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[COLOMBO] The rotational use of different herbicides could significantly increase yields with dry-seeded rice (DSR) farming methods and compares well with traditional wet-seeded rice (WSR), says a new study.
According to a report of the study, published in the July—September issue of Weed Technology, DSR is gaining popularity in Asia although, with this method, weeds can reduce grain yield by 58—79 per cent.
Bhagirath Singh Chauhan, a scientist with the Philippines-based International Rice Research Institute and an author of the report, told SciDev.Net that the study highlights the relevance of using different herbicides to reduce resistance.
"This system is new to Sri Lanka but an emerging trend. Our attempt is to ensure higher yields and to have a practical solution to the resistance evolution phenomenon," Chauhan said.   
The report says that the performance of different herbicides in a DSR system at the Rice Research and Development Institute in Batalegoda, Sri Lanka, indicates higher yields with DSR especially when herbicide rotation is practiced.
Further, the report identifies DSR as one way to reduce water requirement during field preparation and cultivation and suggests further experimentation on weed-management strategies in Sri Lanka.
Additional benefits include reduced labour — since DSR is carried out by tractor-operated machines — and reduction in emissions of methane, a greenhouse gas. DSR is also conducive to subsequent planting of non-rice crops such as maize and pulses, the study says. 
With WSR, seeds are broadcast on the soil surface, making them vulnerable to predation by birds, insects and rats as well as to decay. Since DSR involves planting the seeds in holes drilled into the soil of about 1—2 centimetres in depth, the seeds are safer.
"In WSR, seeds are broadcast (by hand) resulting in non-uniform distribution of plants and aeration in the canopy is absent," says Chauhan. "In the DSR method, seeds are planted in rows and there is better aeration."
M. M. Aheeyar, head of the environment and water resources management division of the Hector Kobbekaduwa Agrarian Research and Training Institute, Colombo, tells SciDev.Net that water allocation has become a problem in Sri Lanka following extreme weather events.
"It is too early to say how replicable the DSR method is, but countries such as Sri Lanka could benefit, especially because it requires much less water," Aheeyar says.
Link to abstract in Weed Technology