Rice cultivation systems: transplanted, direct-seeded, SRI and AWD

The rice ecosystems

Rice is commonly grouped into four ecosystems by water regime:

• Irrigated lowland: bunded fields with controlled irrigation; roughly half the world's rice area but about three-quarters of production, often with two or three crops a year.
• Rain-fed lowland: bunded fields flooded by rain for part of the season, subject to drought and floods.
• Upland: grown like other cereals on unbunded, unflooded fields that depend on rain; low yields, important on hillsides in Asia, Africa and Latin America.
• Flood-prone (deepwater and floating rice): fields flooded deeply for weeks to months; specialised varieties elongate with the rising water.

Rice can grow in flooded soil because air channels (aerenchyma) carry oxygen from leaves to roots. Flooding is not a need of the plant itself but a powerful management tool: it suppresses weeds, buffers temperature, keeps nutrients such as phosphorus more available, and allows several crops a year.

Transplanting into puddled fields

The traditional system across much of Asia:

1. Nursery: seedlings are raised densely in a small nursery bed (wet-bed, dry-bed or the mat nurseries used with transplanting machines).
2. Puddling: the main field is flooded and ploughed or harrowed while wet, churning the soil into a soft mud.
3. Transplanting of seedlings, typically 2 to 4 weeks old, a few per hill, at spacings such as 20 × 15 cm or 20 × 20 cm.
4. Standing water is kept for most of the season.

Why puddle?

• It reduces percolation, sealing the field so water stays on it.
• It controls weeds: buried at puddling, and suppressed by standing water after transplanting.
• It makes transplanting easy in soft soil.

Its costs are heavy: a great deal of labour at transplanting, large water use (much of it percolation and evaporation), methane emissions from continuously flooded soil, and a destroyed soil structure that harms the following upland crop, such as wheat after rice.

Direct seeding

Rising wages, labour shortages and water scarcity have moved many farmers to sow rice directly, skipping the nursery and transplanting.

• Wet direct seeding: pre-germinated seed is broadcast or drilled onto puddled, drained soil. Common in parts of South-East Asia.
• Dry direct seeding: dry seed is drilled into unpuddled, often zero-tilled soil, like wheat, and the field is flooded later or kept aerobic. Widely adopted in the Americas and Australia, and expanding in South Asia.
• Water seeding, sowing into standing water, is used in some mechanised systems.

Benefits: less labour and water at establishment, earlier crops, no damage to soil structure for the next crop, and lower methane emissions. The main problem is weeds, which germinate with the rice instead of being suppressed by standing water; good land levelling, stale seedbeds, competitive varieties, timely herbicides and some hand weeding are needed. Uneven stands, birds, rodents and some soil-borne pests are other risks.

The system of rice intensification (SRI)

SRI was developed in Madagascar in the 1980s by the Jesuit priest Henri de Laulanié. It combines:

• young seedlings (often 8 to 12 days old), transplanted singly, quickly and carefully;
• wide, square spacing, such as 25 × 25 cm;
• intermittent irrigation, keeping the soil moist but not continuously flooded;
• mechanical weeding with rotary weeders, which also aerates the soil;
• generous organic matter.

Supporters report higher yields with less seed and water; critics have argued that many of the largest reported gains came from comparisons with poorly managed conventional fields, and that controlled trials show smaller and more variable effects. A balanced reading is that several SRI elements, such as younger seedlings, wider spacing, water saving and good weeding, can improve yields and resource efficiency in suitable conditions, while SRI's labour demand limits its spread.

Saving water: alternate wetting and drying

Alternate wetting and drying (AWD), developed and promoted by the International Rice Research Institute and national programmes, lets the water in the field fall below the soil surface before re-flooding. In safe AWD, the farmer watches the water level in a perforated tube in the field and irrigates when it drops to about 15 cm below the surface, keeping the field flooded around flowering. It typically saves a substantial share of irrigation water and cuts methane emissions, usually without lowering yield. A related approach, aerobic rice, grows adapted varieties in unflooded, unpuddled soil like an upland crop with irrigation.

Irrigated rice is often said to need around 2,500 litres of water input per kilogram of grain, though much of the water lost by percolation from one field is reused downstream, so the real saving from efficiency measures depends on where the water goes.

Pests, diseases and weeds by system

Flooding and establishment method change the pest picture. Continuous flooding and high nitrogen favour the brown planthopper and sheath blight; dry direct seeding favours weeds and some soil insects; dense, lush crops favour blast in humid weather. Stem borers attack in all systems. Integrated pest management, resistant varieties, balanced nitrogen and conserving natural enemies, especially spiders, are central to rice protection.

Questions people ask

Sources

1. De Datta, S. K. Principles and Practices of Rice Production. Wiley, 1981.
2. Kumar, V. and Ladha, J. K. "Direct seeding of rice: recent developments and future research needs." Advances in Agronomy 111, 297–413 (2011).
3. Bouman, B. A. M., Humphreys, E., Tuong, T. P. and Barker, R. "Rice and water." Advances in Agronomy 92, 187–237 (2007).
4. Stoop, W. A., Uphoff, N. and Kassam, A. "A review of agricultural research issues raised by the system of rice intensification (SRI) from Madagascar." Agricultural Systems 71, 249–274 (2002).
5. Sheehy, J. E. et al. "Fantastic yields in the system of rice intensification: fact or fallacy?" Field Crops Research 88, 1–8 (2004).
6. Lampayan, R. M. et al. "Adoption and economics of alternate wetting and drying water management for irrigated lowland rice." Field Crops Research 170, 95–108 (2015).

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