The rice–wheat cropping system, occupying 24 million hectares of the productive area in South Asia and China, is important for food security. Monitoring long-term changes in crop yields and identifying the factors associated with such changes are essential to maintain and/or improve crop productivity. Long-term experiments (LTE) provide these opportunities. We analyzed 33 rice–wheat LTE in the Indo-Gangetic Plains (IGP) of South Asia, non-IGP in India, and China to investigate the extent of yield stagnation or decline and identify possible causes of yield decline. In treatments where recommended rates of N, P and K were applied, yields of rice and wheat stagnated in 72 and 85% of the LTE, respectively, while 22 and 6% of the LTE showed a significant (P<0.05) declining trend for rice and wheat yields, respectively. In the rice–wheat system, particularly in the IGP, rice yields are declining more rapidly than wheat. The causes of yield decline are mostly location-specific but depletion of soil K seems to be a general cause. In over 90% of the LTE, the fertilizer K rates used were not sufficient to sustain a neutral K input–output balance. Depletion of soil C, N and Zn and reduced availability of P, delays in planting, decreases in solar radiation and increases in minimum temperatures are the other potential causes of yield decline. A more efficient, integrated strategy with detailed data collection is required to identify the specific causes of yield decline. Constant monitoring of LTEs and analysis of the data using improved statistical and simulation tools should be done to unravel the cause–effect relationships of productivity and sustainability of rice–wheat systems.

How extensive are yield declines in long-term rice-wheat experiments in Asia? Field Crops Res

Rice is one of the most important food crops in the world, and staple for more than half of the global population. Looming water crisis, water-intensive nature of rice cultivation and escalating labour costs drive the search for alternative management methods to increase water productivity in rice cultivation. Direct seeded rice (DSR) has received much attention because of its low-input demand. It involves sowing pre-germinated seed into a puddled soil surface (wet seeding), standing water (water seeding) or dry seeding into a prepared seedbed (dry seeding). In Europe, Australia and the United States, DSR is highly mechanised. The development of early-maturing varieties and improved nutrient management techniques along with increased availability of chemical weed control methods has encouraged many farmers in the Philippines, Malaysia, Thailand and India to switch from transplanted to DSR culture. This shift should substantially reduce crop water requirements, soil organic-matter turnover, nutrient relations, carbon sequestering, weed biota and greenhouse-gas emissions. Still, weed infestation can cause large yield losses in DSR. In addition, recent incidences of blast disease, crop lodging, impaired kernel quality and stagnant yields across the years are major challenges in this regard. In this review, we discuss the experiences, potential advantages and problems associated with DSR, and suggest likely future patterns of changes in rice cultivation.

Rice direct seeding: Experiences, challenges and opportunities

How extensive are yield declines in long-term rice-wheat experiments in Asia?

Agricultural crop residue burning contribute towards the emission of greenhouse gases (CO2, N2O, CH4), air pollutants (CO, NH3, NOx, SO2, NMHC, volatile organic compounds), particulates matter and smoke thereby posing threat to human health. In the present study a state-wise inventory of crop residue burnt in India and the air pollutants emitted was prepared using the Inter-Governmental Panel on Climate Change (IPCC) national inventory preparation guidelines for the year 2008–09. Total amount of residue generated in 2008–09 was 620 Mt out of which ~15.9% residue was burnt on farm. Rice straw contributed 40% of the total residue burnt followed by wheat straw (22%) and sugarcane trash (20%). Burning of crop residues emitted 8.57 Mt of CO, 141.15 Mt of CO2, 0.037 Mt of SOx, 0.23 Mt of NOx, 0.12 Mt of NH3 and 1.46 Mt NMVOC, 0.65 Mt of NMHC, 1.21 Mt of particulate matter for the year 2008–09. The variability of 21.46% in annual emission of air pollutants was observed from 1995 to 2009.

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Emission of Air Pollutants from Crop Residue Burning in India

Sub-Saharan Africa (SSA) faces multiple problems. The main one is improving the lives of the 30% of its population that suffers from extreme poverty and food insecurity. As more than 70% of the population lives off farming and related activities, agricultural development will have to play a major role in improving this situation. Fortunately, Africa has an abundant supply of natural resources that can support a huge expansion in food, specifically rice production. Because of strong demand, rice area expansion in SSA is larger than for any other crop. Total milled rice production increased from 2.2 million Mg in 1961 to 9.1 million Mg in 2004. Rice imports into SSA also increased from 0.5 million Mg of milled rice in 1961 to 6.0 million Mg in 2003 and SSA currently accounts for 25% of global rice imports, at a cost of more than US$1.5 billion per year. Therefore, many African governments accord high priority to developing their local rice sector as an important component of national food security, economic growth, and poverty alleviation. The abundant supply of agroclimatically suitable wetlands (~239 million ha) and water resources can support a large expansion in rice area and productivity. Currently, less than 5% of the potentially suitable wetlands are planted with rice because of various constraints. Expansion and intensification of rice cultivation in SSA will not compete with other crops in terms of land and water resources because, during the rainy season, only rice can be grown on low-lying wetlands, including inland valleys. In addition, the labor-intensive nature of rice cultivation will provide additional sources of work and income to the rural poor, especially women. Should labor shortages become acute, however, appropriate mechanization can be considered. Small farmers want to earn money from rice farming, but lack modern inputs and capital to fully exploit their rice lands as these items are limited or not available. This is where an innovative public–private partnership is desirable to support the intensification of rice farming. Rice is cultivated in four ecosystems of SSA: dryland (38% of the cultivated rice area), rainfed wetland (33%), deepwater and mangrove swamps (9%), and irrigated wetland (20%). Many abiotic stresses (drought, flood, and variable rainfall; extreme temperatures; salinity; acidity/alkalinity and poor soils, soil erosion, and high P fixation) and biotic constraints [weeds, blast, Rice yellow mottle virus (RYMV), and African rice gall midge (AfRGM)] limit rice production on the continent. The changing climate is expected to further aggravate the abiotic constraints and reduce rice yields in all ecosystems. Rice production is also restricted by many technical, management, socioeconomic, health, and policy constraints. The constraints to irrigated wetland rice in the Sahel of SSA are similar to those faced by Asian farmers in the 1960s; therefore, well-tested irrigated rice technologies from Asia and elsewhere are being introduced and adapted to local conditions to obtain fast returns on investment. For rice in irrigated wetlands in the humid and moist savanna zones, rainfed wetlands, and drylands, locally developed NERICA (new rice for Africa) varieties and production technologies are being tested in target environments. The progenies of Oryza glaberrima and O. sativa subspecies indica are better adapted to rainfed and irrigated wetlands, while those of O. glaberrima and O. sativa subspecies japonica are more suited to rainfed drylands. In addition, research is ongoing to tackle SSA-specific problems such as RYMV and AfRGM and to develop efficient crop management technologies. Currently available best management practices (integrated crop management options) for different rice ecosystems are shown in Table XV. Additional support through the provision of technical advice through revamped national RD a supply of good-quality seed and other inputs, including farm credit; and enabling policy are needed for profitable and sustainable intensification of rice cultivation in SSA. It is also critical to organize preventive health measures for farmers against wetland-related diseases (malaria, bilharzia, and so on), protect certain natural wetlands (e.g., with bird sanctuaries), preserve mangrove forests in strategic coastal belts and rich peats in inlands, and use chemical inputs efficiently to minimize pollution and maintain environmental quality while intensifying rice production. Anticipatory research is needed to tackle the impacts of changing climate on rice farming and the environment. Modern information and communication technologies (ICTs) can be exploited to reach out to farmers in remote areas and to deploy technologies effectively. In addition, the development of innovative private–public partnerships and the organization of farmers into user-groups will enhance the training, farmer education, and technology adoption required for intensive commercial rice farming.

Ram K. Naresh

Papers

Sustainability of post-green revolution agriculture: the rice-wheat cropping systems of the Indo-Gangetic Plains and China.

Technology Options for Rice in the Rice–Wheat System in South Asia