The Rice-Wheat Consortium’s Example

The Indo-Gangetic Plains of Pakistan, India, Nepal and Bangladesh are endowed with plentiful natural resources, deep productive soils, sufficient good quality water, climatic conditions that permit multiple cropping, high population densities and relatively good infrastructure. The Green Revolution (GR) of the 1970s and 1980s radically changed the traditional agricultural system of this region. Now, about 13.5 million hectares of land are in continuous rotation of irrigated rice and wheat, providing food and livelihoods for many millions. Between 1960 and 1995 rice yields increased from 1.55 to 2.66 tonnes/ha and wheat yields from 0.84 to 2.34 tonnes/ha.

The majority of the farm households have less than 5 ha of land, whilst a minority have more than 20 ha. All farmers use improved varieties of wheat with fertiliser. In rice, some farmers still grow traditional, fine quality varieties like Basmati as they fetch higher market prices. Mechanisation levels are high, especially in the western regions, with resource-poor farmers renting tractors and threshers for tilling and harvesting. Animal power is still common in the eastern regions, but farmers complain of the increasing costs of maintaining draught bullocks. Many farmers are moving to contract ploughing with tractors; dairy cows are acquired in place of draught bullocks.

The main factors for the initial success of the GR and the emergence of the ricewheat system were the introduction of high-yielding, semi-dwarf varieties and chemical fertilisers. Pesticides, investments in irrigation infrastructure, political commitment and policy support played a lesser role. Free irrigation water, cheap agrochemicals, subsidised power supply and low-interest farm credit were some of the crucial supports provided by South Asian governments that made intensive ricewheat production profitable and a safe system for farmers.

However, in the past several years the productivity growth of wheat and rice has declined and the expansion of rice and wheat area has halted due to many reasons (Hobbs and Morris, 1996). Ecological degradation of the natural resource base has occurred as farmers using conventional technologies harvest up to 10 tonnes of cereal per year. Long-term rice-wheat experiments have shown that yield growth declines at constant input levels. Unbalanced use of fertiliser and delayed planting of crops are cited as major factors. Profitability has dropped as more inputs are needed to get the same yield. Input subsidies that favoured the GR have lacked farmlevel incentives for efficient input use. The price of rice and wheat has declined steadily over the last 30 years. Partial removal of subsidies and ecological problems have put stress on the economy of farmers.

Resource degradation in the rice-wheat system can take many forms: loss of organic matter; mining of soil nutrients; build-up of weeds, diseases and pests; waterlogging, salinity and sodicity. Additional problems that reduce system productivity are: low nutrient and water use efficiency associated with delayed crop establishment, driven in turn by inappropriate tillage practices  (delays in sowing wheat after rice can reduce yields as much as 1.5% per day); flat sowing and flood irrigation causing nutrient leaching; puddling leading to formation of a ploughpan, reduced soil permeability and enhanced soil cracking; and restriction of plant root and shoot growth and chlorosis due to temporary water stagnation. To compound these problems, Phalaris minor, the major weed in wheat has developed strong resistance to the commonly used herbicides and farmers have had to shift to new, more expensive herbicides. Excessive pumping from wells is leading to declining water tables in fresh water aquifer zones, while inadequate drainage is causing waterlogging and salinity in others… <more>


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Nitrogen Use Efficiency and Recovery from N Fertilizer under Rice-Based Cropping Systems

Nitrogen fertilization is widely adopted to enhance grain production and improve nitrogen utilization in rice all over the world. Rice is produced under both upland and lowland ecosystems with about 76% of the global rice produced from irrigated-lowland rice systems. Improved nitrogen use efficiency, particularly for N, is an important goal in cropping system development. Determination of N use efficiency in cereal based agro-ecosystems enabled broad assessment of agronomic management and environmental factors related to N use, Grain yield and N accumulation, N in aboveground, N harvest index, and grain N accumulation are the key indicators of N use efficiency. Soil N and biological nitrogen fixation (BNF) by associated organisms are major sources of N for lowland rice. Soil organic N is continually lost through plant removal, leaching, denitrification and ammonia volatilization. An additional concern is that the capacity of soil to supply N may decline with continuous intensive rice cropping under wetland conditions, unless it is replenished by biological N fixation. More than 50% of the N used by flooded rice receiving fertilizer N is derived from the combination of soil organic N and BNF by free-living and rice plant-associated bacteria. The remaining N requirement is normally met with fertilizer. Legumes are used commonly in agricultural systems as a source of N for subsequent crops and for maintaining soil N levels and reducing energy requirements by adding significant amounts of N to the soil (Entz et al., 2002). Reducing fertilizer N use in lowland rice systems while maintaining the native soil N resource and enhancing crop N output is desirable from both environmental and economic perspectives. This may be possible by obtaining N on the land through legume BNF, minimizing soil N losses, and by improved recycling of N through plant residues. Sustainable cropping systems are essential for agronomic, economic, and environmental reasons. Thus the management of indigenous soil N and N derived in situ through legume BNF poses potentials for enhancing the N nutrition and N use efficiency of crops and total N output from a lowland rice-based cropping system. The ability of legumes to fix N and their residual impact on soil N status has long been recognized, but many farmers also realize that the accrued N benefits will vary between different legume systems. To date the fate of N in green manure and productivity of dual-purpose dry season legumes and their effects on soil N dynamics and their contributions to the yield and N uptake of the following rice crop has been studied only a few instances. Cropping systems that include legumes have the potential for contributing N to following crops and may moderate NO3 levels in the soil to avoid potential for NO3 leaching. Grain and forage legumes grown in dry season and their residues could supplement some sort of N source for succeeding crop. Broad bean is used as a winter or spring cover crop, green manure, vegetable and an expensive food legume. It is capable of producing large amounts of dry matter and accumulating large quantities of nitrogen (N) and fixed substantial quantities of N for subsequent crops. Several international studies suggest vetches are efficient N-fixers and accumulate large amounts of N during growth. Hairy vetch not only supply N fixed by leguminous bacteria to the soil but also inhibits the weed growth and decrease the density of insect pest by allelopathy. Broad bean and hairy vetch are used in the rice-based cropping systems in Japan, but scientific information is very little. Therefore the present study was undertaken to the following objectives: (i) to determine N accumulation and quantify N fixed by broad bean and hairy vetches using the 15N natural abundance and N difference method (ii) to quantify N recoveries from rice after broad bean and hairy vetch systems and inorganic fertilizer sources using 15N labeled fertilizer and (iii) to determine the amount of fertilizer N required to optimize rice yield when broad bean and hairy vetch are included in the system…<more>

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Reducing Methane Emissions from Irrigated Rice

Rice is grown on more than 140 million hectares worldwide and is the most heavily consumed staple food on earth. Ninety percent of the world’s rice is produced and consumed in Asia, and 90 percent of rice land is—at least temporarily—flooded. The unique semiaquatic nature of the rice plant allows it to grow productively in places no other crop could exist, but it is also the reason for its emissions of the major greenhouse gas (GHG), methane. 

Methane emissions from rice fields are determined mainly by water regime and organic inputs, but they are also influenced by soil type, weather, tillage management, residues, fertilizers, and rice cultivar. Flooding of the soil is a prerequisite for sustained emissions of methane. Recent assessments of methane emissions from irrigated rice cultivation estimate global emissions for the year 2000 at a level corresponding to 625 million metric tons (mt) of carbon dioxide equivalent (CO2e).

Midseason drainage (a common irrigation practice adopted in major rice growing regions of China and Japan) and intermittent irrigation (common in northwest India) greatly reduce methane emissions. Similarly, rice environments with an insecure supply of water, namely rainfed rice, have a lower emission potential than irrigated rice. Organic inputs stimulate methane emissions as long as fields remain flooded. Therefore, organic inputs should be applied to aerobic soil in an effort to reduce methane emission. In addition to management factors, methane emissions are also affected by soil parameters and climate.

Recent studies suggest that rice cultivation is an important anthropogenic source of not only atmospheric methane but also of N2O. Rice soils that are flooded for long periods of the year tend to accumulate soil organic carbon, even with complete removal of above-ground plant biomass. Significant input of carbon and nitrogen is derived from biological activity in the soil–floodwater system, and conditions are generally more favorable for the formation of conserved soil organic matter. It is currently unknown whether rice systems in the tropics and subtropics truly sequester atmospheric carbon and how soil organic carbon levels will react to a changing climate or new management practices.

Losses of soil organic carbon are of major concern for certain developments in the agricultural sector that are undergoing rapid intensification and diversification of crop land. At the International  Rice Research Institute (IRRI), however, 12 years of continuous rice
cropping in flooded fields did not cause any significant decline in soil  organic carbon. In contrast, the soil organic carbon immediately declined after a shift to a nonflooded system, namely maize. The modification of flooding patterns encompassing one or more dry periods may somehow accelerate decomposition, but—unlike a complete shift to upland systems—the recurring periods of flooding will keep  the overall soil organic carbon at a fairly stable level. Thus, we do not include CO2 emissions in our considerations of mitigation options.

Changing water management appears to be the most promising mitigation option and is particularly suited to reducing emissions in irrigated rice production. Midseason drainage and intermittent irrigation reduce methane emissions by over 40 percent. Shallow flooding provides additional benefits, including water conservation and increased yields. A recent study estimates large potential for additional methane reductions from Chinese rice paddies through modifications of water-management strategies, even though midseason drying is widely practiced there.

Midseason drainage or reduced water use also creates nearly saturated soil conditions, which may promote N2O production. There are conflicting reports on the net global warming potential (GWP) of midseason drainage, but there seems to be a growing consensus that this practice decreases the net GWP of paddy fields as long as nitrogen is applied in appropriate doses. According to an empirical model proposed by Yan et al. (2005), midseason drainage generally tends to be an effective option for mitigating net GWP, although 15 to 20 percent of the benefit gained by decreasing methane emission was offset by the increase in N2O emission… <more>


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