FAQs on Water Management

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What is an Aerobic rice?

Aerobic rice is a production system in which specially developed, input-response rice varieties with "aerobic adaptation" are grown in well-drained, nonpuddled, and nonsaturated soils without ponded water, with a management system aiming at yield levels of 4-6 t ha-1 (and possibly beyond). A nonsaturated soil is also called an "aerobic soil".

More about aerobic rice:

What are aerobic rice varieties?
What is the difference between aerobic rice and upland rice?
Why aerobic rice?
Where aerobic rice?
How to manage aerobic rice?
Is aerobic rice rainfed or irrigated?
Is aerobic rice a “mature” technology?
How about aerobic rice and conservation agriculture?
How sustainable is aerobic rice?

What are aerobic rice varieties?

Varieties adapted to aerobic management systems require the ability to maintain rapid growth in soils with moisture content at or below field capacity. They share this ability with traditional upland rice varieties, which usually have deep root systems and tolerate water stress at both the vegetative and reproductive stages. However, varieties for aerobic production systems also need to be able to produce yields of 4-6 t ha-1 under favorable conditions. Traditional upland varieties, which are usually low-tillering, tall, and have a low harvest index, rarely achieve yields higher than 3 t ha-1 even under the most favorable conditions. Achieving high yields under aerobic soil conditions requires new varieties of “aerobic rice” that combine the drought-resistant characteristics of upland varieties with the high-yielding characteristics of lowland varieties.

Aerobic rice varieties combining high yield potential with tolerance to aerobic soil conditions have usually been derived from breeding programs in which varieties are developed and evaluated under aerobic soil conditions and with fertilizer applications sufficient for a 4-6 t ha-1 yield target. The first generation of varieties that performed well in a wide range of aerobic rice environments (e.g. IR55423-01 (“Apo”) and UPLRI-5 from the Philippines, B6144-MR-6-0-0 from Indonesia, and CT6510-24-1-2 from Colombia) were developed in upland rice breeding programs. They were often derived from crosses between indica and tropical japonica parents, whereas traditional upland varieties are usually derived from the aus or tropical japonica germplasm groups. Some aerobic rice breeding programs, (notably that of the China Agricultural University in Beijing), also have developed successful varieties by crossing high-yielding lowland rice varieties with traditional upland types. In northern China, new elite aerobic varieties were released in the late 1990s such as Han Dao 277, Han Dao 297 and Han Dao 502, with yield potentials of up to 6.5 t ha-1.

What is the difference between aerobic rice and upland rice?

Upland rice is grown in rainfed, naturally well-drained soils with bunded or unbunded fields without surface water accumulation. The general perception about the upland environment is that it is drought-prone, usually sloping land with erosion problems, and has soils with both poor physical and chemical properties. Farmers in these environments are among the poorest and usually can not afford to apply (many) external inputs such as fertilizers. Upland rice varieties are mostly grown as a low-yielding subsistence crop to give stable yields under the adverse environmental conditions of the uplands. Upland rice varieties are drought tolerant, but have a low yield potential and tend to lodge under high levels of external inputs such as fertilizer and supplemental irrigation.

The aerobic rice system is targeted at more favorable environments (see below) where farmers can afford to buy external inputs such as fertilizers and have access to supplementary irrigation if rainfall is not sufficient. Achieving high yields under relatively favorable aerobic soil conditions requires new varieties of “aerobic rice” that combine the drought-resistant characteristics of upland varieties with the high-yielding characteristics of lowland varieties. In essence, aerobic rice can be seen as “favorable” or “high yielding” upland rice. The reason for the introduction of a new term was the need to dissociate the envisioned relatively high-yielding production system from the general perception of extremely harsh and unfavorable conditions of “the uplands”.

Why aerobic rice?

There are two main driving forces for aerobic rice:

The increasing realization that not all “uplands” are “unfavorable”, in the sense that certain uplands may possess soils with good water-holding capacity and high fertility, that they are not always sloping land, that rainfall may be sufficient for a “decent’ crop growth, and that sometimes investments can be made to improve the quality of the uplands. An example of the latter is the terracing of slopes in the hilly and mountainous regions in Yunnan, China. Aerobic rice is seen as a relatively high-yielding production system that optimally exploits the resources available.
The increasing water scarcity in irrigated lowlands. The causes for water scarcity are diverse and location-specific, but include decreasing resources (e.g., falling groundwater tables, silting of reservoirs), decreasing quality (e.g., chemical pollution, salinization), malfunctioning of irrigation systems, and increased competition from other sectors such as urban and industrial users. In extreme cases, water scarcity can be so severe that farmers can not maintain flooded conditions in their fields for even a small part of the growing season, and rice fields are not ponded and saturated with water anymore. However, irrigation water availability is still sufficient for supplementary irrigation to keep the soil water content around field capacity. Under such conditions, lowland rice can not be grown anymore, and aerobic rice becomes a suitable alternative along with upland crops (diversification).

Where aerobic rice?

Aerobic rice can be found, or can be a suitable technology, in the following major rice-growing environments:

So-called “favorable uplands” (see FAQ 3: Why aerobic rice?): areas where the land is flat (or terraced), where rainfall with or without supplemental irrigation is sufficient to frequently bring the soil water content close to field capacity, where no serious soil-chemical limitations such as aluminium toxicity or salinity occur, and where farmers have access to external inputs such as fertilizers. A typical example is in the Cerrado region of Brazil, where farmers grow aerobic rice in rotation with crops such as soybean and fodder on large commercial farms with supplemental sprinkler irrigation on an estimated 250,000 ha of flat lands, realizing yields of 3-4 t ha-1. Another example is rainfed aerobic rice grown in newly-formed terraces in the hills of Yunnan, China, where yields are also typically 3-4 t ha-1.
Fields on upper toposequence locations in undulating so-called “rainfed lowlands”. Quite often, the soils of such upper fields or terraces are relatively coarse-textured and well-drained, so that ponding of water only occurs for a limited (or no) part of the growing season. No widespread examples of aerobic rice in rainfed lowlands are known, but these upper fields have been proposed as target domain for aerobic rice.
Water-short irrigated lowlands (see FAQ 3: Why aerobic rice?): areas where farmers do not have access to water to keep rice fields flooded for a substantial period of time anymore. Water shortage can be encountered in tail-end parts of large-scale surface irrigation systems, in areas where the groundwater has been drawn down so that pumping costs have become very high, in irrigation systems that receive less and less water because of redirected use (cities, industry) or because of reduced stream flow in rivers. A good example is the North China Plain where aerobic rice is grown on about 80,000 ha with supplemental irrigation.
Beside these typical rice-growing environments, aerobic rice can also be found in traditionally non-rice growing areas. Again in the North China Plain, farmers are experimenting with aerobic rice as a means of crop diversification in areas where traditionally maize is the dominant crop.

Aerobic rice can be found in tropical and in temperate climates. Most advances in developing aerobic rice systems, and in adoption by farmers, have been made so far in China and Brazil.

How to manage aerobic rice?

Aerobic rice is basically managed like a wheat or a maize crop. The usual establishment method is dry direct seeding. Before sowing, the land should be dry prepared by ploughing and harrowing to obtain a smooth seed bed. Seeds should be dry seeded at 1-2 cm depth in heavy (clayey) soils and 2-3 cm depth in light-textured (loamy) soils. Optimum seeding rates still need to be established but are probably in the 70-90 kg ha-1 range. In experiments so far, row spacings between 25 and 35 cm gave similar yields. The sowing of the seeds can be done manually (eg dibbling the seeds in slits opened by a stick or a tooth harrow) or using direct seeding machinery. An alternative establishment method is transplanting, where seedlings are transplanted into wet soil that is kept around saturation for a few days to ease transplanting shock. Subsequently the fields dry out to field capacity and beyond. This method of crop establishment can only be done in clay soils with good water-holding capacity.

The total amount of fertilizer N to be applied should aim for the 4-6 t ha-1 yield level, and depends on indigenous soil N supply and other sources of N (such as atmospheric deposition). If no knowledge on local recommendations is available, an amount of 90 kg N ha-1 could be a useful starting point (to be subsequently optimized). Instead of basal application of the first N split, the first application can best be applied 10-12 days after emergence to minimize N losses by leaching (the emerging seedling can’t take up N so fast, so it will easily leach out). Moreover, basal application of N also promotes early weed growth. Second and third split applications of N may be given around maximum tillering and panicle initiation, respectively. With future research, principles of Site-Specific Nutrient Management (SSNM) for aerobic rice should be developed. If the crop is grown in a dry season, a light irrigation application (say 30 mm) should be given after sowing to promote emergence. Subsequent irrigation applications depend on the rainfall pattern, the depth of groundwater, and on the availability and/or cost of irrigation water. Irrigation can be applied by any means as used for upland crops: flash flood, furrow, or sprinkler.

Rice that is not permanently flooded tends to have more weed growth and a broader weed spectrum than rice that is permanently flooded. To control weeds, the use of pre- or post-emergence herbicides is recommended when the weed pressure is high, plus additional manual or mechanical (inter-row cultivation) weeding in the early phases of crop growth.

Is aerobic rice rainfed or irrigated?.

Like wheat or maize, aerobic rice can be rainfed, supplementary irrigated, or fully irrigated. The optimum soil water condition for aerobic rice is around field capacity. If rainfall is insufficient to frequently restore water contents in the soil to field capacity, irrigation can be applied if water resources are available. Irrigation can be applied through flash-flooding, furrow irrigation (or raised beds), or sprinklers. Unlike flooded rice (lowland rice), irrigation - when applied – is not used to flood the soil but to just bring the soil water content in the root zone up to field capacity. The amount of irrigation water should match evaporation from the soil and transpiration by the crop (plus any application inefficiency losses). In lowland rice, the amount of irrigation water should match the same water flows, plus the losses by seepage and percolation.

Is aerobic rice a “mature” technology?.

Aerobic rice can be considered quite a mature technology in temperate and subtropical environments such as northern China and Brazil, where the areas of aerobic rice are estimated at 80,000 ha and 250,000 ha, respectively. In both countries, breeding programs since the 1980s have resulted in the release of several high-yielding “aerobic rice” varieties. On-farm yield levels seem to lie around 3-4 t ha-1, but yields of up to 6 t ha-1 have been recorded as well. Current research focuses on the development of improved management systems and on breeding further improved varieties.

Tropical aerobic rice systems are still very much in the research and development phase. More research is especially needed to breed high-yielding aerobic rice varieties with sufficient aerobic adaptation and to develop sustainable management systems. Without ponded water, rice production is less sustainable than under flooded (lowland) conditions, and typical problems come up that occur in upland crops (see FAQ 10: How sustainable is aerobic rice?). In general, sustainability seems to be more of a problem in tropical areas than in temperate areas such as northern China. Aerobic rice should not be grown consecutively on the same piece of land, and – depending on the cropping history and soil type – low yields can even occur on fields cropped to aerobic rice the very first time.

How about aerobic rice and conservation agriculture?

With aerobic rice, practices of conservation agriculture, such as mulching and minimum tillage as practiced in upland crops, become available to rice farmers as well. In the Indo-Gangetic Plain, farmers are experimenting with minimum tillage practices and permanent raised beds in the rice-wheat system. Pioneering research and development work is being done by the Rice Wheat Consortium (http://www.rwc.cgiar.org/index.asp).

Various methods of mulching (e.g., using dry soil, straw, and plastic sheets) are being experimented with in aerobic rice systems in China. In hilly areas in Shiyan, Hubei Province in China, farmers on an estimated 6000 ha are adopting the use of plastic sheets to cover rice fields in which the soil is kept just below saturation. The proclaimed advantages are: earlier crop establishment (rice is established in early spring when temperatures are still low, and the plastic sheet increases the soil temperature), higher yields, less weed growth, and less water use (important during dry spells). The left-over plastic after harvest may cause environmental degradation if not properly taken care of.

How sustainable is aerobic rice?.

Given assured water supply, lowland rice fields are extremely sustainable and able to produce continuously high yields, even under continuous double or triple-cropping a year. Flooding of rice fields has beneficial effects on soil acidity (pH), soil organic matter buildup, phosphorus, iron, and zinc availability, and biological N fixation that supplies the crop with additional N. When fields are not continuously flooded, such as in aerobic rice, these beneficial effects gradually disappear. A change from flooded to aerobic soil conditions may decrease the soil organic matter content, decrease the soil pH, and decrease the availability of phosphorus, iron, and zinc. Also, problems with micro-nutrient deficiencies have been reported. If field were cropped to rainfed rice with alternate periods of flooding and dry soil, or if fields were previously cropped to upland crops, then the introduction of aerobic will have fewer consequences for these sustainability parameters.

There are indications that soil-borne pests and diseases such as nematodes, root aphids, and fungi occur more in aerobic rice than in flooded rice, especially in the tropics. The current experience is that aerobic rice should not be grown continuously on the same piece of land each year (as can be successfully done with flooded rice) without yield decline. Suitable crop rotations need to be identified, but will be site-specific and responsive to markets.

Current research focuses on determining the causes of yield decline under continuous cropping (biotic, abiotic), on developing “resistant” varieties, on developing suitable management options such as crop rotation, and on developing integrated weed management practices.