Weedy Rices-Origin, Biology, Ecology, and Control

Authors: DELOUCHE, J - MISSISSIPPI STATE UNIV; BURGOS, NILDA - UNIV. OF AR RREC; Gealy, David; DE SAN MARTIN, G - INIA; LABRADA, R - FOOD AND AGRICULTURE ORG; LARINDE, M - FOOD AND AGRICULTURE ORG; ROSELL, C - FOOD AND AGRICULTURE ORG

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 5/15/2007
Publication Date: 6/1/2007
Citation: Delouche, J.C., Burgos, N.R., Gealy, D.R., De San Martin, G.Z., Labrada, R., Larinde, M., Rosell, C. 2007. Weedy Rices-Origin, Biology, Ecology, and Control. FAO Plant Production and Protection Paper 188, FAO Rome. 144 p.

Interpretive Summary: Red rice and ‘weedy rice’ have long been important weeds in direct-seeded production areas such as those in the southern U.S. that are planted using grain drills. Weedy rice is a general term used internationally to describe such species, whether or not they have a red seed coat like that of red rice. In recent years, red/weedy rice has increased in importance worldwide, especially with the adoption of direct- or drill-seeding systems in some of the traditional transplant culture areas. Presently, red/weedy rice infestations are considered to be among the most troublesome, difficult-to-manage and economically damaging pest problems in the rice industry. Infestations increase the labor requirement, increase costs of production, and reduce yield and market quality of the crop. Red rices in the Americas, Europe and North Africa are mostly weedy biotypes of Oryza sativa, the same species as cultivated rice. However, in other parts of the world, other Oryza species also contribute to the weedy rice problem. The red/weedy rices consist of weedy populations of O. sativa and/or other Oryza species that are visually and genetically diverse, very vigorous and competitive, difficult to control, spread rapidly, and reduce yields and gain quality. In most areas there is a diversity and changeability of red/weedy rice populations that results from introductions of seeds from other areas and natural crossing between the weedy biotypes and cultivated varieties. The red/weedy rices are spread from infested areas to non-infested areas primarily as contaminants in planted seeds. They are very successful as weeds because they have key weedy traits and characteristics such as excellent adaptation to most rice production systems; a life cycle closely synchronized with rice; abundant production of seeds that easily shatter to the ground before harvest; rapid emergence followed by vigorous growth and reproductive development; and dormancy that maintains the viability of the shattered seeds in the soil seed bank over long periods of time. Natural crossing of red and cultivated rice provides variability that has been naturally selected to produce ecotypes that are adapted to changes in cropping systems and varieties. The superior vigor and competitiveness of red/weedy rices ensures that they will produce numerous seeds in the midst of the rice crop. Satisfactory control of the red/weedy rices can be achieved using an integrated approach to management that involves well-tested control strategies and the judicious exploitation of new technologies, including prevention of infestations; depletion of red/weedy rice seeds in the soil seed bank; suppression of germination/emergence of red/weedy rice seeds; destruction of red/ weedy rice plants in the rice crop; alternation of rice with other crops; and herbicide-resistance technology. Much has been learned about the diversity, incidence of outcrossing, growth and development, and ecology of red/weedy rices in the last few decades, and modern technological advancements are providing powerful new tools for a much greater understanding of the ecophysiology of these weeds.

Technical Abstract: Red and other weedy rices have long been important weeds in direct-seeded production areas. During the past quarter centry, however, they have increased in importance in these areas and have spread worldwide with the adoption of direct seeding systems in some of the traditional transplant culture areas. Presently, red/weedy rice infestations are considered to be among the most troublesome, difficult-to-manage and economically damaging pest problems by stakeholders in the rice industry from production through marketing. Infestations increase the labor requirement, increase costs of production, reduce yield and market quality of the crop. Red rices in the Americas, Europe and North Africa are mostly weedy biotypes of Oryza sativa. In West Africa and Sub-Saharan Africa , the weedy rices are O. barthii, O. longistaminata , O. punctata, and weedy biotypes of O. glaberrima and O. sativa. In Asia, the area where O. sativa originated, O. rufipogon and other Oryza spp. are typically the sources of the weedy rice pests. Agronomically, the red and weedy rices consist of weedy populations of O. sativa and/or other Oryza spp. that are phenotypically and genotypically diverse, changeable, very vigorous and competitive, exceedingly difficult to control, spread rapidly, and reduce yields and gain quality. In most areas the diversity of the populations and their changeability are the result of introductions from other areas and natural crossing of the weedy biotypes and cultivated varieties. The red and weedy rices have been and still are spread from infested areas to non-infested areas primarily as contaminants in planting seeds. Red and other weedy rices are very successful as weeds because they have most of the traits and characteristics that contribute to the success of weeds in general plus some that are unique. The traits shared with other important weeds are: excellent adaptation to most rice cultural systems; a life cycle closely synchronized with rice; abundant production of seeds that are widely dispersed by early and heavy shattering; rapid emergence followed by vigorous growth and reproductive development; and intense and prolonged dormancy that maintains the viability of the shattered seeds in the soil seed bank during adverse climatic conditions. The unique traits of red rices that contribute to their success as weeds include: morphological and phenological similarity of the plants and seed to those of the rice crop that makes them difficult to recognize and separate; and their close genetic relationship to cultivated rice that precludes use of most selective herbicides. Among these traits, four are critical to the success of red and other weedy rices: the diversity and changeability of the populations produced by natural crossing of the weed and crop; early and heavy shattering; intense and prolonged seed dormancy; and superior vigor and competitiveness as compared to cultivated varieties. Natural crossing of red and cultivated rice provides variability that can be and has been naturally selected to produce ecotypes that are adapted to changes in cropping systems and varieties. Early and heavy shattering assures that most of the red and weedy rice seeds are scattered and dispersed to the surface of the field rather than gathered with the grain for consumption or marketing. Intense and prolonged dormancy maintains the viability of the seeds during the adverse climatic conditions that occur between harvest and the next planting season and permits the establishment of a soil seed bank of weedy rice seeds that can be drawn on for several to many years. The superior vigor and competitiveness of red and other weedy rices ensures that they will produce seeds in the midst of the rice crop. The general and unique traits of the red and weedy rices make them multi-dimensioned pests that produce multi-dimensioned problems in direct seeded