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Experiences and Research Perspectives on Sustainable Development of Rice-Wheat Cropping Systems in the Chengdu Plain, China
Agricultural Sciences in China
September 2010
2010, 9(9): 1317-1325
Experiences and Research Perspectives on Sustainable Development of Rice-Wheat Cropping Systems in the Chengdu Plain, China ZHENG Jia-guo, CHI Zhong-zhi, JIANG Xin-lu, TANG Yong-lu and ZHANG Hong Tillage and Cultivation Research Center, Sichuan Academy of Agricultural Sciences, Chengdu 610066, P.R.China
Abstract The rice and wheat cropping pattern is one of the main cropping systems in the world. A large number of research results showed that successive cropping of rice and wheat resulted in a series of problems such as hindering nutrition absorption, gradual degeneration of soil fertility, decline of soil organic matter, and increased incidence of diseases and pests. In China, especially in the Chengdu plain where rice-wheat cropping system is practiced, productivity and soil fertility was enhanced and sustained. This paper reviews the relevant data and experiences on rice-wheat cropping in the Chengdu Plain from 1977 to 2006. The principal sustainable strategies used for rice-wheat cropping systems in Chengdu Plain included: 1) creating a favorable environment and viable rotations; 2) balanced fertilization for maintenance of sustainable soil productivity; 3) improvement of crop management for higher efficiency; and 4) use the newest cultivars and cultivation techniques to upgrade the production level. Future research is also discussed in the paper as: 1) the constant topic: a highly productive and efficient rice-wheat cropping system for sustainable growth; 2) the future trend: simplified cultivation techniques for the rice-wheat cropping system; 3) the foundation: basic research for continuous innovation needed for intensive cropping. It is concluded that in the rice-wheat cropping system, a scientific and reasonable tillage/cultivation method can not only avoid the degradation of soil productivity, but also maintain sustainable growth in the long run. : rice-wheat cropping system, principle strategies, rotation, sustainable soil productivity, simplification, cultivation techniques, Chengdu Plain
INTRODUCTION The rice-wheat cropping system is the most dominant double-cropping system in Asian subtropical countries such as China, India, Nepal, Bangladesh, and Pakistan, where it is practiced on about 24 million ha (Mahajan and Gupta 2009). However, long-term multi-location experiments conducted in Asia by the International Rice Research Institute (IRRI) indicated a negative relationship between rice yield and the number of successive
crops, with annual yield productivity growth reduced by 0.19% per year (Achim et al. 2000; Dawe et al. 2000). Farmers had to increase fertilizer quantities just to maintain current yield levels, which reduced net incomes (Pagiola 1995). A large number of experiments showed that successive cropping of rice followed by wheat resulted in many problems such as hindering nutrient absorption, gradual soil fertility decline, decline of soil organic matter, and increased disease and pest incidence (Achim et al. 2000). There are concerns as to whether such cropping systems can sustain the de-
Received 24 November, 2009 Accepted 16 January, 2010 Correspondence ZHENG Jia-guo, Professor, Tel: +86-28-84504245, E-mail: [email protected]
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mand of an increasing population demand for food (Ladha et al. 2000). In China, especially in the Chengdu plain where the main cropping system is rice followed by wheat, productivity was maintained and soil fertility was sustained (Zheng 2000; Tang et al. 2000; Tang et al. 2007). This paper reviews relevant data and experiences in the Chengdu Plain from 1977 to 2006 in a systematic way, and discusses and proposes future research needs.
DEVELOPMENT OF CROPPING SYSTEMS AND THEIR PRODUCTION IN THE CHENGDU PLAIN The Chengdu plain is located in the western part of the Sichuan basin, and is an alluvial plain formed by the Mingjiang and Shitingjiang rivers. The plain covers an area of nearly 10 000 km2. The Dujiangyan irrigation system built over 2 000 yr ago enables irrigation for the majority of the area for cropping. It has a sub-tropical climate, annual average temperatures of 15-18°C, annual accumulated temperature (= 10°C) of 4 5005 700°C, annual precipitation of about 1 000 mm, and a non-frost period of 240-300 d. Annual sunshine is only 1 000-1 600 h, the lowest value in China. The thermal condition provides adequate heat for two crops but not enough for three crops a year. Before the 1950s, there were no systematic irrigation systems in most regions. The dominant cropping pattern was one crop a year with a winter fallow followed by a mid-season rice crop. Where irrigation was available, green manure was planted before rice to provide fertility for rice. Later, as demands for food increased, new and higher yielding varieties were developed, fertilizer use increased and the irrigation systems were improved. The proportion of rice land followed by wheat and oilseed also increased. The combination of rice followed by wheat, oilseed and green manure, rose to one third for each crop. In the mid-1970s, the three crop patterns were developed; “green manure-rice-rice” or “wheat/maize-rice” per year. However, temperatures drop in autumn 15 d earlier than other regions with the same latitude, therefore it is necessary to select the rice variety of short growth period, increase farm activities, and thus re-
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sulting in unstable and lower rice yield. Consequently, farmers had lower benefits compared to double cropping. By 1977, the cropping system had to return just rice and wheat (or rapeseed) in a year. At present, crops grown after rice are wheat, oilseed and green manure in the proportion of 63.0, 28.8 and 8.2%, respectively. The green manure field and biomass are later used in the rice seedling bed. The major technical policy from 1985-1995 was to increase the planting of vegetatively propagated crops such as sweet potato and vegetables (Tang et al. 2000). After 1995, a policy to develop a late autumn crop to increase farmers’ income was adopted to realize three crops a year. The new system advanced the harvest of rice, delayed the sowing of wheat to the time after the autumn crop harvest, intercropped potato with oilseed to extend the growing period of the autumn crop. The result was increased yield and profit (Fig.). Research and demonstrations showed that extending the growing period of the autumn crop beyond 85-100 d gave higher yield of the cash crops and ensured satisfactory production and farmer incomes. Guanghan City is located in the middle of Chengdu plain, in a typical rice-wheat cropping area. The Sichuan Academy of Agricultural Sciences, China (SAAS) set up a base there in 1978 to investigate and study the rice-wheat cropping system. Research and demonstrations after that time have shown a sustainable production. Between 1977 and 2006, arable land was reduced from 34 005 to 29 765 ha, whereas the population increased to 590 954 from 468 571, and per capita arable land was reduced from 726 to 504 m2. Rice yields increased by 3 487 kg (76.33%) to 8 054 kg ha -1 from 4 568 kg ha -1 in 1977; wheat yields by 2 906 kg (94.78%) to 5 972 kg ha -1 from 3 066 kg ha -1 ; and oilseeds by 1 093 kg (75.12%) up to 2 548 kg ha -1 from 1 455 kg ha -1 . The total food crop growing area in 2006 was reduced by 10 300 ha over that of 1977, while the total production increased by over 130 000 t (42.7%), equivalent to a unit yield increment of 68.3%. The cropping sector output in 1977 was 59.43 million RMB yuan (adjusted for inflation), while in 2006 it was 1 021.74 million RMB yuan, an increment of 16.2 times (Sichuan Agricultural Statistical Yearbook, Agricultural Department of Sichuan Province 1977-2006).
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Experiences and Research Perspectives on Sustainable Development of Rice-Wheat Cropping Systems in the Chengdu Plain
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Fig. The evolution of cropping systems and crop calendars in the Chengdu plain over time.
THE PRINCIPAL STRATEGIES TO SUSTAIN THE RICE WHEAT CROPPING SYSTEM Creating a favorable environment and viable rotations Improving the growing environment through integrated approaches In production development and reform, large-scale cropping systems and technologies need to be matched to the local physical and ecological conditions. The focus should be to make full use of the local physical conditions and overcome or reduce the influence of unfavorable factors, in order to create a favorable environment for crop growth. For example, wetness is a key factor that adversely restricts food production in the Chengdu plain. Regional construction of irrigation and drainage systems and leveling off land helped relieve this constraint. Farmers were also encouraged to plough and sun-dry the soil to ensure a better soil environment for quality planting and sowing of crops. Use of rotations to reduce unfavorable successive cropping constraints The system of rice and wheat resulted in paddy and dry land crop rotation, and obtained a high yield of wheat planted on paddy fields, but it had some unfavorable effects on subsequent crops. It was desirable to select a mid-maturing rice to allow enough time for growth of subsequent crops in
the rotation. There is now a large winter oilseed growing area in Sichuan, rotated with rice to avoid or reduce the occurrence of diseases, pests and weeds. According to a survey by the Sichuan Academy of Agricultural Sciences (SAAS), three years of rotation with different crops reduced weed numbers with green manure followed by rice (the best), and oilseed followed by rice (the second best); but severe weed numbers occurred if wheat was followed by rice. The three rotation using wheat, oilseed and green manure with rice reduced weeds the most, but also reduced Sclerotinia blight of oilseed (Table 1). It is recommended rotations with different crops be practiced by farmers to reduce biotic constraints. Long term experiments conducted (Zeng and Pang 1991) from 1986 to 1990 further indicated after the 5th year a statistically significant yield reduction, an increase in bulk density and capillary porosity, but a remarkable decline of coarse porosity in the continuous rice-wheat compared to the more diverse rotations. Table 1 Disease and weed incidence of different mid-maturing rice rotations Cropping pattern W-R RS-R GM-R GM-R-W-R-RS-R
Continuous cropping (yr)
Dry matter of weeds in rice season (kg ha-1)
3 3 3
945 ** 534 * 351 339
Infection ratio of Sclerotinia blight in rape-seed (%) 10.6-24.5 ** 2.1-3.5
, extra significant difference. The same as below. W, wheat; R, rice. **
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Besides the inclusion of rape-seed and green manure in the three crops rice wheat rotation, use of vegetables and potatoes after 1990 provided other alternative choices for this system.
Balanced fertilization for maintenance of sustainable soil productivity The Chengdu plain has fairly high soil fertility, with total nitrogen of 0.1527% and organic matter of 2.93%, thus there is less need to increase nitrogen fertilizer use. However, supplementary nitrogen is needed to maintain productivity. If N applications of 150 kg ha-1 are used with or without organic matter each year, the soil N and the organic matter is reduced (Table 2). This result was the same as that reported in Nepal and Bangladesh (Adhikari et al. 1999). Importance of organic matter application and additional application of potash fertilizers Organic fertilizers are traditional sources of nutrients in Sichuan province. Before the 1950s, only organic fertilizers were applied for agricultural production. Since the late 1970s, chemical fertilization increased, but organic application still accounted for 75% of total nutrient additions, with all the potassium coming from organic manures. Up to 1996, N and P chemical fertilizers accounted for 40.8-48.8% of the total nutrient, but 87.7% of potassium came from organic manure. The benefits of organic and potash fertilizers were determined by long-term location experiments (Table 3) from 19811992. The experiments were conducted on acidic purple soil (with available potassium of 21 mg kg-1 and slow
supply potassium of 280 mg kg-1) for a rice and wheat or oilseed rotation over 12 yr. Data indicated that both organic and chemical potash fertilizers affected wheat and oilseed yield more than rice; the effect of organic fertilizer was higher than that for potassium in oilseed, while chemical potassium was better than organic fertilizers in wheat and rice. Promotion of fertilizer efficiency A large number of experimental results on hybrid rice (Tan et al. 1987; Tan et al. 1998; Zheng 1998) indicated that early stage supply of N, basal and tillering fertilization, was advantageous for tillering and subsequent panicle initiation; mid-stage side-dressing of N at the first rachis-branch differentiation stage increased tiller survival, reinforced tillers and promoted panicle formation. The key to large panicles was not due to mid-term fertilization, but rather early N supply to accelerate earlier and strong tillering, and to supply the proper amount of N for meiosis which promoted spikelet differentiation and increased the number and weight of grains. A long-term location experiment conducted by the Institute of Soil and Fertilizer of SAAS ( Hu et al. 1998) indicated that for the rice-wheat cropping system, potash fertilizer was important for increased yield in any crop. However, from a yield increment and economic benefit point of view, potash fertilizers were more beneficial if completely applied to wheat than if half a dose was applied to wheat and rice respectively, and even more so if applied completely to rice. So the application of potash fertilizers should be focused on wheat. Pot experiments (Lv et al. 1990) demonstrated that in manganese (Mn) deficient soils with a pH of 8.03,
Table 2 The N and C balance with two different fertilizer treatments after four years in a rice-wheat system Treatment
Nitrogen (kg ha-1 ) Uptake
Output
172.5 249.0
252.0 285.0
MF HF
Biomass (kg ha-1) Soil
Produce
Remove
-79.5 ** -36.0
16 714.5 17 553.0
14 889.0 14 823.0
Return to soil
Organic matter (%)
1 825.5 * 2 730.0 **
-0.408 * -0.314
MF, medium fertilizer, application N 150 kg, P2O5 90 kg, K2O 180 kg ha-1 each year; HF, high fertilizer, the MF plus application organic N 75 kg ha-1. * , significant difference. The same as below.
Table 3 The effect of potash and organic manure on yield increases in a long-term rice-wheat systems experiment in acidic purple soil Treatments comparison (Manure+NP)-NP NPK-NP Difference
Rice (12 harvests)
Wheat (6 harvests)
Rapeseed (5 harvests)
kg ha -1
%
kg ha -1
%
1 603.5 * 1 894.5 * -291.0
31.6 37.4
1 540.0 ** 1 696.5 ** -156.5
79.1 138.7
kg ha -1 780.0 ** 733.5 ** 46.5
% 96.1 89.7
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Experiences and Research Perspectives on Sustainable Development of Rice-Wheat Cropping Systems in the Chengdu Plain
and 0.6 mg kg-1 of DTPA extractable Mn, application of potash fertilizer further increased Mn deficiency. Hence, a supplemental dose of Mn fertilizer should be applied on such soil.
Improvement of crop management for higher efficiency Introduction of reduced or zero-tillage techniques As life standards improve and rural development progresses, farmers want to improve their working environments and reduce labor intensity. The introduction of reduced and zero-tillage cultivation techniques simplify the production process and reduce production costs. Since the late 1980s, reduced or zerotilled techniques have emerged as popular techniques. At the early stages of introduction, arguments and doubts occurred. After several years of research and farmer practice, advantages in terms of labor saving, cost effectiveness, high yield and efficiency were proven. Since then relevant yield increment mechanisms were unraveled and understood. Wheat yield increased by 10-20% in low-yielding areas, by 5-8% in high-yielding areas, and saved 300-450 RMB yuan in costs (Yu et al. 1998); rice yield generally increased by 5%, saved 2.7-4.7 man d, and saved 405-675 RMB yuan in costs (Du et al. 1992); rapeseed yield increased by 5%, saved 300 RMB yuan in costs, and labor saving of 6-12 man d (Tang et al. 2008). Today, reduced or zero-tillage techniques using on rice, wheat and rapeseed is widely adopted in the Chengdu plain, and replaced most of the farm animals because of the extension of appropriate agro-machinery. Changes of yield and soil condition by long-term reduced or zero-tillage techniques A long-term experiment conducted (Zhuang et al. 1999) on the effects of tillage on rice and wheat yield, soil organic matter and bulk density on a sandy soil showed that reduced tillage had the same yield of rice and wheat compared to traditional tillage, whereas with zero-tillage average wheat yield increased by 5.3% and zerotillage rice yield was reduced by 2.2% from 1985 to 1996. Different tilling methods affected soil bulk density at depths between 7 and 14 cm. The increased soil bulk density was the most typical result for reduced or zero-tillage techniques. The phenomenon of early crop
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senescence in the reduced and zero-tillage soils was a result of the excessive bulk density in the 7-14 cm soil layer. Reduced tillage in the rice season and zero-tillage in the wheat/rapeseed season is generally practiced in the Chengdu plains. If zero-tillage is used in the rice season, it is recommended that rice seedlings are scattered instead of hand-transplanting; or better to still use a zerotillage drill (Zheng et al. 1999).
Use the newest cultivars and cultivation techniques to upgrade the production level Timely replacement of old varieties to increase or maintain productivity Suitable variety selection is of vital importance for yield increases and for quality promotion. Over the last 30 years, the historical move from a conventional rice variety to hybrid rice was successful; hybrids combined increased panicle and spikelet size for higher yield potential. Other advances were made with wild sterile germplasm incorporated into indica-type germplasm; indica-type materials crossed with japonica-type germplasm; winter wheat introgressed to form winter-spring wheat; high yielding traits combined with quality; and conventional rapeseed converted to hybrid rapeseed and normal rapeseed towards quality rapeseed, etc. The major hybrid combinations changed at least five times and the yield increased by 30%. In recent years there were fewer breakthroughs in yield of varieties, but substantial progress in quality, biotic resistance, maturity and growing period, which contributed markedly to the maintenance of crop yield (Cheng and Hu 2008). In the process of extending new varieties, it was important to combine variety with improved management and technical advancement. Based on the cropping system, productivity and market demand, careful selection of suitable varieties combined with improved cultivation techniques adapted to local conditions, utilized the genetic potential of the new varieties. Hybrid rice cultivation techniques represented by the dry rice nursery At the beginning of hybrid rice cultivation, there existed problems of extra tillers, lack of panicles, large panicles and lack of seed setting. The technical strategy of getting 7 500 kg ha-1 in large-scale production in the early 1980s, relied on converting tillers
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to panicles to provide a basic number of plants per unit area, and relied on large panicles at these populations. Since the 1990s, the cultural techniques aimed at a rice yield of 9 000 kg ha-1 and above have been studied. The key issues were selection of varieties bearing heavy panicles, improving population number, increasing the panicle-forming rate and thereby to ensure proper panicle numbers and to increase the filled grains per unit area and hence to increase yield. In 7 ha of paddy rice fields in Liangshan base, Guanghan City, four years of high yields over 9 000 kg ha-1 were obtained (Zheng and Tan 1998). Among the techniques tested, the technique of dry rice seedling nursery method was important, which is a technical improvement on the wet rice nursery and greenhouse nursery techniques. In the dry rice seedling nursery method, the seedbed is built on upland and kept dry during the nursery period. It has the following advantages: earliness (early sowing, emergence and maturity), savings (labor, and nursery bed cost), high yielding, higher efficiency and tolerance to coldness and drought. It increased rice yield by 5% in the highyielding region, and by over 10% in the low-yielding area. It was first demonstrated in Guanghan city in 1992, and became the most important agro-technique followed by farmers after 1995 in the province (Zheng and Tan 1994). Wheat high-yield cultivation using the close small hill planting technique Aiming for the favorable ecological character of large ears and unfavorable multiears, SAAS (Yu et al. 1998; Tang et al. 2000) proposed a systematic research method to attain high-yield. It was important to select suitable varieties with large ears; plant timely; properly reduce sowing seed quantity; adopt close hill spacing by drill; and apply sufficient basal and jointing-stage fertilizers. Within these techniques, the key point is the close hill plant system. It increases the number of hills per unit area and reduces the row- and hill-space, and seeds per hill. Comparing to the traditional method (30 cm × 20 cm, > 30 seeds/hill) by hoe in the plowed field, the density can be controlled in 25 cm × 13 cm, 6-8 seeds/hill by dill in the zero tillage field. These techniques showed high yields of over 7 500 kg ha-1 and at least 6 000 kg ha-1 in demonstration plots. Later on, the planting machines were improved to allow mulching with rice straw dur-
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ing wheat establishment after rice. This high yielding and efficient practice was widely extended to the western plain of Sichuan and other similar regions.
FUTURE DEVELOPMENT PERSPECTIVE The constant topic: a highly productive and efficient rice-wheat cropping system for sustainable growth The average amount of arable land on the Chengdu plain is less than 0.05 ha per capita, and as population continues to grow, this figure will get smaller. This leads to an ever-increasing conflicting issue between the population and land. The multiple-cropping index has increased over time as the cropping pattern on the Chengdu plain evolved from one crop a year to two, then to five crops per two years and finally to three crops a year. It has been shown that as long as the latest developments in crop breeding, tillage and cultivation are utilized, a reasonable and scientific selection and combination of crop varieties are maintained, and crop rotation and intercropping systems are adopted, we can sustain grain and oil production in the area, as well as allocate enough time for growing other cash crops. However, yield levels are already high and it will be difficult to further increase productivity. Other issues worth noting and studying further are the screening of appropriate crop varieties for optimal rotations, and quality. Wheat suffers from heavy rains in spring with serious head blight and unstable and unsatisfactory yields. As long as the super-high yielding rice rely entirely on vegetables for the rotations, it is difficult to popularize. The over-cultivation of “rice-vegetablevegetable” will certainly result in an over-supply of vegetables coupled with increased pollution from excessive use of fertilizers and pesticides. At present, development of super hybrid rice has progressed substantially in China (Cheng et al. 2007); the theory of an ideal rice plant type was described at IRRI and adopted by Chinese scientists (Peng et al. 2005; Chen et al. 2005). Super hybrid rice grain yield cultivated by the System of Rice Intensification (Zheng et al. 2004) reached 12.8 t ha-1 in the Chengdu plain in
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Experiences and Research Perspectives on Sustainable Development of Rice-Wheat Cropping Systems in the Chengdu Plain
2008. The idea of ridge cultivation techniques were also proposed at CIMMYT (Tang et al. 2006). These achievements show promise for increased productivity in the future.
The future trend: simplified cultivation techniques for the rice-wheat cropping system As society progressed, the structure of the rural labor force went through changes and reduced the labor available for agriculture. To promote the sustainable development of the rice-wheat cropping system, China is left with no other choice but to simplify the cultivation process and lighten the workload. The techniques include mechanical direct seeding for rice, no-tillage cultivation for crops, mechanical seedling transplanting, precise surface seeding and mulching straw for wheat, and rice straw mulching with zerotillage for potato. These are being extended in the Chengdu plain in recent years. Direct sowing of rice and mid-season rice + ratooning rice will be popular in the future ( Xiong et al. 2000; Jiang et al. 2007), but some agronomy questions still need to be answered. At the same time, improved agricultural machinery must be introduced for the sowing, transplanting and harvesting stages of crops for better management.
The foundation: basic research for continuous innovation needed for intensive cropping
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and technology advances and society develops, it is necessary to reduce environmental pollution resulting from application of chemical fertilizers and pesticides and to reduce the burning of crop residues in the fields that can be better used to feed the soil biology. There is a need to explore new techniques of improving the productivity of rice-wheat cropping systems but minimize environmental impacts by the conduct of research on new production techniques and extending the results to farmers To realize these objectives, China needs to cooperate with national and international research teams working on similar issues. In south Asia, rice and wheat is the dominant cropping system. IRRI and CIMMYT, two international research centers, support the ricewheat consortium, an eco-regional program across several partner countries and have initiated various joint studies with national program scientists. China would benefit from sharing these experiences.
Acknowledgements This study was funded by the National Key Technologies Research and Development Program of China (2006BAD02A05), and the Science and Technology Project for Public (Agriculture), the Ministry of Agriculture, China (200903050-4).
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wheat yields, soil organic matter and bulk density. Scientia Agricultura Sinica, 32, 39-44. (in Chinese) (Managing editor ZHANG Yimin)
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2010, 9(9): 1317-1325
Experiences and Research Perspectives on Sustainable Development of Rice-Wheat Cropping Systems in the Chengdu Plain, China ZHENG Jia-guo, CHI Zhong-zhi, JIANG Xin-lu, TANG Yong-lu and ZHANG Hong Tillage and Cultivation Research Center, Sichuan Academy of Agricultural Sciences, Chengdu 610066, P.R.China
Abstract The rice and wheat cropping pattern is one of the main cropping systems in the world. A large number of research results showed that successive cropping of rice and wheat resulted in a series of problems such as hindering nutrition absorption, gradual degeneration of soil fertility, decline of soil organic matter, and increased incidence of diseases and pests. In China, especially in the Chengdu plain where rice-wheat cropping system is practiced, productivity and soil fertility was enhanced and sustained. This paper reviews the relevant data and experiences on rice-wheat cropping in the Chengdu Plain from 1977 to 2006. The principal sustainable strategies used for rice-wheat cropping systems in Chengdu Plain included: 1) creating a favorable environment and viable rotations; 2) balanced fertilization for maintenance of sustainable soil productivity; 3) improvement of crop management for higher efficiency; and 4) use the newest cultivars and cultivation techniques to upgrade the production level. Future research is also discussed in the paper as: 1) the constant topic: a highly productive and efficient rice-wheat cropping system for sustainable growth; 2) the future trend: simplified cultivation techniques for the rice-wheat cropping system; 3) the foundation: basic research for continuous innovation needed for intensive cropping. It is concluded that in the rice-wheat cropping system, a scientific and reasonable tillage/cultivation method can not only avoid the degradation of soil productivity, but also maintain sustainable growth in the long run. : rice-wheat cropping system, principle strategies, rotation, sustainable soil productivity, simplification, cultivation techniques, Chengdu Plain
INTRODUCTION The rice-wheat cropping system is the most dominant double-cropping system in Asian subtropical countries such as China, India, Nepal, Bangladesh, and Pakistan, where it is practiced on about 24 million ha (Mahajan and Gupta 2009). However, long-term multi-location experiments conducted in Asia by the International Rice Research Institute (IRRI) indicated a negative relationship between rice yield and the number of successive
crops, with annual yield productivity growth reduced by 0.19% per year (Achim et al. 2000; Dawe et al. 2000). Farmers had to increase fertilizer quantities just to maintain current yield levels, which reduced net incomes (Pagiola 1995). A large number of experiments showed that successive cropping of rice followed by wheat resulted in many problems such as hindering nutrient absorption, gradual soil fertility decline, decline of soil organic matter, and increased disease and pest incidence (Achim et al. 2000). There are concerns as to whether such cropping systems can sustain the de-
Received 24 November, 2009 Accepted 16 January, 2010 Correspondence ZHENG Jia-guo, Professor, Tel: +86-28-84504245, E-mail: [email protected]
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mand of an increasing population demand for food (Ladha et al. 2000). In China, especially in the Chengdu plain where the main cropping system is rice followed by wheat, productivity was maintained and soil fertility was sustained (Zheng 2000; Tang et al. 2000; Tang et al. 2007). This paper reviews relevant data and experiences in the Chengdu Plain from 1977 to 2006 in a systematic way, and discusses and proposes future research needs.
DEVELOPMENT OF CROPPING SYSTEMS AND THEIR PRODUCTION IN THE CHENGDU PLAIN The Chengdu plain is located in the western part of the Sichuan basin, and is an alluvial plain formed by the Mingjiang and Shitingjiang rivers. The plain covers an area of nearly 10 000 km2. The Dujiangyan irrigation system built over 2 000 yr ago enables irrigation for the majority of the area for cropping. It has a sub-tropical climate, annual average temperatures of 15-18°C, annual accumulated temperature (= 10°C) of 4 5005 700°C, annual precipitation of about 1 000 mm, and a non-frost period of 240-300 d. Annual sunshine is only 1 000-1 600 h, the lowest value in China. The thermal condition provides adequate heat for two crops but not enough for three crops a year. Before the 1950s, there were no systematic irrigation systems in most regions. The dominant cropping pattern was one crop a year with a winter fallow followed by a mid-season rice crop. Where irrigation was available, green manure was planted before rice to provide fertility for rice. Later, as demands for food increased, new and higher yielding varieties were developed, fertilizer use increased and the irrigation systems were improved. The proportion of rice land followed by wheat and oilseed also increased. The combination of rice followed by wheat, oilseed and green manure, rose to one third for each crop. In the mid-1970s, the three crop patterns were developed; “green manure-rice-rice” or “wheat/maize-rice” per year. However, temperatures drop in autumn 15 d earlier than other regions with the same latitude, therefore it is necessary to select the rice variety of short growth period, increase farm activities, and thus re-
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sulting in unstable and lower rice yield. Consequently, farmers had lower benefits compared to double cropping. By 1977, the cropping system had to return just rice and wheat (or rapeseed) in a year. At present, crops grown after rice are wheat, oilseed and green manure in the proportion of 63.0, 28.8 and 8.2%, respectively. The green manure field and biomass are later used in the rice seedling bed. The major technical policy from 1985-1995 was to increase the planting of vegetatively propagated crops such as sweet potato and vegetables (Tang et al. 2000). After 1995, a policy to develop a late autumn crop to increase farmers’ income was adopted to realize three crops a year. The new system advanced the harvest of rice, delayed the sowing of wheat to the time after the autumn crop harvest, intercropped potato with oilseed to extend the growing period of the autumn crop. The result was increased yield and profit (Fig.). Research and demonstrations showed that extending the growing period of the autumn crop beyond 85-100 d gave higher yield of the cash crops and ensured satisfactory production and farmer incomes. Guanghan City is located in the middle of Chengdu plain, in a typical rice-wheat cropping area. The Sichuan Academy of Agricultural Sciences, China (SAAS) set up a base there in 1978 to investigate and study the rice-wheat cropping system. Research and demonstrations after that time have shown a sustainable production. Between 1977 and 2006, arable land was reduced from 34 005 to 29 765 ha, whereas the population increased to 590 954 from 468 571, and per capita arable land was reduced from 726 to 504 m2. Rice yields increased by 3 487 kg (76.33%) to 8 054 kg ha -1 from 4 568 kg ha -1 in 1977; wheat yields by 2 906 kg (94.78%) to 5 972 kg ha -1 from 3 066 kg ha -1 ; and oilseeds by 1 093 kg (75.12%) up to 2 548 kg ha -1 from 1 455 kg ha -1 . The total food crop growing area in 2006 was reduced by 10 300 ha over that of 1977, while the total production increased by over 130 000 t (42.7%), equivalent to a unit yield increment of 68.3%. The cropping sector output in 1977 was 59.43 million RMB yuan (adjusted for inflation), while in 2006 it was 1 021.74 million RMB yuan, an increment of 16.2 times (Sichuan Agricultural Statistical Yearbook, Agricultural Department of Sichuan Province 1977-2006).
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Experiences and Research Perspectives on Sustainable Development of Rice-Wheat Cropping Systems in the Chengdu Plain
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Fig. The evolution of cropping systems and crop calendars in the Chengdu plain over time.
THE PRINCIPAL STRATEGIES TO SUSTAIN THE RICE WHEAT CROPPING SYSTEM Creating a favorable environment and viable rotations Improving the growing environment through integrated approaches In production development and reform, large-scale cropping systems and technologies need to be matched to the local physical and ecological conditions. The focus should be to make full use of the local physical conditions and overcome or reduce the influence of unfavorable factors, in order to create a favorable environment for crop growth. For example, wetness is a key factor that adversely restricts food production in the Chengdu plain. Regional construction of irrigation and drainage systems and leveling off land helped relieve this constraint. Farmers were also encouraged to plough and sun-dry the soil to ensure a better soil environment for quality planting and sowing of crops. Use of rotations to reduce unfavorable successive cropping constraints The system of rice and wheat resulted in paddy and dry land crop rotation, and obtained a high yield of wheat planted on paddy fields, but it had some unfavorable effects on subsequent crops. It was desirable to select a mid-maturing rice to allow enough time for growth of subsequent crops in
the rotation. There is now a large winter oilseed growing area in Sichuan, rotated with rice to avoid or reduce the occurrence of diseases, pests and weeds. According to a survey by the Sichuan Academy of Agricultural Sciences (SAAS), three years of rotation with different crops reduced weed numbers with green manure followed by rice (the best), and oilseed followed by rice (the second best); but severe weed numbers occurred if wheat was followed by rice. The three rotation using wheat, oilseed and green manure with rice reduced weeds the most, but also reduced Sclerotinia blight of oilseed (Table 1). It is recommended rotations with different crops be practiced by farmers to reduce biotic constraints. Long term experiments conducted (Zeng and Pang 1991) from 1986 to 1990 further indicated after the 5th year a statistically significant yield reduction, an increase in bulk density and capillary porosity, but a remarkable decline of coarse porosity in the continuous rice-wheat compared to the more diverse rotations. Table 1 Disease and weed incidence of different mid-maturing rice rotations Cropping pattern W-R RS-R GM-R GM-R-W-R-RS-R
Continuous cropping (yr)
Dry matter of weeds in rice season (kg ha-1)
3 3 3
945 ** 534 * 351 339
Infection ratio of Sclerotinia blight in rape-seed (%) 10.6-24.5 ** 2.1-3.5
, extra significant difference. The same as below. W, wheat; R, rice. **
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Besides the inclusion of rape-seed and green manure in the three crops rice wheat rotation, use of vegetables and potatoes after 1990 provided other alternative choices for this system.
Balanced fertilization for maintenance of sustainable soil productivity The Chengdu plain has fairly high soil fertility, with total nitrogen of 0.1527% and organic matter of 2.93%, thus there is less need to increase nitrogen fertilizer use. However, supplementary nitrogen is needed to maintain productivity. If N applications of 150 kg ha-1 are used with or without organic matter each year, the soil N and the organic matter is reduced (Table 2). This result was the same as that reported in Nepal and Bangladesh (Adhikari et al. 1999). Importance of organic matter application and additional application of potash fertilizers Organic fertilizers are traditional sources of nutrients in Sichuan province. Before the 1950s, only organic fertilizers were applied for agricultural production. Since the late 1970s, chemical fertilization increased, but organic application still accounted for 75% of total nutrient additions, with all the potassium coming from organic manures. Up to 1996, N and P chemical fertilizers accounted for 40.8-48.8% of the total nutrient, but 87.7% of potassium came from organic manure. The benefits of organic and potash fertilizers were determined by long-term location experiments (Table 3) from 19811992. The experiments were conducted on acidic purple soil (with available potassium of 21 mg kg-1 and slow
supply potassium of 280 mg kg-1) for a rice and wheat or oilseed rotation over 12 yr. Data indicated that both organic and chemical potash fertilizers affected wheat and oilseed yield more than rice; the effect of organic fertilizer was higher than that for potassium in oilseed, while chemical potassium was better than organic fertilizers in wheat and rice. Promotion of fertilizer efficiency A large number of experimental results on hybrid rice (Tan et al. 1987; Tan et al. 1998; Zheng 1998) indicated that early stage supply of N, basal and tillering fertilization, was advantageous for tillering and subsequent panicle initiation; mid-stage side-dressing of N at the first rachis-branch differentiation stage increased tiller survival, reinforced tillers and promoted panicle formation. The key to large panicles was not due to mid-term fertilization, but rather early N supply to accelerate earlier and strong tillering, and to supply the proper amount of N for meiosis which promoted spikelet differentiation and increased the number and weight of grains. A long-term location experiment conducted by the Institute of Soil and Fertilizer of SAAS ( Hu et al. 1998) indicated that for the rice-wheat cropping system, potash fertilizer was important for increased yield in any crop. However, from a yield increment and economic benefit point of view, potash fertilizers were more beneficial if completely applied to wheat than if half a dose was applied to wheat and rice respectively, and even more so if applied completely to rice. So the application of potash fertilizers should be focused on wheat. Pot experiments (Lv et al. 1990) demonstrated that in manganese (Mn) deficient soils with a pH of 8.03,
Table 2 The N and C balance with two different fertilizer treatments after four years in a rice-wheat system Treatment
Nitrogen (kg ha-1 ) Uptake
Output
172.5 249.0
252.0 285.0
MF HF
Biomass (kg ha-1) Soil
Produce
Remove
-79.5 ** -36.0
16 714.5 17 553.0
14 889.0 14 823.0
Return to soil
Organic matter (%)
1 825.5 * 2 730.0 **
-0.408 * -0.314
MF, medium fertilizer, application N 150 kg, P2O5 90 kg, K2O 180 kg ha-1 each year; HF, high fertilizer, the MF plus application organic N 75 kg ha-1. * , significant difference. The same as below.
Table 3 The effect of potash and organic manure on yield increases in a long-term rice-wheat systems experiment in acidic purple soil Treatments comparison (Manure+NP)-NP NPK-NP Difference
Rice (12 harvests)
Wheat (6 harvests)
Rapeseed (5 harvests)
kg ha -1
%
kg ha -1
%
1 603.5 * 1 894.5 * -291.0
31.6 37.4
1 540.0 ** 1 696.5 ** -156.5
79.1 138.7
kg ha -1 780.0 ** 733.5 ** 46.5
% 96.1 89.7
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Experiences and Research Perspectives on Sustainable Development of Rice-Wheat Cropping Systems in the Chengdu Plain
and 0.6 mg kg-1 of DTPA extractable Mn, application of potash fertilizer further increased Mn deficiency. Hence, a supplemental dose of Mn fertilizer should be applied on such soil.
Improvement of crop management for higher efficiency Introduction of reduced or zero-tillage techniques As life standards improve and rural development progresses, farmers want to improve their working environments and reduce labor intensity. The introduction of reduced and zero-tillage cultivation techniques simplify the production process and reduce production costs. Since the late 1980s, reduced or zerotilled techniques have emerged as popular techniques. At the early stages of introduction, arguments and doubts occurred. After several years of research and farmer practice, advantages in terms of labor saving, cost effectiveness, high yield and efficiency were proven. Since then relevant yield increment mechanisms were unraveled and understood. Wheat yield increased by 10-20% in low-yielding areas, by 5-8% in high-yielding areas, and saved 300-450 RMB yuan in costs (Yu et al. 1998); rice yield generally increased by 5%, saved 2.7-4.7 man d, and saved 405-675 RMB yuan in costs (Du et al. 1992); rapeseed yield increased by 5%, saved 300 RMB yuan in costs, and labor saving of 6-12 man d (Tang et al. 2008). Today, reduced or zero-tillage techniques using on rice, wheat and rapeseed is widely adopted in the Chengdu plain, and replaced most of the farm animals because of the extension of appropriate agro-machinery. Changes of yield and soil condition by long-term reduced or zero-tillage techniques A long-term experiment conducted (Zhuang et al. 1999) on the effects of tillage on rice and wheat yield, soil organic matter and bulk density on a sandy soil showed that reduced tillage had the same yield of rice and wheat compared to traditional tillage, whereas with zero-tillage average wheat yield increased by 5.3% and zerotillage rice yield was reduced by 2.2% from 1985 to 1996. Different tilling methods affected soil bulk density at depths between 7 and 14 cm. The increased soil bulk density was the most typical result for reduced or zero-tillage techniques. The phenomenon of early crop
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senescence in the reduced and zero-tillage soils was a result of the excessive bulk density in the 7-14 cm soil layer. Reduced tillage in the rice season and zero-tillage in the wheat/rapeseed season is generally practiced in the Chengdu plains. If zero-tillage is used in the rice season, it is recommended that rice seedlings are scattered instead of hand-transplanting; or better to still use a zerotillage drill (Zheng et al. 1999).
Use the newest cultivars and cultivation techniques to upgrade the production level Timely replacement of old varieties to increase or maintain productivity Suitable variety selection is of vital importance for yield increases and for quality promotion. Over the last 30 years, the historical move from a conventional rice variety to hybrid rice was successful; hybrids combined increased panicle and spikelet size for higher yield potential. Other advances were made with wild sterile germplasm incorporated into indica-type germplasm; indica-type materials crossed with japonica-type germplasm; winter wheat introgressed to form winter-spring wheat; high yielding traits combined with quality; and conventional rapeseed converted to hybrid rapeseed and normal rapeseed towards quality rapeseed, etc. The major hybrid combinations changed at least five times and the yield increased by 30%. In recent years there were fewer breakthroughs in yield of varieties, but substantial progress in quality, biotic resistance, maturity and growing period, which contributed markedly to the maintenance of crop yield (Cheng and Hu 2008). In the process of extending new varieties, it was important to combine variety with improved management and technical advancement. Based on the cropping system, productivity and market demand, careful selection of suitable varieties combined with improved cultivation techniques adapted to local conditions, utilized the genetic potential of the new varieties. Hybrid rice cultivation techniques represented by the dry rice nursery At the beginning of hybrid rice cultivation, there existed problems of extra tillers, lack of panicles, large panicles and lack of seed setting. The technical strategy of getting 7 500 kg ha-1 in large-scale production in the early 1980s, relied on converting tillers
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to panicles to provide a basic number of plants per unit area, and relied on large panicles at these populations. Since the 1990s, the cultural techniques aimed at a rice yield of 9 000 kg ha-1 and above have been studied. The key issues were selection of varieties bearing heavy panicles, improving population number, increasing the panicle-forming rate and thereby to ensure proper panicle numbers and to increase the filled grains per unit area and hence to increase yield. In 7 ha of paddy rice fields in Liangshan base, Guanghan City, four years of high yields over 9 000 kg ha-1 were obtained (Zheng and Tan 1998). Among the techniques tested, the technique of dry rice seedling nursery method was important, which is a technical improvement on the wet rice nursery and greenhouse nursery techniques. In the dry rice seedling nursery method, the seedbed is built on upland and kept dry during the nursery period. It has the following advantages: earliness (early sowing, emergence and maturity), savings (labor, and nursery bed cost), high yielding, higher efficiency and tolerance to coldness and drought. It increased rice yield by 5% in the highyielding region, and by over 10% in the low-yielding area. It was first demonstrated in Guanghan city in 1992, and became the most important agro-technique followed by farmers after 1995 in the province (Zheng and Tan 1994). Wheat high-yield cultivation using the close small hill planting technique Aiming for the favorable ecological character of large ears and unfavorable multiears, SAAS (Yu et al. 1998; Tang et al. 2000) proposed a systematic research method to attain high-yield. It was important to select suitable varieties with large ears; plant timely; properly reduce sowing seed quantity; adopt close hill spacing by drill; and apply sufficient basal and jointing-stage fertilizers. Within these techniques, the key point is the close hill plant system. It increases the number of hills per unit area and reduces the row- and hill-space, and seeds per hill. Comparing to the traditional method (30 cm × 20 cm, > 30 seeds/hill) by hoe in the plowed field, the density can be controlled in 25 cm × 13 cm, 6-8 seeds/hill by dill in the zero tillage field. These techniques showed high yields of over 7 500 kg ha-1 and at least 6 000 kg ha-1 in demonstration plots. Later on, the planting machines were improved to allow mulching with rice straw dur-
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ing wheat establishment after rice. This high yielding and efficient practice was widely extended to the western plain of Sichuan and other similar regions.
FUTURE DEVELOPMENT PERSPECTIVE The constant topic: a highly productive and efficient rice-wheat cropping system for sustainable growth The average amount of arable land on the Chengdu plain is less than 0.05 ha per capita, and as population continues to grow, this figure will get smaller. This leads to an ever-increasing conflicting issue between the population and land. The multiple-cropping index has increased over time as the cropping pattern on the Chengdu plain evolved from one crop a year to two, then to five crops per two years and finally to three crops a year. It has been shown that as long as the latest developments in crop breeding, tillage and cultivation are utilized, a reasonable and scientific selection and combination of crop varieties are maintained, and crop rotation and intercropping systems are adopted, we can sustain grain and oil production in the area, as well as allocate enough time for growing other cash crops. However, yield levels are already high and it will be difficult to further increase productivity. Other issues worth noting and studying further are the screening of appropriate crop varieties for optimal rotations, and quality. Wheat suffers from heavy rains in spring with serious head blight and unstable and unsatisfactory yields. As long as the super-high yielding rice rely entirely on vegetables for the rotations, it is difficult to popularize. The over-cultivation of “rice-vegetablevegetable” will certainly result in an over-supply of vegetables coupled with increased pollution from excessive use of fertilizers and pesticides. At present, development of super hybrid rice has progressed substantially in China (Cheng et al. 2007); the theory of an ideal rice plant type was described at IRRI and adopted by Chinese scientists (Peng et al. 2005; Chen et al. 2005). Super hybrid rice grain yield cultivated by the System of Rice Intensification (Zheng et al. 2004) reached 12.8 t ha-1 in the Chengdu plain in
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Experiences and Research Perspectives on Sustainable Development of Rice-Wheat Cropping Systems in the Chengdu Plain
2008. The idea of ridge cultivation techniques were also proposed at CIMMYT (Tang et al. 2006). These achievements show promise for increased productivity in the future.
The future trend: simplified cultivation techniques for the rice-wheat cropping system As society progressed, the structure of the rural labor force went through changes and reduced the labor available for agriculture. To promote the sustainable development of the rice-wheat cropping system, China is left with no other choice but to simplify the cultivation process and lighten the workload. The techniques include mechanical direct seeding for rice, no-tillage cultivation for crops, mechanical seedling transplanting, precise surface seeding and mulching straw for wheat, and rice straw mulching with zerotillage for potato. These are being extended in the Chengdu plain in recent years. Direct sowing of rice and mid-season rice + ratooning rice will be popular in the future ( Xiong et al. 2000; Jiang et al. 2007), but some agronomy questions still need to be answered. At the same time, improved agricultural machinery must be introduced for the sowing, transplanting and harvesting stages of crops for better management.
The foundation: basic research for continuous innovation needed for intensive cropping
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and technology advances and society develops, it is necessary to reduce environmental pollution resulting from application of chemical fertilizers and pesticides and to reduce the burning of crop residues in the fields that can be better used to feed the soil biology. There is a need to explore new techniques of improving the productivity of rice-wheat cropping systems but minimize environmental impacts by the conduct of research on new production techniques and extending the results to farmers To realize these objectives, China needs to cooperate with national and international research teams working on similar issues. In south Asia, rice and wheat is the dominant cropping system. IRRI and CIMMYT, two international research centers, support the ricewheat consortium, an eco-regional program across several partner countries and have initiated various joint studies with national program scientists. China would benefit from sharing these experiences.
Acknowledgements This study was funded by the National Key Technologies Research and Development Program of China (2006BAD02A05), and the Science and Technology Project for Public (Agriculture), the Ministry of Agriculture, China (200903050-4).
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