Dr. Madonna C. Casimero, acting deputy executive director for research of PhilRice (Philippine Rice Research Institute), said this weed is called weedy rice, as it is closely related to cultivated rice. As such, it is difficult to differentiate the weed and rice plants during the vegetative stage, most especially in direct-seeded rice.
"It could be a serious threat in direct-seeded rice, as it would be quite difficult to distinguish it from rice plants during early vegetative growth," said the lady weed control expert. The cost of controlling weedy rice could be staggering once it has invaded a community of rice fields, she added.
Research reports indicate that if 35 percent of a rice crop is composed of weedy rice plants, grain yield losses can reach 50 to 60 percent. What’s more, greater weedy rice infestation can result in total crop loss because of competition and crop lodging. Weedy rice prevents rice plants to use all the fertilizer nutrients intended for the latter since it competes with cultivated rice for soil nutrients and sunlight.
The seeds of weedy rice easily shatter. A slight touch on almost ripe panicles causes the seeds to shatter and fall to the ground where they become dormant for some time. The seeds become part of the harvest of early maturing varieties harvested by mechanical harvesters.
If weedy rice grains are mixed with the crop harvest, the milled rice becomes contaminated with red to cream weedy rice grains, resulting in a great reduction of its market value.
Scientists claim that this weed could be a natural hybrid of cultivated rice and the wild rice species Oryza rufipogon and Oryza nivara. It is known as padi angin in Malaysia, lua lon in Vietnam, lutao in China, akamai in Japan, sharei in Korea, khao pa in Laos, khao nok in Thailand, and jhora dhan in Bangladesh. However, the Philippines has no name for this weed.
Weedy rice seeds that fall on the ground do not germinate until some time later. Its seed dormancy varies, but a large portion of the seeds normally germinates within three months after shattering. Thus, the next crop in double cropping areas would have severe weed problems.
Weedy rice can be taller or shorter than or even as tall as cultivated rice. The flag leaves are either erect or droopy. The panicles can either be closed or open like those of cultivated rice. The grains may not have awns, but some variants can have either short or long awns.
This weed also grows vigorously and matures early. Farmers, therefore, must learn to distinguish weedy rice from cultivated rice during its vegetative phase to be able to remove them before these could produce panicles.
To prevent weedy rice seeds from being introduced into rice fields, Dr. Casimero advise farmers to:
• Use certified seeds or "clean seeds" from a source known to be not invaded by weedy rice. Avoid rice seeds contaminated by weedy rice.
• Make sure that machinery or implements coming from infested fields must be thoroughly cleaned before these are used in uncontaminated fields.
• Clear irrigation canals of weedy rice infestation to prevent weedy rice seeds from invading clean fields.
If infestation is not avoided, remove the weedy rice plants manually once these are recognized. For those that have escaped manual weeding, remove them before these produce panicles or before the panicles mature. This can help prevent more serious infestations in future crops.
Practice stale seedbed technique during land preparation to encourage the weedy rice seeds to germinate when irrigation water is allowed into the field or when it rains. Plow the seedlings under the soil or apply nonselective herbicide. Repeat this process several times to deplete the soil seed bank of weedy rice.
If irrigation water is immediately available, practice manual or mechanical transplanting to attain better control. On the other hand, practice crop rotation if upland crops can be grown in the field after rice, as these crops allow manual weeding and herbicide application.
If weedy rice has already started to infest a rice field, spread the rice uniformly in the field and burn it to destroy the seeds that have fallen on the ground.