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New Agriculturist: Focus on... Rice - learning from the past?
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Rice - learning from the past?

In ancient times, the economy of the great city of Angkor, located in present-day Cambodia, was based on rice, with a complex system of waterways for irrigation. But despite the success of the city (it was home to about 750,000 people and covered some 1,000 square kilometres), population pressures and water shortages eventually resulted in people migrating south, and the city's decline. Eight hundred years later, rice is still the most important crop for millions of smallholder farmers. And, although the complex systems for irrigation of old no longer exist, rice remains a water-intensive crop; 90 per cent of agricultural water use in Asia is currently for rice production. As the archaeological secrets of Angkor are further revealed, what cautionary tales from this once mighty city can be communicated to the modern world and to future generations of rice farmers?

Using single animal plough on rice field in Cambodia (© FAO)
Using single animal plough on rice field in Cambodia

Angkor was the capital of the successful Khmer Empire and its Hindu kings, who ruled over large areas of Southeast Asia for more than five hundred years. The city came to an abrupt end when it was sacked by Siamese invaders in 1431, but latest research reported by CNN has proposed that the city's infrastructure was already beginning to fail. Archaeological evidence seems to indicate that the network of reservoirs and canals built for irrigation of rice, as well as for trade and travel, became too elaborate to sustain. Rivers had been diverted, directional flow changed, and man-made channels engineered but, as forest was felled to clear more land for rice, problems of sedimentation resulted in siltation of the complex system, a problem that eventually became insoluble.

With increasing concern over water usage in the coming decades, the development of rice production systems that use less water is critical. The International Rice Research Institute (IRRI) estimates that it currently takes 5,000 litres of water to produce one kilogram of rice, almost twice that needed to grow most other crops. Intensive irrigation, much of it for rice production, is leading to the demise of Central Asia's Aral Sea. According to researchers at the International Water Management Institute (IMWI), the Aral Sea will be completely dry by the year 2020 if current water management practices are continued. And yet, if part (132,000 ha) of the saline rice-producing areas were taken out of production, water flow in feeder rivers could be doubled.

Dryland vs submerged rice

Prior to domestication of rice in China and the introduction of puddling and transplanting, early rice cultivation relied on direct seeding and it was mostly a dry, upland crop. It was not until much later in the history of rice in Southeast Asia that the crop began to be grown in the vast river deltas. There is no doubt that water efficiency in current rice culture can be improved and, with the added challenge of maintaining production yields, researchers are being pressured to develop new techniques that require less water. As direct seeding of pre-germinated rice becomes a growing modern practice in several Asian countries, it is possible that dryland methods could be re-introduced. It is estimated that this practice helps to reduce water use by 15-20 per cent over traditional transplanting techniques.

On the other hand, the practice of puddling land also aims to reduce water loss by breaking down soil particles to reduce percolation. Transplanting also allows field durations to be shortened, thereby maximising available water supplies. Submerged rice cultivation is also important for groundwater recharge and use of field bunds helps to control flooding during heavy rains. Water efficiency for submerged rice cultivation can be improved through levelling of rice fields, but this requires use of draught animal power or mechanisation.

Flooding prior to planting of rice is also a means to control weeds. It has been particularly important for control of red rice, which is botanically the same species as cultivated white rice. A lack of chemical control options has led to the 'flood and plant' system being used in parts of the United States. This results in anaerobic conditions in which red rice is unable to survive but also often affects affects water quality and leads to soil loss. A new herbicide recently developed by BASF has allowed farmers to control the weed without flooding. Development of a new rice seed by BASF has also allowed Colombian farmers to reduce planting intervals between crops. Traditionally, rice fields are left bare for 4 - 6 weeks, which allows control of red rice after it has emerged and before replanting the field. With the new variety the crop can make use of field moisture previously taken up by the emerging weed.

For any new developments for growing water with less rice, farmers will need to be given clear and careful advice if any long-term impact on water use is to be achieved and, as FAO warns, the benefits of these new techniques will have to weighed against the time-proven benefits of submerged rice culture. However, without addressing the need to curtail future consumption of water, Angkor's may not be the only rice economy to collapse.

Date published: July 2004


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