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Toward increasing nutrient-use efficiency in rice cropping systems: the next generation of technology
ELSEVIER
Field Crops Research 56 (1998) 1-6
Field Crops Research
Toward increasing nutrient-use efficiency in rice cropping systems: the next generation of technology K.S. Fischer
*
International Rice Research Institute, P.O. Box 933, 1099 Manila, Philippines
Abstract Rice production must increase dramatically to meet the demand projected for 2025. If the technologies that affect nutrient utilization by the rice crop remain unchanged, that production increase will require almost 300% more than the present application rate of N alone in irrigated environments. This is an undesirable amount economically and environmentally. The nutrient-use efficiency of rice cropping systems must be improved, along with yield potential of rice cultivars, in order to improve profitability of rice production and prevent environmental degradation in irrigated areas. Part of the projected demand for rice will be met through production increases in rainfed environments, where yields need to double from their present average of 2.0 t ha- 1. The productivity of these systems is governed by interactions between water availability and the nutritional status of the crop. Cultivar improvement for tolerance to abiotic stress must be combined with management approaches that improve nutrient-use efficiency in such rainfed environments. © 1998 Elsevier Science B.V. Keywords: Modem cultivars; Nutrient X water interaction; Nutrient-use efficiency; Rice; Yield gap
1. The demand for rice in 2020 The projected global requirement for rice over the next 25 years has been described in IRRI planning documents (IRRI, 1997): Rice remains the staple food for nearly half the world's people, most of them living in Asia, many of them among the poorest of the poor in the world. The rice research community led by IRRI has been successful, so far, in helping provide this staple food to expanding populations.
* Corresponding author. 0378-4290/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0378-4290(97)00139-1
But alarming indications of continuing poverty and malnutrition, unabated environmental degradation, and high population growth are again placing pressure on the fine balance between supply and demand for this important staple grain. (p. 1) The world population continues to increase by 85 million people a year. Although population growth rate has been declining in most countries of the developing world, the absolute yearly increase in the number of people during this decade will be the highest in modem history because the population base has been expanding. The developing world will be adding another 2.3 billion people over the next three decades compared to an increase of 2.1 billion over the previous three
2
K.S. Fischer/Field Crops Research 56 (1998) 1-6
decades. Population growth will in fact be higher in regions characterized by pervasive poverty and malnutrition such as South Asia and sub-Saharan Africa where the per capita grain consumption is expected to increase further because of the large unmet demand for food. The challenge to the scientific community is therefore to continuously increase food supplies with limited natural resources, in a manner that protects the soil, water, and biotic resource base from which all food must come. (p. 2)
2. T o w a r d m o r e information-intensive m a n a g e ment
The introduction of semi-dwarf, fertilizer-responsive, high-yielding rice cultivars in the late 1960s that heralded the Green Revolution in rice cultivation has so far helped most Asian countries to meet their food needs. Through the progressive introduction of these high-yielding cultivars over the last three decades, rice yield has doubled and foodgrain production has, until recently, outpaced population growth. Sustaining the growth in rice supply is, however, becoming difficult, because the technologies have reached mostly irrigated and favorable rainfed environments. These farmers now use modem cultivars whose yields approach the maximum scientists can achieve using today's knowledge. The original Green Revolution technology was criticized as being biased against the poor and against ecological sustainability. Although subsequent empirical research has proved that modern high-yielding cultivars are scale-neutral and do benefit poor farmers, concern is growing that the intensive monoculture of rice and heavy use of agrochemicals deteriorate soil and water quality, and that a few modern cultivars have displaced many traditional cultivars with diverse traits, thus, shrinking the genepool. Expansion of modern agriculture initiated by the Green Revolution and the integration of rural society into the modern market economy have drawn attention to environmental impact and equity aspects of agricultural development and the challenge of 'sustainability'. The challenge to agricultural scientists today is how to contribute to the transition to a
doubly Green Revolution that will help sustain adequate growth in grain production to enable food to reach the poor at affordable prices (the first green) in a manner that protects soil and water quality and preserves biodiversity (the second green). The issues that agricultural development will face at the dawn of the 21st century are much different from those faced in the 1960s. IRRI (1997) (p. 2) has identified the emerging issues as "...increasing scarcity of labor; water scarcity; the trend to commercialize farms; pressure to liberalize rice trade; pressure on the resource base; addressing locationspecific problems; changing food consumption patterns; and the role of women in research and development." Interactions between rice production and the environment have been the subject of considerable discussion (IRRI, 1997; p. 4): Almost every component of modern agriculture raises ecological concerns: the hypothesis that soil fertility is declining under intensification; the potential decrease in the efficiency of fertilizer conversion into usable output, due to deterioration of the soil's nitrogen-supplying capacity; the reduction in biodiversity of rice cultivars and the longterm problems of managing pests, weeds, and diseases in highly productive ecosystems. These are not, however, inevitable if we learn how to manage the natural resources. In exploring ways to raise yields even further, from limited soil, water, and biological resources, scientists must guard against worsening these ecological concerns. A particular challenge is how to extend information-intensive natural resource management technologies to the numerous small-scale and marginal farmers. In seed-based technology, scientific knowledge is embodied in new cultivars that do not necessarily require farmers to gain additional knowledge to ensure their success. Technologies that enhance input efficiency and conserve natural resources contain much technical information derived from scientific research about crops, soilwater-pest interactions, and other complex relationships. Adaptation and adoption of these technologies demand much more farmer education
K.S. Fischer/Field Crops Research 56 (1998) 1-6
than is the case with improved cultivars, and need much more effective research-extension linkages. However, with advances in modem communication and information systems, opportunities for moving many 'on-the-shelf technologies out to farmers will increase.
3. Evolving rice agroecosystems needed to meet the demand Rice is grown in agroecosystems that range from uplands to the flood-prone and coastal wetlands. Thus, its adaptive mechanisms are diverse, and the rice agroecosystem can be characterized by the supply of water as depicted in Fig. 1.
Deficit ~
Upland Level to steeply sloping fields; rarely flooded, aerobic soil; rice direct seeded on plowed dry soil or dibbled in wet, nonpuddled soil
3
Approximately 75% of the world's rice is currently produced under irrigated systems. Modem irrigated rice culture in tropical and subtropical environments is perhaps the most intensive cereal production system in the world. This system, originating in the rice valleys and deltas, remained relatively unchanged for more than 2000 years until the 1960s, at which time it underwent massive intensification. This intensification led to more tillage operations in puddled soil, an increase in the amount of time soils remain submerged, and greater amounts of applied irrigation and water and fertilizer nutrients, especially N fertilizers, in double- and triple-crop rice systems. Given this rapid change in a relatively short period, we are only now beginning to quantify and understand the impact of intensification on the natural resource base.
Water
Rainfed lowland Level to slightly sloping, bundled fields; noncontinuous flooding of variable depth and duration; submergence not exceeding 50 cm for more than l0 consecutive days; rice transplanted in puddled soil or direct-seeded on puddled or plowed dry soil; alternating aerobic to anaerobic soil of variable frequency and duration.
~__~ Surplus
Irrigated Leveled bundled fields with water control; rice transptanted or direct seeded in puddled soiI; shallow flooded with anaerobic soil during crop growth.
Flood prone Level to slightly sloping or depressed fields; more than l0 consecutive days of medium to very deep flooding (50 to more than 500 cm) during crop growth; rice transplanted in puddled soil or direct seeded on plowed dry soil; aerobic to anaerobic soil; soil salinity or toxicity in tidal areas.
Fig. 1. Rice a g r o e c o s y s t e m characteristics. These a g r o s y s t e m s are characterized b y the natural resources o f water a n d land, and b y the adaptation o f the rice plant to them. Irrigated rice m a y be f o u n d at a n y point in a t o p o s e q u e n c e if controlled w a t e r delivery and d r a i n a g e are available. Source: IRRI (1993).
4
K.S. Fischer/Field Crops Research 56 (1998) 1-6
Table 1 Rice area (106 ha), yield (t ha-l), and total production (106 t, rough rice) in 1991 and estimates for all of Asiaa in 2025 Rice system
Irrigated Rainfed lowland Upland Deepwater Total
1991
2025
Area
Average yield
Production
Area
Average yield
Production
73.9 38.7 10.5 10.0 133.3
4.89 2.30 1.07 1.53 3.58
361.6 89.2 11.2 15.3 477.3
73.9 38.7 10.5 10.0 133.3
8.00 3.50 1.07 1.53 5.66
591.9 135.5 11.2 15.3 753.9
Source: Cassman and Pingali (1995). alncludes China, India, Indonesia, Pakistan, Bangladesh, Vietnam, Philippines, Thailand, Myanmar, Nepal, Malaysia, Sri Lanka, Cambodia, Laos, Japan, Korea, and Taiwan.
But now, these systems must again increase their output to meet an expected annual demand of 755 Mt of rice by 2025 (Hossain and Fischer, 1995). This output will require an average yield of 8.0 t ha-1 on irrigated lands, assuming the same harvested area of about 74 Mha in 1991 (Table 1). Although present rice cultivars have sufficient yield potential to achieve these yield levels, the exploitable yield gap between farmers' average yield and the biological yield potential of current cultivars will disappear rapidly early in the next century. Thus, there is a need for rice cultivars with increased yield potential (Khush, 1995). Another challenge is to achieve increased input-use efficiency as well. This is particularly important for fertilizer-N inputs, because N not acquired by the plant can be lost to the atmosphere as a 'greenhouse gas' (Neue et al., 1995), leached through the soil profile to contaminate groundwater, or discarded as runoff to pollute surface waterways. Like most response curves, however, rice yield response to N follows a diminishing return function at higher yield levels. In fact, yields can decrease as the result of lodging and increased disease when N fertilizer is applied at rates that exceed crop demand. To achieve a yield of 4.9 t ha - l , for example, farmers typically apply 100 kg N ha -1, assuming a soil N contribution of about 47 kg N h a - l to crop uptake, and a grain yield of 3.3 t ha -~ without fertilizer-N, as found in 11 long-term experiments in five Asian countries (IRRI, 1994), and a fertilizer-N recovery by the rice crop of 33% in farmers' fields. Based on the same assumptions, a yield of 8.0 t ha- l requires 280 kg N ha -1 with the same recovery efficiency (Cassman and Pingali, 1995). Thus, fertilizer N applied to irrigated rice in Asia would i n -
crease from 7.0 Mt (elemental N basis, 15.5 Mt as urea at 45% N) to 19.6 Mt (43.6 Mt urea) to support average yields of 8.0 t h a - l - - n e a r l y a 300% increase in N addition for a 63% increase in yield by 2025. With good management on research stations, it is possible to achieve a recovery efficiency of 50 to 60%, and it should be possible to further increase efficiency by developing improved techniques to predict soil N supply and in-season plant N status (Cassman et al., 1994). To achieve a yield of 8.0 t ha -1 with 50% recovery efficiency, the N-input requirement decreases by one-third. Clearly, it will be necessary to increase both yield and N-use efficiency in tandem over the next 30 years to avoid environmental problems associated with high rates of N addition, and to improve profitability of rice production. Conventional plant breeding has improved the N-use efficiency as seen in Fig. 2. Cultivars released in 1965 produced less than 40 kg grain k g - 1 N taken up by the plant. The cultivars released in 1995 are producing almost 55 kg of grain for the same amount of N taken up by the c r o p - - m o r e than 35% increase in efficiency. Moreover, the balance between N inputs and input requirements for other macro- and micronutrients will become more important as yields are pushed higher and nutrient removal with harvested grain increases in rice systems. The same arguments can be made for increasing the efficiency of water and energy inputs to rice systems, and for reducing pesticide use. There are other opportunities to increase the efficiency of N fertilization in rice systems. Ladha et al. (1996) reported that free-living a n d / o r associated
K.S. Fischer/Field Crops Research 56 (1998) 1-6
~-
55 IR72
K z
°
50
"7 t~ IE
~
c-
45
~
40
"e-
35
IR8
P z
30 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year
Fig. 2. Efficiency of nitrogen use of rice cultivars vs. their year of release. All entries were grown at IRRI under the same field conditions. Source: S. Peng (personal communication).
phototrophs and heterotrophs can fix 50 to 100 kg N h a - l in irrigated rice paddies, and that future biological N fixation systems may further increase that supply. While most of the rice supplies will still come from irrigated rice systems, production in unimproved rainfed systems, which account for over half of the land under rice cultivation, must also increase. In fact, we must find ways to double their mean yield of less than 2.0 t ha-1 achieved today to 4.0 t ha -1 in the next 25 years. In these variable and heterogenous systems, the fertility status of a crop interacts with environmental stresses. A proportion of the rainfed system does not encounter serious soil fertility constraints other than moderate requirements for N and, less frequently, P and K fertilizers. Other major areas, however, experience a range of serious chemical imbalances that may seriously affect productivity and influence cultivar adaptation. More than 50% of the shallow, rainfed lowland rice in South Asia and 75% of that in Southeast Asia is grown on soils with potentially major fertility constraints.
5
The main soil problems in rainfed systems are salinity, alkalinity, Fe toxicity, P and Zn deficiencies, and organic and acid sulfate conditions (Sehgal and Abrol, 1994; Sekhbon and Bilbao, 1994). Soils with low cation exchange capacity are common, necessitating exact N management (Garrity et al., 1986). With increasing population pressures and land-use intensity, less farmyard manure is being applied as organic fertilizer (Fujisaka, 1990), and even though it has been shown that green manure legumes are very good sources of biologically fixed N, adoption of the technology has been virtually nil (Garrity and Flinn, 1988). Low use of mineral fertilizer in rainfed lowlands is attributable to poverty, lack of credit, unavailability of fertilizers, inadequate infrastructure, and low use efficiency. Improving fertilizer use requires alleviating these constraints. Agricultural research can develop responsive and adapted cultivars that could help justify policy changes affecting rainfed lowland areas, in much the same way that government policies were modified in response to modern wheat and rice technologies that became available in the late 1960s. The variable moisture regime of rainfed lowlands contributes to their uncertain fertility (De Datta, 1986). They experience repeated wetting and drying, accompanied by radical changes in soil chemistry caused by the shift from an oxidative to a reducing environment. Fertilizer placement and timing may improve nutrient-use efficiency (De Datta, 1986), but unless the cultivar is responsive, there is little likelihood that farmers will adopt more labor-intensive or costly practices (Misra et al., 1986).
4. Research outputs for the next generation of technology
The research needed to address the complex set of issues related to rice supply was the subject of a workshop on nutrient-use efficiency held at IRRI in December, 1995. Conference organizers were K.G. Cassman and H.U. Neue. Papers by IRRI scientists presented the Institute's current research agenda, and short papers by external reviewers provided perspectives on nutrient-use efficiency in other cropping systems. The outcome of that review is also reported in order to guide the direction of future research
6
K.S. Fischer~Field Crops Research 56 (1998) 1-6
(Lafitte, 1998). Technologies to improve nutrient-use efficiency must provide better congruence between plant demand for nutrients and the pattern of supply from both soil and fertilizer sources. At the same time, cultivar improvement is required to develop plants that are more buffered in their response to the fluctuating supply of available nutrients and, in rainfed environments, water. Because the next generation of technologies will be more specific to particular environments than past management practices, more precise systems of environmental classification will be required to orient both research activities and the extension of technologies to farmers. And, the new technologies will be assessed not only for agricultural productivity, but also for their interaction with the environment. One of the members of the review team was Professor Horst Marschner. He has contributed much to the understanding of plant nutrition, and specifically, to the direction of rice research at IRRI. This review was the occasion of his last visit to IRRI. He died tragically from an illness he contracted while stimulating research in Africa. We dedicate this series of papers to his memory. May our capacity to improve the well-being of present and future generations of rice farmers and consumers, particularly those with low incomes, be guided by his legacy to science.
References Cassman, K.G., Pingali, P.L., 1995. Intensification of irrigated rice systems: learning from the past to meet future challenges. GeoJournal 35, 299-305. Cassman, K.G., Kropff, M.J., Zhende, Y., 1994. A conceptual framework for nitrogen management of irrigated rice in highyield environments. In: Virmani, S.S. (Ed.), Selected Papers from the International Rice Research Conference 1994, International Rice Research Institute, Los Bafios, Philippines, pp. 81-96. De Datta, S.K., 1986. Tolerance of rice varieties for stagnant
flooding. Progress in Rainfed Lowland Rice. International Rice Research Institute, Los Bafios, Philippines, pp. 201-206. Fujisaka, S., 1990. Rainfed lowland rice: building research on farmer practice and technical knowledge. Agric. Ecosyst. Environ. 33, 57-74. Garrity, D.P., Flinn, J.C., 1988. Farm-level management systems for green manure crops in Asian rice environments. Sustainable Agriculture: Green Manure in Rice Farming. International Rice Research Institute, Los Bafios, Philippines, pp. 111-130. Garrity, D.P., Oldeman, L.R., Morris, R.A., 1986. Rainfed lowland rice ecosystems: characterization and distribution. Progress in Rainfed Lowland Rice. International Rice Research Institute, Los Bafios, Philippines, pp. 3-23. Hossain, M., Fischer, K.S., 1995. Rice research for food security and sustainable agricultural development in Asia: achievements and future challenges. GeoJoumal 35, 286-298. IRRI, 1993. Rice research in a time of change. IRRI's MediumTerm Plan for 1994-1998. International Rice Research Institute, Los BalSos, Philippines. IRRI, 1994. Program Report for 1993. International Rice Research Institute, Los Bafios, Philippines. 1RRI, 1997. Sustaining food security beyond the year 2000: a global partnership for rice research. Medium-term plan 19982000. International Rice Research Institute, Los Bafios, Philippines. Khush, G.S., 1995. Modem varieties--their real contribution to food supplies and equity. GeoJoumal 35, 275-285. Ladha, J.K., Kundu, D.K., Angelo-van Coppenolle, M.G., Peoples, M.B., Carangal, V.R., Dart, P.J., 1996. Legume productivity and soil nitrogen dynamics in lowland rice-based cropping systems. Soil Sci. Soc. Am. J. 60, 183-192. Lafitte, H.R., 1998. Research opportunities to improve nutrient-use efficiency in rice cropping systems. Field Crop Res. 56, 223-238. Misra, B., Mukhopadhyay, S.K., Flinn, J.C., 1986. Production constraints of rainfed lowland rice in eastern India. Progress in Rainfed Lowland Rice. International Rice Research Institute, Los Bafios, Philippines, pp. 191-200. Neue, H.U., Ziska, L., Matthews, R.B., Dai, Q., 1995. Reducing global wanning--the role of rice. GeoJoumai 35, 351-362. Sehgal, J.L., Abrol, I.P., 1994. Soil degradation in India--Status and impact. In: Etchevers B., J.D. (Ed.), Transactions of the 15th World Congress of Soil Science, July 10-16, 1994, Acapulco, Mexico. Int. Soc. Soil Sci., Mexico, pp. 188-212. Sekhbon, G.S., Bilbao, G.A., 1994. Sustaining high levels of productivity in South Asia and Latin America. In: Etchevers B., J.D. (Ed.), Transactions of the 15th World Congress of Soil Science, July 10-16, 1994, Acapulco, Mexico, 1994. Int. Soc. Soil Sci., Mexico, pp. 213-226.
Field Crops Research 56 (1998) 1-6
Field Crops Research
Toward increasing nutrient-use efficiency in rice cropping systems: the next generation of technology K.S. Fischer
*
International Rice Research Institute, P.O. Box 933, 1099 Manila, Philippines
Abstract Rice production must increase dramatically to meet the demand projected for 2025. If the technologies that affect nutrient utilization by the rice crop remain unchanged, that production increase will require almost 300% more than the present application rate of N alone in irrigated environments. This is an undesirable amount economically and environmentally. The nutrient-use efficiency of rice cropping systems must be improved, along with yield potential of rice cultivars, in order to improve profitability of rice production and prevent environmental degradation in irrigated areas. Part of the projected demand for rice will be met through production increases in rainfed environments, where yields need to double from their present average of 2.0 t ha- 1. The productivity of these systems is governed by interactions between water availability and the nutritional status of the crop. Cultivar improvement for tolerance to abiotic stress must be combined with management approaches that improve nutrient-use efficiency in such rainfed environments. © 1998 Elsevier Science B.V. Keywords: Modem cultivars; Nutrient X water interaction; Nutrient-use efficiency; Rice; Yield gap
1. The demand for rice in 2020 The projected global requirement for rice over the next 25 years has been described in IRRI planning documents (IRRI, 1997): Rice remains the staple food for nearly half the world's people, most of them living in Asia, many of them among the poorest of the poor in the world. The rice research community led by IRRI has been successful, so far, in helping provide this staple food to expanding populations.
* Corresponding author. 0378-4290/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0378-4290(97)00139-1
But alarming indications of continuing poverty and malnutrition, unabated environmental degradation, and high population growth are again placing pressure on the fine balance between supply and demand for this important staple grain. (p. 1) The world population continues to increase by 85 million people a year. Although population growth rate has been declining in most countries of the developing world, the absolute yearly increase in the number of people during this decade will be the highest in modem history because the population base has been expanding. The developing world will be adding another 2.3 billion people over the next three decades compared to an increase of 2.1 billion over the previous three
2
K.S. Fischer/Field Crops Research 56 (1998) 1-6
decades. Population growth will in fact be higher in regions characterized by pervasive poverty and malnutrition such as South Asia and sub-Saharan Africa where the per capita grain consumption is expected to increase further because of the large unmet demand for food. The challenge to the scientific community is therefore to continuously increase food supplies with limited natural resources, in a manner that protects the soil, water, and biotic resource base from which all food must come. (p. 2)
2. T o w a r d m o r e information-intensive m a n a g e ment
The introduction of semi-dwarf, fertilizer-responsive, high-yielding rice cultivars in the late 1960s that heralded the Green Revolution in rice cultivation has so far helped most Asian countries to meet their food needs. Through the progressive introduction of these high-yielding cultivars over the last three decades, rice yield has doubled and foodgrain production has, until recently, outpaced population growth. Sustaining the growth in rice supply is, however, becoming difficult, because the technologies have reached mostly irrigated and favorable rainfed environments. These farmers now use modem cultivars whose yields approach the maximum scientists can achieve using today's knowledge. The original Green Revolution technology was criticized as being biased against the poor and against ecological sustainability. Although subsequent empirical research has proved that modern high-yielding cultivars are scale-neutral and do benefit poor farmers, concern is growing that the intensive monoculture of rice and heavy use of agrochemicals deteriorate soil and water quality, and that a few modern cultivars have displaced many traditional cultivars with diverse traits, thus, shrinking the genepool. Expansion of modern agriculture initiated by the Green Revolution and the integration of rural society into the modern market economy have drawn attention to environmental impact and equity aspects of agricultural development and the challenge of 'sustainability'. The challenge to agricultural scientists today is how to contribute to the transition to a
doubly Green Revolution that will help sustain adequate growth in grain production to enable food to reach the poor at affordable prices (the first green) in a manner that protects soil and water quality and preserves biodiversity (the second green). The issues that agricultural development will face at the dawn of the 21st century are much different from those faced in the 1960s. IRRI (1997) (p. 2) has identified the emerging issues as "...increasing scarcity of labor; water scarcity; the trend to commercialize farms; pressure to liberalize rice trade; pressure on the resource base; addressing locationspecific problems; changing food consumption patterns; and the role of women in research and development." Interactions between rice production and the environment have been the subject of considerable discussion (IRRI, 1997; p. 4): Almost every component of modern agriculture raises ecological concerns: the hypothesis that soil fertility is declining under intensification; the potential decrease in the efficiency of fertilizer conversion into usable output, due to deterioration of the soil's nitrogen-supplying capacity; the reduction in biodiversity of rice cultivars and the longterm problems of managing pests, weeds, and diseases in highly productive ecosystems. These are not, however, inevitable if we learn how to manage the natural resources. In exploring ways to raise yields even further, from limited soil, water, and biological resources, scientists must guard against worsening these ecological concerns. A particular challenge is how to extend information-intensive natural resource management technologies to the numerous small-scale and marginal farmers. In seed-based technology, scientific knowledge is embodied in new cultivars that do not necessarily require farmers to gain additional knowledge to ensure their success. Technologies that enhance input efficiency and conserve natural resources contain much technical information derived from scientific research about crops, soilwater-pest interactions, and other complex relationships. Adaptation and adoption of these technologies demand much more farmer education
K.S. Fischer/Field Crops Research 56 (1998) 1-6
than is the case with improved cultivars, and need much more effective research-extension linkages. However, with advances in modem communication and information systems, opportunities for moving many 'on-the-shelf technologies out to farmers will increase.
3. Evolving rice agroecosystems needed to meet the demand Rice is grown in agroecosystems that range from uplands to the flood-prone and coastal wetlands. Thus, its adaptive mechanisms are diverse, and the rice agroecosystem can be characterized by the supply of water as depicted in Fig. 1.
Deficit ~
Upland Level to steeply sloping fields; rarely flooded, aerobic soil; rice direct seeded on plowed dry soil or dibbled in wet, nonpuddled soil
3
Approximately 75% of the world's rice is currently produced under irrigated systems. Modem irrigated rice culture in tropical and subtropical environments is perhaps the most intensive cereal production system in the world. This system, originating in the rice valleys and deltas, remained relatively unchanged for more than 2000 years until the 1960s, at which time it underwent massive intensification. This intensification led to more tillage operations in puddled soil, an increase in the amount of time soils remain submerged, and greater amounts of applied irrigation and water and fertilizer nutrients, especially N fertilizers, in double- and triple-crop rice systems. Given this rapid change in a relatively short period, we are only now beginning to quantify and understand the impact of intensification on the natural resource base.
Water
Rainfed lowland Level to slightly sloping, bundled fields; noncontinuous flooding of variable depth and duration; submergence not exceeding 50 cm for more than l0 consecutive days; rice transplanted in puddled soil or direct-seeded on puddled or plowed dry soil; alternating aerobic to anaerobic soil of variable frequency and duration.
~__~ Surplus
Irrigated Leveled bundled fields with water control; rice transptanted or direct seeded in puddled soiI; shallow flooded with anaerobic soil during crop growth.
Flood prone Level to slightly sloping or depressed fields; more than l0 consecutive days of medium to very deep flooding (50 to more than 500 cm) during crop growth; rice transplanted in puddled soil or direct seeded on plowed dry soil; aerobic to anaerobic soil; soil salinity or toxicity in tidal areas.
Fig. 1. Rice a g r o e c o s y s t e m characteristics. These a g r o s y s t e m s are characterized b y the natural resources o f water a n d land, and b y the adaptation o f the rice plant to them. Irrigated rice m a y be f o u n d at a n y point in a t o p o s e q u e n c e if controlled w a t e r delivery and d r a i n a g e are available. Source: IRRI (1993).
4
K.S. Fischer/Field Crops Research 56 (1998) 1-6
Table 1 Rice area (106 ha), yield (t ha-l), and total production (106 t, rough rice) in 1991 and estimates for all of Asiaa in 2025 Rice system
Irrigated Rainfed lowland Upland Deepwater Total
1991
2025
Area
Average yield
Production
Area
Average yield
Production
73.9 38.7 10.5 10.0 133.3
4.89 2.30 1.07 1.53 3.58
361.6 89.2 11.2 15.3 477.3
73.9 38.7 10.5 10.0 133.3
8.00 3.50 1.07 1.53 5.66
591.9 135.5 11.2 15.3 753.9
Source: Cassman and Pingali (1995). alncludes China, India, Indonesia, Pakistan, Bangladesh, Vietnam, Philippines, Thailand, Myanmar, Nepal, Malaysia, Sri Lanka, Cambodia, Laos, Japan, Korea, and Taiwan.
But now, these systems must again increase their output to meet an expected annual demand of 755 Mt of rice by 2025 (Hossain and Fischer, 1995). This output will require an average yield of 8.0 t ha-1 on irrigated lands, assuming the same harvested area of about 74 Mha in 1991 (Table 1). Although present rice cultivars have sufficient yield potential to achieve these yield levels, the exploitable yield gap between farmers' average yield and the biological yield potential of current cultivars will disappear rapidly early in the next century. Thus, there is a need for rice cultivars with increased yield potential (Khush, 1995). Another challenge is to achieve increased input-use efficiency as well. This is particularly important for fertilizer-N inputs, because N not acquired by the plant can be lost to the atmosphere as a 'greenhouse gas' (Neue et al., 1995), leached through the soil profile to contaminate groundwater, or discarded as runoff to pollute surface waterways. Like most response curves, however, rice yield response to N follows a diminishing return function at higher yield levels. In fact, yields can decrease as the result of lodging and increased disease when N fertilizer is applied at rates that exceed crop demand. To achieve a yield of 4.9 t ha - l , for example, farmers typically apply 100 kg N ha -1, assuming a soil N contribution of about 47 kg N h a - l to crop uptake, and a grain yield of 3.3 t ha -~ without fertilizer-N, as found in 11 long-term experiments in five Asian countries (IRRI, 1994), and a fertilizer-N recovery by the rice crop of 33% in farmers' fields. Based on the same assumptions, a yield of 8.0 t ha- l requires 280 kg N ha -1 with the same recovery efficiency (Cassman and Pingali, 1995). Thus, fertilizer N applied to irrigated rice in Asia would i n -
crease from 7.0 Mt (elemental N basis, 15.5 Mt as urea at 45% N) to 19.6 Mt (43.6 Mt urea) to support average yields of 8.0 t h a - l - - n e a r l y a 300% increase in N addition for a 63% increase in yield by 2025. With good management on research stations, it is possible to achieve a recovery efficiency of 50 to 60%, and it should be possible to further increase efficiency by developing improved techniques to predict soil N supply and in-season plant N status (Cassman et al., 1994). To achieve a yield of 8.0 t ha -1 with 50% recovery efficiency, the N-input requirement decreases by one-third. Clearly, it will be necessary to increase both yield and N-use efficiency in tandem over the next 30 years to avoid environmental problems associated with high rates of N addition, and to improve profitability of rice production. Conventional plant breeding has improved the N-use efficiency as seen in Fig. 2. Cultivars released in 1965 produced less than 40 kg grain k g - 1 N taken up by the plant. The cultivars released in 1995 are producing almost 55 kg of grain for the same amount of N taken up by the c r o p - - m o r e than 35% increase in efficiency. Moreover, the balance between N inputs and input requirements for other macro- and micronutrients will become more important as yields are pushed higher and nutrient removal with harvested grain increases in rice systems. The same arguments can be made for increasing the efficiency of water and energy inputs to rice systems, and for reducing pesticide use. There are other opportunities to increase the efficiency of N fertilization in rice systems. Ladha et al. (1996) reported that free-living a n d / o r associated
K.S. Fischer/Field Crops Research 56 (1998) 1-6
~-
55 IR72
K z
°
50
"7 t~ IE
~
c-
45
~
40
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35
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30 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year
Fig. 2. Efficiency of nitrogen use of rice cultivars vs. their year of release. All entries were grown at IRRI under the same field conditions. Source: S. Peng (personal communication).
phototrophs and heterotrophs can fix 50 to 100 kg N h a - l in irrigated rice paddies, and that future biological N fixation systems may further increase that supply. While most of the rice supplies will still come from irrigated rice systems, production in unimproved rainfed systems, which account for over half of the land under rice cultivation, must also increase. In fact, we must find ways to double their mean yield of less than 2.0 t ha-1 achieved today to 4.0 t ha -1 in the next 25 years. In these variable and heterogenous systems, the fertility status of a crop interacts with environmental stresses. A proportion of the rainfed system does not encounter serious soil fertility constraints other than moderate requirements for N and, less frequently, P and K fertilizers. Other major areas, however, experience a range of serious chemical imbalances that may seriously affect productivity and influence cultivar adaptation. More than 50% of the shallow, rainfed lowland rice in South Asia and 75% of that in Southeast Asia is grown on soils with potentially major fertility constraints.
5
The main soil problems in rainfed systems are salinity, alkalinity, Fe toxicity, P and Zn deficiencies, and organic and acid sulfate conditions (Sehgal and Abrol, 1994; Sekhbon and Bilbao, 1994). Soils with low cation exchange capacity are common, necessitating exact N management (Garrity et al., 1986). With increasing population pressures and land-use intensity, less farmyard manure is being applied as organic fertilizer (Fujisaka, 1990), and even though it has been shown that green manure legumes are very good sources of biologically fixed N, adoption of the technology has been virtually nil (Garrity and Flinn, 1988). Low use of mineral fertilizer in rainfed lowlands is attributable to poverty, lack of credit, unavailability of fertilizers, inadequate infrastructure, and low use efficiency. Improving fertilizer use requires alleviating these constraints. Agricultural research can develop responsive and adapted cultivars that could help justify policy changes affecting rainfed lowland areas, in much the same way that government policies were modified in response to modern wheat and rice technologies that became available in the late 1960s. The variable moisture regime of rainfed lowlands contributes to their uncertain fertility (De Datta, 1986). They experience repeated wetting and drying, accompanied by radical changes in soil chemistry caused by the shift from an oxidative to a reducing environment. Fertilizer placement and timing may improve nutrient-use efficiency (De Datta, 1986), but unless the cultivar is responsive, there is little likelihood that farmers will adopt more labor-intensive or costly practices (Misra et al., 1986).
4. Research outputs for the next generation of technology
The research needed to address the complex set of issues related to rice supply was the subject of a workshop on nutrient-use efficiency held at IRRI in December, 1995. Conference organizers were K.G. Cassman and H.U. Neue. Papers by IRRI scientists presented the Institute's current research agenda, and short papers by external reviewers provided perspectives on nutrient-use efficiency in other cropping systems. The outcome of that review is also reported in order to guide the direction of future research
6
K.S. Fischer~Field Crops Research 56 (1998) 1-6
(Lafitte, 1998). Technologies to improve nutrient-use efficiency must provide better congruence between plant demand for nutrients and the pattern of supply from both soil and fertilizer sources. At the same time, cultivar improvement is required to develop plants that are more buffered in their response to the fluctuating supply of available nutrients and, in rainfed environments, water. Because the next generation of technologies will be more specific to particular environments than past management practices, more precise systems of environmental classification will be required to orient both research activities and the extension of technologies to farmers. And, the new technologies will be assessed not only for agricultural productivity, but also for their interaction with the environment. One of the members of the review team was Professor Horst Marschner. He has contributed much to the understanding of plant nutrition, and specifically, to the direction of rice research at IRRI. This review was the occasion of his last visit to IRRI. He died tragically from an illness he contracted while stimulating research in Africa. We dedicate this series of papers to his memory. May our capacity to improve the well-being of present and future generations of rice farmers and consumers, particularly those with low incomes, be guided by his legacy to science.
References Cassman, K.G., Pingali, P.L., 1995. Intensification of irrigated rice systems: learning from the past to meet future challenges. GeoJournal 35, 299-305. Cassman, K.G., Kropff, M.J., Zhende, Y., 1994. A conceptual framework for nitrogen management of irrigated rice in highyield environments. In: Virmani, S.S. (Ed.), Selected Papers from the International Rice Research Conference 1994, International Rice Research Institute, Los Bafios, Philippines, pp. 81-96. De Datta, S.K., 1986. Tolerance of rice varieties for stagnant
flooding. Progress in Rainfed Lowland Rice. International Rice Research Institute, Los Bafios, Philippines, pp. 201-206. Fujisaka, S., 1990. Rainfed lowland rice: building research on farmer practice and technical knowledge. Agric. Ecosyst. Environ. 33, 57-74. Garrity, D.P., Flinn, J.C., 1988. Farm-level management systems for green manure crops in Asian rice environments. Sustainable Agriculture: Green Manure in Rice Farming. International Rice Research Institute, Los Bafios, Philippines, pp. 111-130. Garrity, D.P., Oldeman, L.R., Morris, R.A., 1986. Rainfed lowland rice ecosystems: characterization and distribution. Progress in Rainfed Lowland Rice. International Rice Research Institute, Los Bafios, Philippines, pp. 3-23. Hossain, M., Fischer, K.S., 1995. Rice research for food security and sustainable agricultural development in Asia: achievements and future challenges. GeoJoumal 35, 286-298. IRRI, 1993. Rice research in a time of change. IRRI's MediumTerm Plan for 1994-1998. International Rice Research Institute, Los BalSos, Philippines. IRRI, 1994. Program Report for 1993. International Rice Research Institute, Los Bafios, Philippines. 1RRI, 1997. Sustaining food security beyond the year 2000: a global partnership for rice research. Medium-term plan 19982000. International Rice Research Institute, Los Bafios, Philippines. Khush, G.S., 1995. Modem varieties--their real contribution to food supplies and equity. GeoJoumal 35, 275-285. Ladha, J.K., Kundu, D.K., Angelo-van Coppenolle, M.G., Peoples, M.B., Carangal, V.R., Dart, P.J., 1996. Legume productivity and soil nitrogen dynamics in lowland rice-based cropping systems. Soil Sci. Soc. Am. J. 60, 183-192. Lafitte, H.R., 1998. Research opportunities to improve nutrient-use efficiency in rice cropping systems. Field Crop Res. 56, 223-238. Misra, B., Mukhopadhyay, S.K., Flinn, J.C., 1986. Production constraints of rainfed lowland rice in eastern India. Progress in Rainfed Lowland Rice. International Rice Research Institute, Los Bafios, Philippines, pp. 191-200. Neue, H.U., Ziska, L., Matthews, R.B., Dai, Q., 1995. Reducing global wanning--the role of rice. GeoJoumai 35, 351-362. Sehgal, J.L., Abrol, I.P., 1994. Soil degradation in India--Status and impact. In: Etchevers B., J.D. (Ed.), Transactions of the 15th World Congress of Soil Science, July 10-16, 1994, Acapulco, Mexico. Int. Soc. Soil Sci., Mexico, pp. 188-212. Sekhbon, G.S., Bilbao, G.A., 1994. Sustaining high levels of productivity in South Asia and Latin America. In: Etchevers B., J.D. (Ed.), Transactions of the 15th World Congress of Soil Science, July 10-16, 1994, Acapulco, Mexico, 1994. Int. Soc. Soil Sci., Mexico, pp. 213-226.