SUMMARYThe lack of control of barnyardgrass in flooded rice cultivated with imidazolinone-resistant rice cultivars is challenging the utilization of this system, which is continuously expanding for new rice areas worldwide. The objectives of the present study were to evaluate the frequency, distribution and mechanisms of imidazolinone resistance in barnyardgrass to establish the best practices to control and prevent this problem. The distribution of resistance was evaluated in 624 populations collected in Southern Brazil. The frequency of imidazolinone-herbicide resistance was 0·81, broadly distributed in all sampled regions. Resistance to quinclorac was also found in 0·19 of the populations, but all of the evaluated populations were susceptible to cyhalofop-butyl. Further studies were conducted in six populations. The enhanced metabolism was assessed with the metabolic inhibitors that reversed the resistance to quinclorac from 0·54 to 1·00 in two populations and the resistance to imazethapyr from 0·15 to 0·41 in three populations. The acetolactase synthase (ALS) enzyme activity also indicated the occurrence of altered target site resistance in two populations caused by the ALS gene mutations Trp574Leu and Ser653Asn, which is a novel finding in this species. The herbicide resistance in barnyardgrass in Southern Brazil presented a complex basis of resistance because it is associated with resistance to multiple herbicides due to multiple mechanisms and with multiple mutations of the ALS gene. This indicates that it is necessary to adopt specific measures to prevent and control the evolution of multiple herbicide resistance in this species.

SUMMARYGene flow from herbicide-resistant rice (Oryza sativaL.) cultivars can affect the biodiversity ofOryzaspp. and can result in the lack of opportunity to control weedy rice through selective herbicides. The aim of the present study was to quantify the outcrossing rate from the herbicide-resistant red rice and rice cultivars carrying three different ALS (acetolactate synthase) alleles using a single plant pollen donor approach. A field experiment was performed using the encircle population combination technique. The main plots comprised the pollen-receptor IRGA 417 cultivar or a susceptible biotype of weedy rice, and the subplots comprised the pollen-donor inbred cultivars IRGA 422 CL and PUITÁ INTA CL, the hybrid SATOR CL or a resistant biotype of weedy rice. Among the pollen-donors, the outcrossing rate for pollen receptor susceptible weedy rice and the IRGA 417 cultivar was 0.0344% and 0.0142%, respectively. Rice cultivars carrying theALSgene mutations Ala122Thr, Ser653Asn and Gly654Asn showed a similar outcrossing rate of 0.0243%. The outcrossing rate decreased over a distance of up to 3.5 m from the pollen-donor and was not affected by the wind cardinal direction. The risk of gene flow of herbicide resistance from rice to weedy rice should be reduced through the development of new strategies to contain and mitigate gene flow and of the elimination of weedy rice escapees.