7 Other land 4,088.0 Food production Per capita/day Share in nutritional intake (%/day) Crop Area Produc- Food Calories Protein Calories Protein (million tion (mil- (million t) (kcal) (g) (kcal) (g) ha) lion t) Rice (rough) 158.5 684.6 531.9 65% milling rate 536 10.1 18.9 12.7 Maize 158.8 819.2 114.0 80% for feed 141 3.4 5.0 4.3 Wheat 224.6 686.6 439.4 70% milling rate 532 16.2 18.8 20.4 Millet and sorghum 74.2 83.0 47.2 30% milling rate 59 1.7 2.1 2.1 Barley and rye 60.8 169.9 12.0 70% milling rate 13 0.4 0.5 0.5 Oats 10.2 23.2 3.6 65% milling rate 3 0.1 0.1 0.1 Potatoes 18.7 332.1 217.3 60% for feed 61 1.4 2.2 1.8 Sweet potatoes and yams 13.0 150.9 81.0 50% for feed 33 0.4 1.2 0.5 Subtotal 1,378 33.7 48.7 42.5 All foods 2,831 79.3 2,831 79.3 Source: Compiled by IRRI from FAO database. Table 1.3. Percentage of national rice and wheat consumption by the poorest and richest quintiles of the population.a Country (survey year) Rice Wheat Poorest Richest Poorest Richest Bangladesh (2005) 18 21 9 45 Indonesia (1999) 17 19 6 43 Philippines (1999-2000) 18 22 15 27 a Percentages are calculated on the basis of consumption quantities (kg), not value. Sources of data: BBS (2007) for Bangladesh, BPS (2000) for Indonesia, and BAS (2001) for the Philippines. reverse does not appear to be true in areas content is modest. Unmilled (brown) rice where wheat is the staple food, for example, of 17,587 cultivars in the IRRI germplasm Pakistan and the wheat-eating provinces collection averages 9.5% protein content, in China. Thus, rice is clearly the world’s ranging from 4.3% to 18.2%. most important food crop for the poor. The Environmental factors (soil fertility, geographic pattern of rice production and wet or dry season, solar radiation, and consumption is further described in Chapter 3. temperature during grain development) Rice provided 19% of global human per and crop management (added N fertilizer, capita energy and 13% of per capita protein plant spacing) affect rice protein content. in 2009. Although rice protein ranks high Breeding efforts to increase protein have in nutritional quality among cereals, protein been largely unsuccessful because of the 12 Rice almanac considerable effects of environment and fuel for cooking, (2) it may cause digestive because of complex inheritance properties disturbances, and (3) oil in the bran layer in the triploid endosperm tissue. tends to turn rancid during storage even at Rice also provides minerals, vitamins, moderate temperatures. and fiber, although all constituents except In contrast, parboiling rough rice carbohydrates are reduced by milling. before milling, as is common in India and Milling removes roughly 80% of the Bangladesh, allows a portion of the vitamins thiamine from brown rice. A precook and minerals in the bran to permeate the rinse or a boiling of milled rice results in endosperm and be retained in the polished additional loss of vitamins, especially B1. rice. This treatment also lowers protein loss Where rice is the main item of the during milling and increases whole-grain diet, it is frequently the basic ingredient recovery. of every meal and is normally prepared Even though rice diets are often by boiling or steaming. In Asia, bean marginally deficient in protein, vitamins, curd, fish, vegetables, meat, and spices are and minerals, clinical manifestations of added depending on local availability and deficiency are not common among people economic situation. A small proportion of whose diets are otherwise adequate in rice is consumed in the form of noodles, calories. The exception is when people do which serve as a bed for various, often heavy labor and their higher calorie demand highly spiced, specialties or as the bulk is met by an increase in rice without a ingredient in soups. corresponding increase in other foods such Most rice is consumed in its polished as legumes or fish. Under these conditions, state. When such rice constitutes a high there is danger of beriberi, which is related proportion of food, dietary deficiencies may to a deficiency of thiamine or vitamin B1. result. Despite the dramatic losses in food Research is under way to fortify rice value resulting from milling, brown rice with micronutrients in areas where these are is unpopular because (1) it requires more inadequate in the diet. Vitamin A is an Rice husks can be used for fuel, bedding, and incubation material. Introduction and Setting 13 important one—a severe lack causes In much of Tanzania, rice is used for irreversible blindness—and has now been making bread; in the south, it is also used incorporated in experimental lines known in ceremonies. In West Africa, rice bread, as Golden Rice. Other new varieties are rice cake, and rice porridge are used for rich in iron and zinc, micronutrients often ceremonies such as funerals and weddings. deficient in people consuming mainly rice. Some “old” varieties (most likely O. These fortified rice varieties are being tested glaberrima) are used in traditional religious in nutrition trials before farmers grow them rituals in West Africa, while certain parts of commercially. some varieties are used as medicines in the traditional treatment of illnesses. Rice contains many compounds in the Specialty uses of rice grains that promote shiny hair and good skin. Several countries are now making Glutinous rice plays an important role in face washes, liquid shower soaps, and some cultures. In Laos and northeastern hair products from rice, including Japan, Thailand, for example, glutinous rice is the Republic of Korea, the Philippines, and staple food. In other cultures, it is prepared Thailand. Also, in Thailand and the US, in a sweetened form for snacks, desserts, milk is made from rice for lactose-intolerant or special foods for religious or ceremonial people. occasions. In a few areas, glutinous rice An extensive list of other ways of using is pounded and roasted to be eaten as a rice is given by the Food and Agriculture breakfast cereal. Organization of the United Nations (FAO): Alcoholic beverages made from rice are • Milled rice is marketed precooked, found throughout the rice-producing world. canned, dried, and puffed for The most common is a rice beer produced breakfast cereals as rice flour; by boiling husked rice, inoculating the mix extrusion-cooked foods; puddings and with a bit of yeast cake, and allowing the breads; cakes and crackers; noodles mixture to ferment for a short period. and rice paper; fermented foods and The mash left at the bottom of the vinegars; rice starch; and syrups. container is often prized. Among the Ifugao • Rice bran, which forms 5% to 8% of of the Philippines, the mash is frequently the grain weight, is used as livestock reserved for the village priest. Among the feed, a pickling medium, a medium Kachins of Myanmar, it is the first food for growing mushrooms, and as a offered to a recently captured and hungry growing medium for some enzymes, wild elephant. Kachins believe that the as well as for flours, concentrates, elephant will be loyal forever to the person oils, and dietary fiber. who first provides such a meal. • Hulls and husks, about 20% of the Sake is widely consumed in Japan, as is grain weight, are used for fuel, wang-tsiu in China. These rice-based wine- bedding, and incubation material, like beverages are served warm and featured and as a seedbed medium, as well at ceremonial feasts. as being sometimes incorporated in In some parts of the world, especially livestock feeds, concrete blocks, tiles, in North America and Europe, rice is fiberboard, ceramics, cement, filters, developing a new market niche as a staple charcoal briquettes, and cooking gas and as a gourmet food. This trend appears production. to be related to the arrival of large numbers • Rice straw, more or less equivalent of immigrants from Southeast Asia, who in production weight to grain, is used introduced aromatic rice to markets where as fuel for cooking, roofing material, it was previously unknown. It has been livestock feed, fertilizer, and a adopted by a food quality-conscious public medium for growing mushrooms. over the past several years. 14 Rice almanac Introduction and Setting Chapter 2 Rice and the environment 15 Rice fields near the Oyunahara Shrine, Hongu Wakayama Prefecture in Japan. R ice grows in a wide range of environments. More than 90% of global rice production is harvested from irrigated plants are transplanted. Direct seeding on wet or dry soil is also widely practiced and has largely replaced transplanted or rainfed lowland rice fields. Awareness irrigated rice in Southeast Asia. Deepwater is growing that lowland rice environments rice and floating rice are found in flood- provide a rich variety of ecosystem prone environments, where the fields services. Rice production also has environ- suffer periodically from excess water and mental impacts, largely by releasing or uncontrolled, deep flooding. Upland rice is sequestering gases and compounds to/from grown under dryland conditions (no ponded the atmosphere and troposphere and by water) without irrigation and without changing the chemical composition of the puddling (harrowing or rototilling under water flowing through the rice fields. shallow submerged conditions), usually in nonbunded fields. Figure 2.1 shows the major growing areas of the three basic Rice environments and cropping systems. systems Rice grows in a wide range of environments Irrigated environments and is productive in many situations where Worldwide, about 93 million ha of irrigated other crops would fail. Most classifications lowland rice provide 75% of the world’s of rice environments are based on rice production. Some 56% of the world’s hydrological characteristics. Irrigated irrigated area of all crops is in Asia, where lowland rice is grown in bunded fields with rice accounts for 40–46% of the irrigated ensured irrigation for one or more crops a area of all crops. Rice occupies 64–83% of year. Farmers generally try to maintain 5–10 the irrigated area in Southeast Asia, 46–52% cm of water (“floodwater”) on the field. in East Asia, and 30–35% in South Asia. At Rainfed lowland rice is grown in bunded the field level, rice receives up to 2–3 times fields that are flooded with rainwater for at more water per hectare than other irrigated least part of the cropping season to water crops, but an unknown portion of the water depths that exceed 100 cm for no more than losses is reused by other fields downstream. 10 days. Assuming a reuse rate of 25%, we estimate In both irrigated and rainfed lowlands, that irrigated rice receives 34–43% of the fields are predominantly puddled, and 16 Rice almanac Fig. 2.1. Major global rice-growing areas and ecosystems. world’s irrigation water and 24–30% of the world’s total rice production. Rainfed rice world’s developed freshwater resources. environments experience multiple abiotic Irrigated rice is grown mostly with stresses and high uncertainty in the timing, supplementary irrigation in the wet season duration, and intensity of rainfall. Some and is reliant entirely on irrigation in the 27 million ha of rainfed rice are frequently dry season. The proportion of the Asian affected by drought, the largest, most rice area that is irrigated (excluding China, frequently, and most severely affected areas where essentially all rice is irrigated) being eastern India (about 20 million ha) increased substantially from the late 1970s and northeastern Thailand and Lao PDR (7 (35%) to 2010 (55%) because of an increase million ha). Drought is also widespread in in the irrigated area coupled with a large Central and West Africa. Further constraints decline in upland and deepwater rice arise from the widespread incidence of cultivation. In many irrigated areas, rice problem soils with poor physical and is grown as a monoculture with two crops chemical properties. Country average rice a year. However, significant areas of rice yields are only some 2.3 t/ha in the lowlands are also grown in rotation with a range of and 1 t/ha in the uplands. other crops, including 15–20 million ha In rainfed lowlands, small to moderate of rice-wheat systems. At the turn of the topographic differences can have important millennium, country average irrigated consequences for water availability, rice yields in Asia ranged from 3 to 9 tons soil fertility, and flooding risk. The (t)/ha, with an overall average of about 5 t/ha, unpredictability of rainfall often results in which increased to 5.3–5.4 t/ha by 2012. field conditions that are too dry or too wet. Besides imposing water-related stresses on Rainfed environments crop growth, these conditions prevent timely Worldwide, about 52 million ha of rainfed and effective management operations such lowlands supply about 19% of the world’s as land preparation, transplanting, weed rice production, and 15 million ha of control, and fertilizer application. If such rainfed uplands contribute about 4% of the operations are delayed or skipped, yield Rice and the environment 17 losses can be large, even though the plants have not suffered physiological water stress. Rainfed uplands are highly heterogeneous, with climates ranging from humid to subhumid, soils from relatively fertile to highly infertile, and topography from flat to steeply sloping. With low population density and limited market access, shifting cultivation with long (more than 15 years) fallow periods Upland rice in Lao PDR. was historically the Flood-prone environments dominant land-use system. Increasing Flood-prone environments include population and improved market access deepwater areas submerged under more have put pressure on these systems, but than 100 cm of water from 10 days to a shifting cultivation with 3–5-year fallow few months, areas that are affected by flash periods still accounts for 14% of the Asian floods of longer than 10 days, extensive low- upland rice area, mainly in northeastern lying coastal areas where plants are subject India, Lao PDR, and Vietnam. However, to daily tidal submergence, and areas with some 70% of Asia’s upland rice areas have problem soils (acid-sulfate and sodicity) made the transition to permanent systems where the problem is often excess water but in which rice is grown every year and is not necessarily prolonged submergence. closely integrated with other crops and Altogether, 11 million ha of flood-prone rice livestock. In Central and West Africa, the areas have average yields of more than 1.5 rice belt of Africa, upland areas represent t/ha. about 35% of the area under rice cultivation and employ about 70% of the region’s rice Salinity-prone environments farmers. As market access remains limited, Salinity is widespread in coastal areas, and most of the world’s upland rice farmers tend salinity, alkalinity, or sodicity is widespread to be self-sufficient by producing a range of in inland areas of arid regions. These agricultural outputs. problems occur in both irrigated and rainfed Research efforts to increase yields and environments. In coastal areas, rice can suffer yield stability in rainfed environments, from salinity because of seawater ingress limited in the past, have been intensified during high tides. In inland areas, salinity in the past 10–15 years, especially in the arises from salt deposits present in the soil lowlands. Together with socioeconomic or bedrock or from the use of salty irrigation developments, this has considerably water. In the mid-1980s, an estimated 1.3 improved the potential of rainfed systems million ha of rice-growing areas were through better access to information and affected by salinity or alkalinity. The extent markets for inputs and outputs, more of affected land has since increased greatly, opportunities for off-farm income, improved with some 3.8 million ha affected in India varieties, and (partial) mechanization. alone. 18 Rice almanac Soils The presence of “aquic” soil conditions is indicated by redoximorphic features such Most lowland rice soils are wetland soils as zones of accumulation and depletion of that are grown to rice. Wetlands are iron and manganese. Plowing and puddling defined as having free water at or near the often result in the development of a dense surface for at least the major part of the layer below the cultivated topsoil. growing season of arable crops, or for at Three types of water saturation occur least 2 months of the growing season of in rice soils: (1) endosaturation, in which perennial crops, grasslands, forests, or other the entire soil is saturated with water; (2) vegetation. The floodwater is sufficiently episaturation, in which upper soil layers shallow to allow the growth of a crop or are saturated but underlain by unsaturated of natural vegetation rooted in the soil. subsoil layers; and (3) anthric saturation, Free surface water may occur naturally, or a variant of episaturation with controlled rainfall, runoff, or irrigation water may be flooding and puddled surface soil. retained by field bunds, puddled plow layers, The properties of a typical soil profile or traffic pans. of a flooded rice soil during the middle of a Wetlands have at least one wet growing growing season are shown in Table 2.1. season, but may be dry, moist, or without surface water in other seasons. Wetland soils Water use and water productivity may therefore alternately support wetland and upland crops when cultivated. More than 90% of the world’s rice The transition from wetlands to uplands production is harvested from irrigated or is often gradual. It may fluctuate from rainfed lowland rice fields. Traditionally, year to year, depending on variations in lowland rice is raised in a seedbed and then precipitation, runoff, or irrigation. If water transplanted into a main field that is kept (both drainage and irrigation) can be fully under continuous or intermittent ponded controlled, farmers can choose to establish water conditions to help control weeds wetlands or uplands. But, in most wetlands, and pests. Land preparation consists of drainage capacities are insufficient to soaking, plowing, and puddling. Puddling prevent soil submergence during the rainy is also done to control weeds, to reduce soil season, particularly in the lowlands of the permeability and percolation losses, and to humid tropics. ease field leveling and transplanting. The Rice field irrigation using a water pump. Rice and the environment 19 water balance of lowland rice, because of Table 2.1. Typical profile of a flooded rice soil. its flooded nature, is different from that of Horizon Description other cereals such as wheat. Dry or wet direct seeding in irrigated Ofw A layer of standing water that becomes rice offers the advantages of more efficient the habitat of bacteria, phytoplankton, water use and higher tolerance of water macrophytes (submerged and floating deficit, as well as faster and easier planting, weeds), zooplankton, and aquatic reduced labor, earlier crop maturity by invertebrates and vertebrates. The 7–10 days, and lower methane emission, chemical status of the floodwater depends and it also eliminates operations related on the water source, soil, nature and to nursery preparation for transplanting. biomass of aquatic fauna and flora, Wet direct seeding involves sowing pre- cultural practices, and rice growth. The germinated seed, either broadcast or drilled, pH of the standing water is determined by onto puddled wet soil. In dry seeding, rice the alkalinity of the water source, soil pH, is broadcast or drilled into dry soil and the algal activity, and fertilization. Because of seed is then covered. the growth of algae and aquatic weeds, Total seasonal water input to rice fields the pH and O2 content undergo marked (rainfall plus irrigation, but excluding diurnal fluctuations. During daytime, the capillary rise, which is rarely quantified) pH may increase to 11 and the standing is up to 2–3 times more than that for other water becomes oversaturated with O2 cereals. It varies from as little as 400 because of photosynthesis of the aquatic millimeters (mm) per field in heavy clay biomass. Standing water stabilizes the soils with shallow groundwater tables soilwater regime, moderates the soil temp- that supply water for crop transpiration erature regime, prevents soil erosion, and by capillary rise to more than 2,000 mm enhances C and N supply. in coarse-textured (sandy or loamy) soils Apox The floodwater-soil interface that receives with deep groundwater tables. About 1,300 sufficient O2 from the floodwater to mm seems to be a typical average value maintain a pE + pH value above the range for irrigated rice in Asia. Nonproductive below which NH4+ is the most stable outflows of water by runoff, seepage, and form of N. The thickness of the layer may percolation are 25–50% of all water input range from several millimeters to several in heavy soils with shallow water tables of centimeters, depending on perturbation 20–50-cm depth and 50–85% in coarse- by soil fauna and the percolation rate of textured soils with deep water tables of water. 1.5-m depth or more (see Box 2.1). Though Apg The reduced puddled layer is charac- runoff, seepage, and percolation are losses terized by the absence of free O2 in the soil at the field level, they are often captured and solution and a pE + pH value below the reused downstream and do not necessarily range at which Fe(III) is reduced. lead to true water depletion at the irrigation Apx A layer that has increased bulk den- area or basin scale. However, the proportion sity, high mechanical strength, and low and magnitude of reuse of these flows are permeability. It is frequently referred to as not generally known. a plow or traffic pan. Modern rice varieties, when grown B The characteristics of the B horizon under flooded conditions, are similar in depend highly on water regime. In epiaquic transpiration efficiency to other C3 cereals, moisture regimes, the horizon generally such as wheat, at about 2 kilograms (kg) remains oxidized, and mottling occurs grain per cubic meter of water transpired. along cracks and in wide pores. In aquic What few data are available indicate that moisture regimes, the whole horizon, or water productivity of rice as measured by at least the interior of soil peds, remains evapotranspiration is also similar to that of reduced during most years. wheat, ranging from 0.6 to 1.6 kg grain per cubic meter of evapotranspired water, with 20 Rice almanac a mean of 1.1 kg grain per cubic meter. The higher evaporation rates from the water layer in rice than from the underlying soil in wheat are apparently compensated for by the higher yields of rice. Maize, as a C4 crop, has a higher evapotranspiration efficiency (ranging from 1.1 kg to 2.7 kg grain per cubic meter of water, with a mean of 1.8). The water productivity of rice for total water input (irrigation plus rainfall) ranges from 0.2 kg to 1.2 kg grain per cubic meter of Plowing and puddling often result in the devel- water, with an average of 0.4, less than half opment of a dense layer beneath the cultivated topsoil. that of wheat. Box 2.1. Water flows from a rice field Ecosystem services For lowland rice, water is needed to prepare Though only a few studies have been the land and to match the outflows of conducted so far, awareness is growing seepage, percolation, and evapotranspiration that lowland rice environments provide during crop growth. The amount of water an unusually rich variety of ecosystem used for wetland preparation can be as low services. Studies on the value of rice as 100–150 mm when the time lag between ecosystems beyond crop production have soaking and transplanting is only a few days recently received a boost by the threat to or when the crop is directly wet seeded. But, rice price supports and trade restrictions in it can be as high as 940 mm in large-scale many countries presented by multilateral irrigation systems with poor water control, trade negotiations under the World Trade where the time lag between soaking and Organization. transplanting is as long as 2 months. After the crop is established, the soil Provisioning functions is usually kept ponded until shortly before The most important provisioning function harvest. Seepage is the lateral subsurface of the rice environment is the production flow of water, and percolation is the flow of rice. Irrigated rice culture has been of water down below the root zone. Typical sustained for thousands of years in various combined values for seepage and percolation parts of Asia. Recent findings of 30 long- vary from 1–5 mm/day in heavy clay soils term continuous cropping experiments to 25–30 mm/day in sandy and sandy at 24 sites in Asia confirm that, with an loam soils. Evaporation is water lost into assured water supply, lowland rice fields the air as vapor from the ponded water are extremely sustainable and able to layer or from the surface of the soil, and produce continuously high yields. Flooding transpiration is water released into the air as has beneficial effects on soil acidity; vapor through the plants. Typical combined phosphorus, iron, and zinc availability; evapotranspiration rates of rice fields are and biological nitrogen fixation. Other 4–5 mm/day in the wet season and 6–7 mm/ provisioning services are the raising of day in the dry season, but can be as high as fish and ducks in rice fields, ponds, or 10–11 mm/day in subtropical regions before canals. Frogs and snails are collected for the onset of the monsoon. Over-bund flow consumption in some countries. or surface runoff is the spillover when water depths rise above the bunds of the fields. Regulating services Seepage, percolation, evaporation, and over- Bunded rice fields may increase the water bund flow are all nonproductive flows of water storage capacity of catchments and river and are considered losses at the field level. basins, lower the peak flow of rivers, and increase groundwater flow. The many Rice and the environment 21 irrigation canals and reservoirs associated income for rural people for centuries if not with the lowland rice landscape have a millennia. Many old kingdoms as well as similar buffering function. small communities have been founded on Other regulatory services of the construction of irrigation facilities to bunded rice fields and terraces include stabilize rice production. The collective trapping of sediments and nutrients approach needed to invest in rice systems and the prevention or mitigation of land (construction of terraces, tank systems for subsidence, soil erosion, and landslides. irrigation) and operation and maintenance Percolation from rice fields, canals, and (terraces, but also cropping calendar) storage reservoirs recharges groundwater requires strong community efforts. Rice systems. Such recharge may also provide affects daily life in many ways, and a means of sharing water equitably among the social concept of rice culture gives farmers, who can pump from shallow meaning to rice beyond its role as an item aquifers at relatively low cost rather than of production and consumption. Many suffer from inequitably shared or poorly traditional festivals and religious practices managed surface irrigation systems. The are associated with rice cultivation, and rice moderation of air temperature by rice fields are valued for their scenic beauty. fields has been recognized as a regulating Rice is also an integral part of the history service in peri-urban areas where paddy and culture of Africa, where it has been and urban land are intermingled. This grown for more than 3,000 years. function is attributed to relatively high evapotranspiration rates that lower the ambient temperature of the surrounding Managing pests in the rice area in the summer and result in ecosystem lateral heat emission from the water body in winter. Rice can be used as a Rice fields harbor a tremendous diversity desalinization crop because of its ability of animals, plants, and microorganisms, to grow well under flooded conditions some of which are harmful to the rice crop where continuously percolating water and many of which are beneficial. The leaches salts from the topsoil. goal of many scientists at IRRI and other institutions is to manage rice pests in ways Supporting services that are safe, sustainable, and economical. As a supporting service, flooded rice Emphasis is placed on breeding rice fields and irrigation channels form a varieties with resistance to insect pests comprehensive water network, which and diseases and on minimizing the use together with their contiguous dry land of pesticides to promote natural biological provides a complex mosaic of landscapes. control by beneficial insects, spiders, The Ramsar Convention on wetlands and microorganisms. The importance of classified irrigated rice land as a human- biological control in rice was dramatically made wetland. Surveys show that such demonstrated in the 1970s, when the landscapes sustain a rich biodiversity, indiscriminate use of broad-spectrum including unique and threatened species, insecticides devastated populations of and enhance biodiversity in urban and beneficial insects and spiders and led to peri-urban areas. In parts of the United huge outbreaks of the brown planthopper, States such as California, rice fields are which had previously been a minor pest. ponded in winter and used to provide A rice pest is any organism that causes habitat for ducks and other water birds. economic loss in rice production, including The cultural services of rice arthropods (insects and mites), pathogens environments are especially valued in (bacteria, fungi, and viruses), weeds, Asian countries, where rice has been mollusks (snails), and vertebrates (rodents the main staple food and the single most and birds). Some common pests are shown important source of employment and in Table 2.2. The damage they do ranges 22 Rice almanac Table 2.2. Examples of organisms that may harm or compete with the rice crop. Insect pests Stem borers African rice gall midge Orseolia oryzivora (Harris & Gagne) Yellow stem borer Scirpophaga incertulas (Walker) White stem borer S. innotata (Walker) Striped stem borer Chilo suppressalis (Walker) Dark-headed rice borer C. polychrysus (Meyrick) Defoliators Rice leaffolders Cnaphalocrocis medinalis (Guenée) and others Rice caseworm Nymphula depunctalis (Guenée) Leafhoppers Yellow stem borer is a serious pest of rice. Green leafhopper Nephotettix virescens (Distant) from severing stems or killing tissue to N. nigropictus (Stål) competing with the crop for nutrients and N. parvus Ishihara et Kawase sunlight. N. cincticeps (Uhler) Weeds are an almost universal Planthoppers companion of rice in the tropics. In many Brown planthopper Nilaparvata lugens (Stål) situations, weed growth is prolific and Whitebacked Sogatella furcifera Horvath planthopper weeds are a major constraint to crop yield. Rice bugs Weeding is a major production cost, with Malayan black Scotinophara coarctata estimates of 50–150 person-days per hectare rice bug (Fabricius) Rice grain bug Leptocorisa oratorius required for manual weeding, depending (Fabricius) on the number of weedings and type of Rodents rice culture. For many farmers, weeding Rice field rats Rattus argentiventer (Rob. & Kloss) requires the greatest labor input during the R. tanezumi (Temminck) agricultural cycle, and labor is often not Diseases available when weeds are most damaging to Viral diseases and their vectors Rice tungro Nephotettix virescens the crop. Upland rice more than any other (Distant) crop shows the ravages of a lack of proper N. nigropictus (Stål) weeding. Sometimes, when the land is too Ragged stunt Nilaparvata lugens (Stål) Rice yellow mottle Chaetocnema pulla weedy, the crop is abandoned. ChapiusTrichispa sericia The demands of transplanting and (Guérin) manual weeding and increasing shortages of Bacterial diseases and their causal agents labor have encouraged the move to direct- Bacterial blight Xanthomonas oryzae pv. seeding in irrigated and rainfed lowlands. oryzae (Uyeda ex Ishiyama 1922) Swings et al 1990 Weeds become a major problem in direct- seeding systems because rice and weeds Fungal diseases and their causal agents Blast Pyricularia oryzae Cav. emerge at the same time, and weed control Sheath blight Rhizoctonia solani by flooding is difficult in seeded rice. (Thanatephorus cucumeris With weeding a major cost in both [Frank] Donk) Weeds transplanted and direct-seeding systems, Ageratum conyzoides L. herbicide use to control weeds is increasing Cyperus difformis L. rapidly. As a result, herbicide-resistant C. iria L. Echinochloa colona (L.) Link weeds and pollution are emerging problems E. crus-galli (L.) P. Beauv. in rice production. Fimbristylis miliacea (L.) Vahl Insects attack all parts of the rice plant. Monochoria vaginalis (Burm. f.) Presl Hundreds of species feed on rice, but only Rice and the environment 23 a few cause yield loss. The most important resistant varieties for many years. However, and widely distributed pest species are stem the evolution of resistance-breaking strains borers, leaffolders, planthoppers, and gall of these pathogens has necessitated the midge. Stem borers are chronic pests, found continuing release of new resistant varieties. in every field in every season, but generally Strong sources of resistance for sheath at low numbers. Planthoppers and gall blight have not been identified in rice midge usually create localized outbreaks, germplasm. Sheath blight is a particularly causing high yield losses in relatively small important disease in intensive rice-growing areas. Biological control by natural enemies conditions where high amounts of nitrogen plays a critical role in the management of fertilizer are applied. all insect pests. Resistant rice varieties are of importance in the control of planthoppers and gall midge. No strong sources of Environmental impacts resistance to stem borers have been found Rice production mainly affects the in rice germplasm, although modern environment by releasing or sequestering semidwarf rice varieties generally have gases or compounds that are active in the less stem borer damage than the traditional atmosphere or troposphere and by changing varieties they replaced. Insecticides are the chemical composition of the water used extensively against planthoppers flowing through rice fields. Rice is in turn in temperate areas of Asia, where mass affected by environmental changes, such as immigration of planthoppers from tropical global climate change. areas is a frequent problem. Disease. Bacterial blight, blast, and Ammonia volatilization sheath blight are the most important Ammonia volatilization is the major diseases of rice and they have a worldwide pathway of nitrogen loss from applied distribution. Three insect-vectored viral nitrogen fertilizer in rice systems. Across diseases are also of importance: tungro in irrigated environments in Asia, nitrogen Asia, hoja blanca in South America, and rice fertilizer input averages 120 ± 40 kg/ha, yellow mottle in Africa. Bacterial blight and with the highest amounts in southern China, blast have been successfully controlled by at up to 250 kg/ha. In tropical transplanted Farmers hand weed their rice field in the uplands of Lao PDR. 24 Rice almanac 24 Rice almanac rice, nitrogen losses from ammonia Nitrous oxide. In irrigated rice systems volatilization can be 50% or higher, while with good water control, nitrous oxide in direct-seeded rice in temperate regions, emissions are small except when nitrogen losses are generally negligible because fertilizer rates are excessively high. In most of the fertilizer is incorporated into irrigated rice fields, the bulk of nitrous oxide the soil before flooding. Ammonia-nitrogen emissions occur during fallow periods and volatilizations from lowland rice fields immediately after flooding of the soil at the are estimated at 3.6 teragrams (Tg) a year end of the fallow period. In rainfed systems, (compared with 9 Tg a year emitted from however, nitrate accumulation in aerobic all agricultural fields worldwide), which is phases might contribute to considerable 5–8% of the estimated 45–75 Tg of globally emission of nitrous oxide. emitted ammonia-nitrogen each year. Methane. In the early 1980s, it was The magnitude of ammonia volatilization estimated that lowland rice fields emitted depends largely on climatic conditions, field 50–100 Tg of methane per year, or 10–20% water management, and method of nitrogen of the then-estimated global methane fertilizer application. Volatilized ammonium emissions. Recent measurements show can be deposited on the earth by rain. This that many rice fields emit substantially less can be a beneficial source of (free) nitrogen methane, especially in northern India and fertilizer in agricultural lands, but it can China, both because methane emissions also lead to soil acidification and unintended have decreased with changes in rice nitrogen inputs into natural ecosystems. production systems and because techniques for measuring greenhouse gas emissions Greenhouse gases have improved with the use of simulation Of the three main greenhouse gases, rice models coupled with geographic information production sequesters carbon from carbon systems (GIS) data based on soil and land dioxide and likely increases emissions of use (Fig. 2.2). nitrous oxide and methane, though by how There is more uncertainty about the much is not reliably known. amount of methane emissions from rice Carbon sequestration. Rice soils that fields than from most other sources in the are flooded for long periods of the year global methane budget. Current estimates of tend to sequester carbon, even with the annual methane emissions from rice fields complete removal of aboveground plant are in the range of 20–60 Tg, or 3–10% of biomass. Significant carbon accumulation global emissions of about 600 Tg. Estimates results from biological activity in the soil- of annual methane emissions from the floodwater system. Average soil organic principal rice producers China and India carbon content in irrigated double and are 10–30 Tg. The magnitude and pattern triple rice systems in Asia is 14–15 g/kg of methane emissions from rice fields are in the upper 20–25 cm of soil. Assuming determined mainly by the water regime an average bulk density of about 1.25 t per and organic inputs and to a lesser extent cubic meter of soil and a physical land area by soil type, weather, tillage practices, of about 24 million ha, these monoculture residue management, fertilizer use, and systems alone store about 45 t/ha of carbon rice cultivar. The use of organic manure or a total of 1.1 petagrams of carbon (109 t) generally increases methane emissions. in the topsoil. Additional carbon is stored in Flooding of the soil is a prerequisite for other irrigated rice systems (such as single sustained emissions of methane. Mid- rice and rice-maize), although typically season drainage, a common irrigation in smaller amounts than in monoculture practice in the major rice-growing regions of systems. However, reliable information on China and Japan, greatly reduces methane soil carbon stocks is not available for rice emissions. Similarly, rice environments systems in most countries, and it is not with an uneven supply of water, such as known how soil organic carbon amounts rainfed environments, have a lower emission will change in response to changing climate potential than environments where rice is or management practices. continuously flooded. Rice and the environment 25 Fig. 2.2. Methane emissions from rice production in Asia. Map drawn by K. Sumfleth. Water pollution in parts of northern China, and to reclaim The changes in water quality associated sodic soils when used in combination with with rice production can be positive or gypsum, as in parts of the northwest Indo- negative, depending on the quality of Gangetic Plain. the incoming water and on management practices relating to fertilizer and biocide use, among others. The quality of the water Rice and health—pollution and leaving rice fields may be improved by nutrition the capacity of the wetland ecosystem to remove nitrogen and phosphorus. On the Many of the rural poor in Asia obtain water other side of the ledger, nitrogen transfers for drinking and household use from shallow from flooded rice fields by direct flows of aquifers under agricultural land. Among the dissolved nitrogen in floodwater through agrochemicals that pose the greatest threats to runoff or drainage warrant more attention. domestic use of groundwater are nitrate and The pollution of groundwater is covered biocide residues. In addition, contamination below in the discussion of human health. of groundwater with arsenic has recently emerged as a major health issue in Asia. Salinization Other health aspects concern malnutrition Percolating water from lowland rice fields and vector-borne diseases related to rice usually raises the water table. Where the production. groundwater is saline, this can salinize the root zone of nonrice crops in the area Nitrates and cause waterlogging and salinity in Nitrate leaching from flooded rice fields lower areas in the landscape, such as in is normally negligible because of rapid parts of Australia and the northwest Indo- denitrification under anaerobic conditions. Gangetic Plain. Where irrigation water is In the Philippines, for example, nitrate relatively fresh, flooded rice can be used pollution of groundwater under rice-based in combination with adequate drainage to cropping systems exceeded the 10 mg/ leach salts that had previously accumulated liter (mg/L) limit for safe drinking water under nonrice crops out of the root zone, as only when highly fertilized vegetables 26 Rice almanac were included in the cropping system. In pollution by biocides is greatly affected by the Indian Punjab, however, an increase field water management. Different water in nitrate of almost 2 mg/L was recorded regimes result in different pest and weed between 1982 and 1988, with a simultaneous populations and densities, which farmers increase in nitrogen fertilizer use from 56 may combat with different amounts and kg/ha to 188 kg/ha, most of it on combined types of biocides. In traditional rice systems, rice-wheat cultivation. The relative relatively few herbicides are used because contribution to this increase from rice, puddling, transplanting, and ponding water however, is not clear. are effective weed control measures. Biocides Arsenic Mean biocide use in irrigated rice systems Arsenic in groundwater has been reported in varies from 0.4 kg/ha of active ingredients several countries in Asia. Severe problems in Tamil Nadu, India, to 3.8 kg/ha in of arsenicosis occur in rural areas in Zhejiang Province, China. In the warm Bangladesh and in West Bengal in India. In and humid conditions of the tropics, the past two decades, the number of shallow volatilization is the major process of tubewells for irrigation in these areas has biocide loss, especially when biocides are increased dramatically, and dry-season applied on the water surface or on wet soil. rice production (boro rice) depends heavily Relatively high temperatures favor rapid on groundwater. It is unclear whether transformation of the remaining biocides by groundwater extraction for irrigation photochemical and microbial degradation, influences arsenic behavior in the shallow but little is known about the toxicity of the aquifers, but irrigation from arsenic- residual components. In case studies in the contaminated aquifers may pose several Philippines, mean biocide concentrations risks. Arsenic accumulates in the topsoil as in groundwater under irrigated rice-based a result of irrigation water input. Because cropping systems were one to two orders of rice fields receive higher inputs of irrigation magnitude below the single (0.1 microgram water than other crops, they accumulate [μg]/L) and multiple (0.5 μg/L) biocide limits more arsenic than other fields. Moreover, for safe drinking water, although temporary arsenic is potentially more bioavailable peak concentrations of 1.14–4.17 μg/L were under flooded than nonflooded conditions. measured. It is not yet possible to predict arsenic Biocides and their residues may be uptake by plants from the soil, and directly transferred to open water bodies significant correlations are not often found through drainage water flowing overland between total arsenic in the soil and in from rice fields. The potential for water plants. Arsenic that is taken up by rice is Few herbicides are needed for weed control in systems using puddling and transplanting. 27 Rice and the environment 27 found mostly in roots and shoot tissue, and whether such increases in the endosperm very little in the grains. In Bangladesh, are sufficient to significantly affect no milled rice samples have been found to human nutrition. To drive the adoption of contain more arsenic than the government micronutrient-rich varieties, however, the threshold of 1 part per million for safe improved traits will need to be combined consumption, although straw samples have, with other traits that are attractive to raising concerns about arsenic toxicity in farmers, such as tolerance of drought, animal feed. salinity, or submergence. Arsenic in the soil may also affect crop production, but this aspect has not received Vector-borne diseases much attention yet, and understanding of the Irrigated rice fields can serve as breeding long-term aspects of arsenic in agriculture sites for mosquitoes and snails, intermediate is too limited to assess the risks. Water- hosts capable of transmitting human saving irrigation techniques for rice (such as parasites. In particular, before transplanting alternate wetting and drying irrigation and and after harvest, puddles in rice fields are aerobic rice) reduce the irrigation inputs and attractive breeding grounds for the mosquito arsenic contamination risk of the topsoil. As Anopheles gambiae, Africa’s most efficient the soil becomes more aerobic, the solubility malaria vector. Factors that determine and uptake of arsenic also decline. whether the introduction of irrigated rice increases or reduces the incidence of Nutrition malaria are known, and technical options Human micronutrient deficiencies are exist to mitigate this impact, including relatively severe in areas where rice is the alternate wetting and drying irrigation. major staple. Increasing the density of Moreover, countries such as Sri Lanka provitamin A carotenoid, iron, and zinc have made great strides in controlling in rice can alleviate these deficiencies, epidemics through broad-based public especially among the urban and rural poor health campaigns. Japanese B-encephalitis who have little access to alternatives such is highly correlated with rice irrigation in as enriched foods and diversified diets. Asia, especially where pigs are also reared, Promising examples are the development as in China and Vietnam. Again, alternate of Golden Rice to combat vitamin A wetting and drying can help reduce the deficiency and of iron-rich rice to combat breeding of vectors. iron deficiency, although it is still debated Golden Rice grains with other beta carotene-rich (vitamin A) foods. 28 Rice almanac Chapter 3 Rice in the economy 29
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