Direct approach to weeds in rice
Transplanting rice seedlings into standing water is a long established way of giving the crop a competitive advantage over weeds. Historically, rice plants were tall, and their shading provided additional competition to weeds. Weeds that survived the competition were weeded by hand two or three times. But modern rice varieties, progeny of the Green Revolution, are short and fertiliser-responsive, but are not effective shade plants. Transplanting and hand weeding are laborious, and the cost of pumping water for flooding rice fields is rising. Weeds, the major source of yield loss in rice, remain and so rice growers, faced with rising fuel, labour and input costs, and lower prices for their crop, need less costly approaches to weed control.
The most promising option for the future is to adopt direct sowing of rice in place of transplanting, reducing its dependence on labour and water, and for farmers to become familiar with the correct use of herbicides. Two research projects (in Bangladesh and India) were initiated in 1999 to develop low-cost, labour-efficient methods of weed management for both direct seeding and transplanting systems. Lessons learned may apply elsewhere in South and South East Asia.
Direct seeding can refer to either wet or dry methods, depending on the manner of crop establishment. Wet-seeding involves sowing pre-germinated seed, either broadcast or drilled, on to puddled wet soil, and then gradually flooding the land. In dry-seeding, rice is broadcast or drilled into dry soil and the seed is then covered. But both methods result in weeds starting growth at the same time as the rice and therefore competing from an early stage. Therefore, research has focused on matching the sowing of early maturing varieties with correct chemical herbicide use. Where successful, this provides farmers a bonus; it permits the planting of a post-rice crop in the residual moisture, increasing total crop yields and overall farm productivity.
Trials over the past four years have revealed that yields from direct-seeded rice were higher than in transplanted crops, provided that weeds were adequately controlled. And the costs were significantly less for direct-seeding, coming not only from savings in labour, but also from the decreased need for irrigation water. There is also less land preparation. But, good weed control is essential. Although with direct-seeded rice broadleaved weeds are not a problem, grasses and sedges are, including Ischaemum rugosum, Echinachloa spp., Leptochloa chinensis and Cyperus spp. Without any weed control, yield losses can be 3t/ha in dry-seeded and 4t/ha in wet-seeded crops.
Clearly, effective use of herbicides is an essential element in these new approaches to rice culture and promoting this will require good extension. The two projects, funded by DFID, have numerous partners including IRRI, Universities in India and Bangladesh, NRI-UK, NGOs active in agriculture, and a major herbicide manufacturer, Syngenta. The findings of the project are key to increasing productivity in what is the rice basket of India and for the staple crop of Bangladesh. With a large and growing number of urban poor throughout South and South East Asia, low-cost rice is essential. The only way that rice growers can supply this is to cut their costs of production. Direct-seeding has the potential to help cut costs, and the diverse collaborators in this project surely have the knowledge, skills and resources to communicate the effective use of direct seeding and herbicides to the millions of small-scale rice producers in the region.
Date published: March 2004
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