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Factors affecting wetland rice production and the classification of wetlands for agricultural production

Wetlands are the important world's natural resources. They support the growth and development of a wide varieties of natural vegetation and serve as breeding ground for many wildlife and fish species. They are also suitable for development for food production. The development of wetlands for agricultural production is very intensive and extensive in Asia. In Africa, there are large wetland areas but most of them are still unexploited or underutilized. Recently, in recognition of the limitations of upland production systems to provide sustainable food security to their populations, many Sub-Sahara African countries have promoted the development of wetland areas for agricultural production. So far, although many other crops can be grown quite productively on wetland soils with adequate water management, most of wetland areas in the world have been developed for rice-based production systems. The wetland rice area, especially in Africa, has steadily increased during the period from 1975 to 1995 ( 1). Paddy production from wetland ecologies in 1995 accounted for about 97 percent of the world's paddy output, providing staple food for more than 3 billion people, especially in developing countries. The production, processing and marketing of wetland rice provide employment opportunities and incomes for several hundred millions families worldwide. This paper attempts to describe the factors affecting wetland rice production and their application in the classification of wetland areas in order to assist to the development of a classification system of wetlands for agricultural development.

Factors affecting wetland rice production

In 1995, rice is grown on about 148.7 million hectares in 117 countries with the highest recorded national yield of 8,544 kg/ha and the lowest one of 719 kg/ha, showing a wide variation in yield performance from country to country. Followings are the factors contributing to the large difference in rice yields obtained by different world's rice producing countries.

Geographic Factors

Wetland rice cultivation extends from 45º North to 40º South. Data in Figure 2 indicate that, in general, yields of wetland rice planted in the areas between the Tropic of Cancer and the Topic of Capricorn (TCTC) zone are lower than those of rice planted in areas outside of this zone. Most of the rice crops in the five world's highest yielding countries in 1995 were planted outside the TCTC zone (Table 1). Latitude alone is not responsible for high yield, although it is possible that latitude may have an effect on yield through resultant differences in temperature, solar radiation,day length

Van Nguu Nguyen  
Technical Officer, Crop and Grassland Service (AGPC), FAO, Rome 

Harvested areas of wetland rice, World and Africa, 1975-1995

Rice producing countries based on yield in 1995

and other. Wetland rice is also grown at altitude up to 1 500 m above sea level or more, but the actual area planted to rice at high altitude is comparatively small. Lower temperatures may affect growth and yields of rice planted at high altitudes.

Lower and Upper latitudes of the world's five top rice yielding countries in 1995


Lower latitude

Upper latitude

National yield (kg/ha)


39º South

11º South

8 544


22º North

33º North

8 137


35º North

42º North

7 961


30º North

45º North

6 343


25º North

49º North

6 300

*   Most of the rice in Australia is grown in New South Wales in the southern part of the country

** Most of the rice in Egypt is grown in Lower Nile valley in the northern part of the country

Climatic Factors

Temperature regimes greatly influence not only the growth duration but also the growth pattern of the rice plants. Rice varieties from Japan which has low temperatures during the cropping season when grown in Indonesia which has moderate to high temperatures during the cropping season, for example, generally mature early whereas Indonesian varieties when grown in Japan generally extend growth duration. In temperate countries, generally, low temperature regimes limit rice cropping to only one season. On the other hand, it is well known that respiration is low at low temperatures. The low respiration due to low night temperatures during the grain development phases of rice plants planted in areas outside of the TCTC zone may favor grain development and filling, thus high yields. Table 2 provides the critical temperatures for different development phases of the rice plants.

Critical Temperatures at different growth stages of the rice plant (Adapted from Yoshida 1978)

Growth Stage

Critical Temperature (ºC)









Seedling emergence












Panicle initiation




Panicle differentiation












In many rice growing areas, the year is divided fairly into distinct wet and dry seasons. Without irrigation, rice production can be practiced only during the wet season and the rice cropping season is determined by the length and the pattern of rainfall. In areas which receive 1 200 mm of rainfall or more per year, the water supply is adequate for at least one rainfed wetland rice crop provided the rainfall is concentrated in one season and its distribution is reasonably uniform. Rainfall and its distribution, however, are very variable in both space and time. Variability in the amount and distribution of rainfall are the most important factors affecting yield of rainfed wetland rice.

Solar radiation is essential for photosynthetic activity. As such, the growth, development and yield of rice plants are affected by the level of solar radiation. Rice yields are closely correlated to the solar radiation during the reproductive and ripening phases of the rice plants (Figure 3). The average solar radiation for the rice growing season in Mediterranean countries, Australia, and USA is about 100 cal/cm2 per day greater than in monsoon tropical countries. In the tropical climate, solar radiation during dry season is normally much higher than during wet season.

Day-length was an important factor affecting wetland rice production. With the availability of an increasing number of high yielding and non-photoperiod-sensitive rice varieties, the effect of day-length on wetland rice production is becoming less and less important. However, the long day and short night regimes coupled with high level of solar radiation during reproductive and ripening phases of rice, planted in countries having temperate climate, are generally favorable for high yield.

Winds and relative humidity may affect growth and production of rice plants. Winds at high speeds during the typhoons are very detrimental to the growth and production of rice plants, especially when they occur during the flowering and ripening phases of rice.

Land And Soil Factors

So far, most of the wetland rice fields were developed in river valleys, basins, deltas, estuaries, lake fringes and coastal plains. The surface of most of these areas are either level or gently undulating with slopes varying from 0 to 8%. The distribution of water supply to rice crops in these areas is most feasible. On dissected plains with less than 50% of surface area having 0 to 8% slope, the topography interferes with the distribution of water supplies making them moderately suitable for wetland rice cultivation. Wetland rice cultivation is limited on undulating to rolling terrain where less than 30% of the surface have slopes from 0 to 8%, as great effort is required to construct terraces and problems with water supply distribution are usually beyond the scope of farmers.

Wetland rice is grown on practically all types of soils, from sandy loam to heavy clay. However, it is well established that the heavy soil characteristic of river valleys and deltas are better suited to wetland rice production than lighter soils. An ideal rice soil should contain up to 50-60% of finer fractions of silt and clay. Variations in soil conditions and the extension of rice cultivation to unsuitable soils are among the factors causing wide disparity in yields. Many wetland rice farmers practice puddling during land preparation to level rice fields and to conserve water. Puddling, however, may create highly unsuitable conditions for upland crops grown in rotation with rice. Soils which have an ability to Bconvert to granular aggregated structure after drying are suitable for wetland-rice based cropping systems.

Nutrient removal of a rice crop (Variety IR8) yielding 7.9 tons/ha at Maligaya Rice Research and Training Center, Philippines, 1979 dry season (Adapted from De Datta 1981)

Nutrient element

Total amount of mineral removed by
the crop at harvest (kg)



























Table 3 shows that an IR8 rice crop yielding 7.9 tons/ha removed about 123 kg N, 21.4 kg P, 120 kg K and substantial amounts of other nutrient elements from the growth medium. Therefore, yield of wetland rice is influenced by the level of soil fertility. The deficiency of nitrogen is the most common constraint to wetland rice production. Uncertainty, however, exists as to factors affecting the nitrogen supply capacity of rice soils under flooded conditions. It had been commonly believed that the total soil nitrogen could be an adequate guide to nitrogen release. Recently, however, results from long-term experiments have provided evidence that soil N supply is governed more by the chemical qualities than the amounts of soil organic matter and soil N (Cassman et al, 1997). Although the availability of phosphorus is generally improved under flooded conditions, phosphorus deficiency is the next commonly observed constraint to wetland rice production, especially in acidic soils with light texture. Increasing evidences of potash, zinc, sulfur deficiencies have been obtained in wetland areas under continuous cropping with high yielding rice varieties. Iron toxicity is one major constraints of wetland rice production, especially in inland valley swamps in West Africa (Jamin and Andriesse, 1993). The availability of soil nutrients, however, changes with the moisture regimes of soils. The mineral stresses in wetland rice-growing soils associated with changes in the moisture regimes of rice soils are shown in Table 4.

Influence of soil and time of drying on mineral stress in wetland rice (Adapted from Ponnamperuma and Ikehashi, 1979)






During drying

After flooding

Neutral soils


Fe deficiency, P deficiency

N deficiency



Fe deficiency


Strong acid soils


P deficiency

Iron toxicity



Mn toxicity


Acid sulfate soils


Al toxicity, P deficiency

Fe toxicity



Al toxicity


Calcareous soils


Fe deficiency

Zn deficiency, N deficiency



Fe deficiency


Alkali soils


Fe deficiency, Ca deficiency

Zn deficiency, N deficiency



Fe deficiency


Saline soils


Severe osmotic stress

Osmotic stress



Osmotic stress

Osmotic stress

It is remarkable that rice can tolerate a wide range of pH values in soils. However, it thrives better in neutral to slightly acid soils than in definitely alkaline and acidic soils. Acidity, especially acid sulfate, is a main constraint to high yields of rice in several million hectares. In sub-humid and semi-arid monsoon climate, the possibilities for a wetland rice crop in the coastal or estuarial plains inland from a mangrove bands depend largely on the period over which it is possible to keep land flooded or saturated with freshwater. The potential productivity of many hundred thousands of hectares of tropical peat lands still needs to be established. Also, salinity and alkalinity are an important constraint to rice production in several million hectares of poorly drained irrigated areas.

Water Supply Factors

The rice plants generally have shallow root systems. Water deficiency or drought stress has been recognized as the most important limiting factor to wetland rice production. Water deficiency may happen at any time during the cropping season of rainfed wetland rice. The damage on rice crops varies with the developmental stage during which water deficiency occurs and the duration of the water deficiency. Damage is usually heavy and irreparable when intensive water deficiency occurs during the reproductive and flowering phases of rice crops. Yield losses of 1 ton/ha or more may occur after 10 days with continuous water deficiency during these phases. Water deficiency is the main cause of low and variable yields in rainfed wetland rice.

Although rice plants are well know for their ability to transport oxygen from air to their root systems, flooding with consequent crop submergence may severely damage rice crop. Most of wetland rice cultivars, including deep-water ones, can stand complete submergence for at least 6 days before 50% of crop die, whereas 100% mortality occurs in all cultivars within 14 days of complete submergence (Setter et al 1995). Also, stagnant water or waterloggong cause excessive soil reduction which alters the chemistry of wetland soils and usually causes nutrient deficiency or toxicity or both to rice crops. Adequate water movement through rice fields is recognized by most rice farmers as essential. Incoming water is normally aerated and its oxygen prevents the soil redox potential from falling too far. Rice yields in millions of hectares with prolonged stagnant flood with depth of 50 cm or more are generally low. In these areas, the depth, duration and frequency of flooding are important factors affecting yields of wetland rice.

In addition to rainfall, interflows and streams activated by local rains are common sources of water for wetland rice in undulating terrain. In basin, deltas, estuaries and lake fringes the water table rises during the rainy season and comes within easy reach of rice roots. Under rainfed conditions, therefore, the water supply to wetland rice fields depends on both rainfall and the hydrology of the rice fields. Water supply to wetland rice under rainfed conditions, however, is neither adequate nor fully controlled, due to variability in rainfall and its distribution, topography, land form and soil physical characteristics.

Adequate and controllable water supply to wetland rice is possible only with the development of irrigation and drainage infrastructures. Run-off water from watershed areas can be stored with the construction of dams and reservoirs and then supplied to rice fields. Water in major rivers can be diverted to rice fields with construction of diversion dams and/or be lifted to rice field using either modern or traditional water lifting devices. Water in underground aquifers can be lifted through installation of tube/deep wells and utilization of pumps. In addition to the removal of the negative effects of water deficiency on rice growth and yield, irrigation has also indirect beneficial effects such as weed control, timely crop establishment, efficient fertilizer management, etc. Therefore, the availability of water sources and their potentials for irrigation development for wetland rice cultivation are important factors affecting the development of wetland rice production. In fact, it is well recognized that the development of the irrigation infrastructures is one of the main pillars of the successful Green Revolution in may Asian rice producing countries during 1970s and 1980s.

Farming Practices

The environmental conditions and the preference of the population on grain quality determine the selection of rice varieties for planting. The diversity of planted rice varieties partly explains the differences in yields obtained from different countries. Figure 4 shows that Oryza sativa var. japonica is principal rice type in areas outside the zone between the TCTC zone, whereas most rice varieties planted inside the TCTC zone belong to Oryza sativa var. indica type. The Oryza glaberima spp was widely grown in West Africa. In many of these areas, however, Oryza glaberima spp has been replaced by Oryza sativa var. indica. The highest yield of pureline indica (10.3 tons/ha) was obtained with the improved rice variety IR8 planted at the experimental farms of the International Rice Research Institute in the Philippines (De Datta, 1981) whereas a yield of 13 tons/ha was obtained with the pureline japonica rice variety Koshihikari planted in Yanco, Australia (Horie et al, 1994). The recently developed hybrid rice varieties have yield potentials which are about 15 to 30% higher than the yield potentials of best pureline rice varieties. The

Rice producing countries by dominant rice type in 1995

Rice producing countries based on number of agricultural tractors in use in 1995

super rice varieties which are being developed by IRRI are reported to have a yielding potential of about 15 tons/ha (Fischer, 1994) while the yield potential of an ideal japonica rice is estimated to be about 18 tons/ha (Horie et al, 1994). Yields of rice varieties with superior grain quality, for example Basmati rice varieties, are generally low.

Good, clean and healthy seeds of improved rice varieties are basic conditions for healthy growth and development of rice plants, at least during their early phases. The level of the development of the local seed industry and distribution, therefore, has great influence on the performance and yield of wetland rice. Utilization of seed of poor quality is one of the main factors responsible for the low yields obtained by many farmers in tropical Asia and Africa.

Land preparation by itself may not be necessary for successful wetland rice production. However, land preparation indirectly affects rice yield through resultant better field conditions. Land preparation is recognized as an effective weed control practice. Also, good land preparation facilitates better water management and to a lesser extent fertilizer management in wetland rice production. Land preparation using the hand hoe is widely practiced by women in small inland swamps in West Africa whereas animal traction and/or small motocultors/tractors is popular in tropical Asian and African developing countries where the area of wetland rice fields per household varies from a few thousand square meters to a few hectares. On the other hand, in many areas in America, Australia, and Europe, where the wetland rice area per household ranges from several tens to several hundreds of hectares, land preparation and leveling are mainly done with the use of laser-guided tractors and weed control is done with herbicides. The available tools and equipment, the sex of the person who is responsible for land preparation and the size of the fields interactively affect the quality of land preparation and thus the performance and yield of wetland rice. Figure 5 shows the degree of mechanization in the world's rice producing countries.

The rice crop can be established either by direct seeding or by transplanting of seedlings which were raised in separate nursery beds. Theoretically, there is no advantage in any of the above mentioned methods of crop establishment on rice yields. Transplanted rice crops, however, have better competitive edge over weeds. Also, when transplanting is done in regular patterns, it facilitates weeding by hand and/or mechanical means such as rotary/push weeders. Transplanting also require less seeds than direct seeding. In many developing countries where labour is plentiful and cheap, transplanting is widely practiced. However, in areas where labour is scarce and costly direct seeding is usually practiced. Direct seeding is usually a popular method of crop establishment in deep-water rice areas. The period for crop establishment in these areas is short, as the flood water usually arrives soon after the onset of the monsoon.

The rates of fertilizer applied to rice planted in European countries, Japan, and USA are in general higher than those applied to wetland rice in tropical Asia and Africa. Also, farmers applied more fertilizer to irrigated rice than to rainfed rice. In the developing countries, farmers usually applied mostly major nutrient elements, especially nitrogen and to a lesser extent phosphorus and potassium. In developed countries, more balanced fertilizer application is practiced as the support facilities such as extension services and the laboratories for soil and plant analysis are readily available to farmers. Unbalanced fertilizer application in intensive rice production, however, has led to increasing deficiency in other nutrient elements which in turn limits rice yield.

In countries with high rice yields such as Australia, Egypt, and USA, rice is generally planted in rotation with other crops. On a given field, rice usually is planted only one in every 2 to 3 years in Australia and only one in every 1 to 2 years in Egypt and USA. The continuous planting of rice (2 crops or more per year) on the same field is practiced in many irrigated areas in tropical Asia,. There are increasing evidences showing a decline in yield of rice planted under intensive-monoculture rice cultivation. At 3 locations in the Philippines, for example, rice yields under intensive-mono-rice culture (2 rice crops or more per year) have declined by 0.1 to 0.3 tons/ha per year over 20 years (Cassman et al, 1997). The rice-rice cropping system is also widely practiced in several irrigated schemes in tropical Africa. The intensive mono-rice cropping systems observed in tropical Asia and Africa may be due to the population's preference for rice and the environmental factors. They, however, may also be due to the defects in the design for land and irrigation development.

Weeds, diseases, insect pests and other biological agents such as rats, birds, etc. are common constraints to wetland rice production. Weed growth and competition increase when soils are subject to alternating wet and dry conditions. Bird and rat damages are particularly important when rice fields are small and isolated. However, the limited available information does not allow a good comparison on the yield losses due to these constraints in different countries. On the other hand, intensive use of chemical products for insect, disease, and weed control in wetland rice production has changed the status of the economic importance of pests and diseases in wetland rice production from low to high. Integrated Pest Management in wetland rice production, therefore, has been popularized to minimize the harmful effect of excessive use of pesticides on wetland rice production and on the environment. The major insects and diseases of wetland rice production in Southern Africa are shown in Table 5.

Common diseases and insect pests of rice in some SADC countries (Adapted from Jorge and Mabbayad, 1992)



Insect pests


Blast, brown spot

Stem borers, Spodoptera species


Blast, sheath rot, brown spot, glume discoloration, bacterial leaf blight

Diopsis species, stem borers, rice hispa, mole crickets, grasshoppers


Blast, bacterial leaf blight, narrow brown spot, white tip

Nematodes, Diopsis species, stem borers, Spodoptera species


Blast, brown spot

Leaffolders, Trchispa sericea (rice beetles)


Blast, glume discoloration


Socio-economic Factors

As discussed above, farmers' farming practices and their intensity are, in great part, determined by farmers' socio-economic conditions. The other socio-economic factors affecting wetland rice production are land tenure, credit availability and market accessibility.Evidences have indicated that wetland rice generally yields higher when it is cultivated by the land owner. It is normally more interesting for the land owner to invest in the improvement of his land and for buying the tools and equipment needed for farming. Land tenure has been recognized as a major constraint of wetland rice production in inland swamps in West Africa (Jamin and Andriesse, 1993). The success of wetland rice production, as that of the production of other crops, depends greatly on the existence of viable credit facilities and systems for purchasing seeds, inputs, equipment. Also, the access to markets for the purchase of inputs as well as to sell the produces and the competition for time and resources stemming different production activities are often factors determining farmers' choice on crops to be planted, crop cultivation practices and crop rotation.

Farming practices and their quality depend greatly on farmers' knowledge on wetland rice cultivation, while the rice production technologies available to farmers depend on the level of research and extension services in the concerned location/country. The presence of water born diseases such as river blindness, malaria, schistosomiasis, etc. may limit farming activities in wetland rice production.

The classification of wetland for rice production

Considerable effort has been devoted to the classification of land for agricultural development in general and for rice production in particular. FAO has developed the Agro-ecological Land Resources Assessment for Agricultural Development Planning, a comprehensive land classification, especially for rainfed upland agricultural production systems. Detailed descriptions of Agro-ecological zones in different countries have been published by FAO. Recently, this approach has been successfully used to classify the resource bases and land productivity in Kenya (Kassam et al, 1991). In West Africa, both the International Institute for Tropical Agriculture (IITA) and the Inland Valley Consortium/West Africa Rice Development Association (IVC/WARDA) have carried out the Agro-ecological characterization of the land resources in West Africa. The objective of IITA's exercise is to enable researchers to identify the characteristics of inland valleys in the region best suited for cultivation and those best suited for resource conservation or protection (Jagtap, 1993), whereas the objective of IVC/WARDA's exercise is to identify the constraints to agricultural development of the inland valleys and priorities of research programmes (Andriesse et al, 1993). The Consultative Group on International Agricultural Research (CGIAR) has also initiated the Ecological Programme for the Humid and Sub-humid Tropics of Sub-Saharan Africa (EPHTA) to promote collaborative efforts in the region, including the land characterization/classification for agricultural development.

In order to promote collaboration among institutions and organizations involved in rice research and development, the International Rice Research Institute convened in 1982 an International Rice Research Conference to classify the rice growing environments. Factors affecting rice production were used (IRRI, 1984) and wetland rice was classified as following:

1.  Irrigated Rice: Irrigated rice areas have adequate water supply throughout the growing season. In much of the irrigated rice areas, rainfall supplements irrigation water. Irrigated areas are subdivided into three categories:

2.  Rainfed Lowland Rice: Rainfed lowland rice areas have a greatly diversity of growing conditions that vary by amount and duration of rainfall, depth of standing water, duration of standing water, flooding frequency, time of flooding, soil type and topography. Rainfed lowland has five categories:

3.  Deep-water Rice: Deep-water rice areas are flooded with a water depth which is above 50cm for major part of the growing season.

4.  Tidal Wetland Rice: Tidal wetland rice areas are near to the seacoasts and inland estuaries that are directly or indirectly influenced by tides. Tidal wetland areas are divided into four categories:

Indonesia is the world's third largest rice producer. In 1995, Indonesia produced about 49.7 million tons of paddy from about 11.4 million hectares obtaining an average yield of 4 348 kg/ha. The Indonesian system of classification of wetland for rice production (Suryatna et al, 1979) is one of the most comprehensive and practical systems. This system classifies the land in the country in:

1.  Class P-II: Restrictions to wetland rice production are slight. More than 50% of the unit land surface is level to gently undulating and conditions of soil texture, permeability, and fertility are well suited to paddy rice production. Soils respond well to fertilizers notably those containing nitrogen. There are no problem with drainage or flooding. Water usually is sufficient for at least one crop of rice and prospects of continual irrigation to support year-round cropping are often good.

2.  Class P-III: Land moderately well suited to wetland rice production:

3.  Class P-IV: Land poorly suited to wetland rice production:

4.  Class P-V: Land generally unsuitable for wetland rice production:

More simple and practical systems of classification of wetlands have also been used for agricultural development in several developing countries. In Nigeria, in recognition of the effect of climate, soil and local hydrological conditions on their formation, the inland swamps are groups into V and U shaped. In Burkina Faso, based on the levels which they have been developed, the inland swamps are generally classified into improved swamps,simply improved swamps, traditional swamps, etc. In Bangladesh, the information provided by FAO's Agro-ecological Land Resources Assessment for Agricultural Development Planning was used to classified land of the country into major agro-ecological zones (AEZ). Surveys using Participatory Rural Appraisal approach (PRA surveys) were then carried out in each AEZ and based on the results of the surveys, land in each AEZ was classified based on the topographical position (e.g. high, medium-high, medium-low, low wetlands) and soil texture (e.g. light and heavy). Information resulting from this classification was used together with information on the socio-economic conditions and the farming practices obtained during the PRA surveys as well as the results on responses of crops to management practices from research institutions to design the rice-based farming systems for extension activities. The FAO's projects in Sierra Leone and Democratic Republic of Congo (ex-Zaire) used data/information on the size and the accessibility of the inland valley swamps, the local hydrological conditions, local land form and topography, the socio-economic conditions, the farming systems practiced by concerned communities (which were generated through agro-economic surveys), together with information provided by earlier land classifications (e.g. climate, soil association, length of growing season, natural vegetation, etc.), to classify inland swamps for the selection of developmental options. Recently, the Special Programme for Food Security of the FAO (SFPS) has used the participatory approach, involving expertise of the local farmers and local population and national and international experts in different disciplines in the selection of land areas or sites in many Low Income and Food Deficit countries for its activities. The agro-socio-economic surveys were then conducted at the selected sites for determining the technological packages to be introduced for increasing food production as well as for generating incomes and employment opportunities for rural communities.

Summary and conclusions

Wetland areas in the world have been extensively and intensively developed for food production systems, especially those which are rice-based. The yield and performance of wetland rice planted in different countries still exhibit wide variations due to the varying climate, land and soil, water supply, farming practices and socio-economic conditions.

These factors affecting wetland rice production, especially the climate, land and soil and water supply, have been used in the past by many national, regional and international institutions to classify wetlands for agricultural development or for rice-based production systems as well as for research purposes. Considerable information on the suitability of different wetland areas for agricultural development in general and for rice-based production systems in particular, therefore, is available at least at the macro-level.

A detailed characterization and/or classification of land is desirable for agricultural development. This, however, may require time and effort which may lead to a considerable delay in the land development.

Practical systems of land classification using information provided by earlier efforts and information generated through participatory agro-socio-economic surveys have been proven to be viable alternatives for the classification of wetlands for agricultural development. They could be used to expedite the development of wetlands for producing food in order to satisfy the urgent demand by the population and to improve their food security situation.


The valuable comments and suggestions provided by Mr. D.V. Tran, Senior Rice Agronomist at FAO, during the preparation of this paper are highly appreciated.


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