Weedy rice attacks Asia's direct-seeded rice

A Malaysian farmer manually removes weedy rice panicles by hand. In Malaysia, weedy rice or padi angin was detected in 1988. It is estimated to be causing crop losses of about US$25 million a year in Peninsular Malaysia. (Photo by D. Johnson)

Asia’s rice farmers —already reeling from a host of problems, including less land, labor, and water to grow the region’s most important crop, as well as the looming impact of climate change —are facing a cancerous new threat called weedy rice.

Just like a cancer growing among human cells, weedy rice grows alongside conventional rice, making it very difficult to see and deal with. In Thailand, it was reported to have infested almost 200,000 hectares of rice in 2005, whereas, in the Philippines, the highest infestation rate has reached 90% in Iloilo Province.

Weedy rice plants are weedy forms of rice that have—in recent years— invaded the world’s major rice-growing areas, including North America, the Caribbean, Africa, and Asia. But it is in Asia that it has caused the most damage.

The exact origins of weedy rice in the region are still a mystery, although it is believed to be a natural hybrid of cultivated (Oryza sativa) and wild rice species (O. rufipogon and O. nivara), or a result of the degeneration of cultivated rice.

It is especially a problem in the direct-seeded rice areas of Asia. Weed scientist Madonna Casimero of the Philippine Rice Research Institute (PhilRice) says that, since it was first found in the Philippines in the 1960s, it has become a big problem because it cannot be controlled by herbicides. “Weed scientists and rice farmers, particularly in Iloilo Province, are all struggling to find a sustainable solution but it is a tough challenge,” Dr. Casimero says.

At the crop’s early growth stages, removing wild rice seedlings by hand weeding is difficult, since they look a lot like the seedlings of cultivated rice. “Some farmers with small farms do manual weeding or roguing, and remove the rice panicles by hand,” Dr. Casimero says. “But such strategies are not feasible if the farmer has 3 hectares or more, especially if there is heavy infestation, because the labor cost is too high.”

Dr. Casimero says weedy rice is finally recognizable at harvest time because it tends to be taller, has grains with long awns, and some plants have red grains seen only after milling. “The bad news is that farmers cannot harvest the grains because they shatter easily. To make things worse, the grains left on the ground will germinate when conditions are favorable and produce even more weedy rice.”

Malaysia and Thailand now have the largest areas in Asia in terms of weedy rice infestastion. “When the weed first appeared in Malaysia in 1988, farmers did not recognize it as a problem and did little to check its spread,” says weed scientist Azmi bin Man of the Malaysian Agricultural Research and Development Institute. “Now, most rice fields in Peninsular Malaysia and Sabah are affected, and it is estimated that weedy rice is causing crop losses valued at about $25 million a year on the peninsula.” Dr. Chanya Maneechote, a weed scientist from Thailand’s agriculture department, has reported that 200,000 hectares were seriously damaged due to weedy rice infestation in 2005.

Farmers and scientists inspect a weedy rice field in Thailand. Weedy rice is classified into three different groups in Thailand according to appearance: Khao Hang (rice with awn), Khao Deed (jumping rice), and Khao Daeng (red rice). (Photo by D. Johnson)

Once farmers’ rice fields are infested, the control costs for weedy rice are high. At present, no single management technique can effectively control the problem.

One way to prevent weedy rice is to use clean, certified seed from a known source and then keep machinery and canals free from seeds and infestations. Manual weeding and good land preparation can also reduce initial infestations of weedy rice. Other reported control methods include water seeding (sowing pregerminated seeds in water), manual or mechanical transplanting, crop rotation, burning of rice straw in dry rice fields, application of pretilachlor with fenclorim during final land preparation, and higher seeding rates of more than 150 kilograms per hectare.

The battle against weedy rice in Asia has been joined by the Irrigated Rice Research Consortium (IRRC) based at the International Rice Research Institute (IRRI) in the Philippines (www.irri.org/irrc). The IRRC has developed a brochure that describes weedy rice’s traits, the problems it causes, and its preventive measures and control. This brochure is now being distributed widely in Asian countries and can be downloaded from the IRRC’s Web site.

IRRI weed scientists David Johnson and Joel Janiya agree that “A major step toward winning the battle against weedy rice will be to make farmers more aware of the problem, and we hope the brochure will help achieve this.”

In the Philippines, PhilRice, in collaboration with the Australian Centre for International Agricultural Research, has started studies to examine and characterize the biology of weedy rice to develop control strategies. A team, led by Dr. Casimero, has begun surveying all provinces in Central Luzon and parts of the Western Visayas to generate a map showing where weedy rice can be found. Starting next cropping season, they will be testing an integrated weed management strategy in Iloilo Province involving longer land preparation time and water management to reduce weedy rice populations.

In the Philippines, the highest infestation rate has reached 90% in Iloilo Province. (PhilRice photo)

Trina Mendoza (t.mendoza@cgiar.org)

Research streams archive