Herbicide resistant rice (Oryza sativa L.): Global implications for weedy rice and weed management

Rice cultivars resistant to broad-spectrum herbicides have been developed a nd their commercial release is imminent, especially for imidazolinone and g lufosinate resistant varieties in the USA and Latin America. Glyphosate-res istant rice should follow within a few years. Rice growers throughout the w orld could benefit from the introduction of herbicide-resistant lice cultiv ars that would allow in-crop, selective control of weedy Oryza species. Oth er perceived benefits are the possibility to control 'hard-to-kill' weed sp ecies and weed populations that have already evolved resistance to herbicid es currently used in rice production, especially those of the Echinochloa s pecies complex. Weed management could also be improved by more efficient po st-emergence control. Introduction of herbicide resistant rice could also b rine, areas heavily infested with weedy rice that have been abandoned back to rice production, allow longer term crop rotations, reduce consumption of fossil fuels, promote the replacement of traditional chemicals by more env ironmentally benign products, and provide more rice grain without adding ne w land to production. There are also concerns, however, about the impact of releasing herbicide-resistant rice on weed problems. Of most concern is th e possibility of rapid transfer of the resistance trait to compatible weedy Oryza species. Development of such herbicide resistant weedy rice populati ons would substantially limit the chemical weed management options for farm ers. Herbicide-resistant rice volunteers also could become problematic, and added selection pressure to weed populations could aggravate already serio us weed resistance problems. Because of the risk of weedy Oryza species bec oming resistant to broad-spectrum herbicides, mitigating measures to preven t gene flow, eventually attainable by both conventional breeding and molecu lar genetics, have been proposed. With commercialisation of the first herbi cide resistant varieties planned for 2001, these mitigating measures will n ot be available for use with this first generation of herbicide resistant r ice products. Release of herbicide resistant rice should depend on a thorou gh risk assessment especially in areas infested with con-specific weedy ric e or intercrossing weedy Oryza species. Regulators will have to balance ris ks and benefits based on local needs and conditions before allowing commerc ialisation of herbicide-resistant rice varieties. If accepted, these variet ies should be considered as components of integrated weed management, and a rational herbicide use and weedy rice control should be promoted to preven t losing this novel tool.