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The evolution of herbicide-resistant weeds is the major challenge for chemical management worldwide, increasing production costs, and reducing yield. This work aimed to evaluate the putative resistance of the Amaranthus hybridus population from Candido Mota (CMT) to glyphosate and imazethapyr and to investigate the non-target site mechanisms involved. Dose–response studies were conducted under greenhouse conditions and the control and biomass reduction were evaluated 28 days after application (DAA). Absorption, translocation, and metabolization studies were evaluated at 72 h after treatment (HAT) using radiometric techniques. The dose–response results show different responses among populations to glyphosate and imazethapyr. The CMT population was not controlled with labeled herbicide doses. Based on biomass reduction, the resistance factor was 16.4 and 9.4 to glyphosate and imazethapyr, respectively. The CMT absorbed 66% of 14C-glyphosate and 23% of 14C-imazethapyr at 72 HAT. Although the CMT population absorbed more glyphosate than the susceptible population (12.6%), translocation was impaired for both herbicides in the CMT when compared to the SUS population. There was no evidence that herbicide metabolization was involved in CMT resistance to the herbicides studied. Understanding the mechanisms endowing resistance allows better decision-making. This is the first study that describes non-target-site resistance mechanisms in an Amaranthus hybridus population from Brazil. [ABSTRACT FROM AUTHOR]

Conyza sumatrensis was reported to be associated with 20 cases of herbicide resistance worldwide, with a recent report of multiple drug resistance to paraquat, glyphosate, and chlorimuron in Brazil. In Paraguay, there were no reports of cases of resistance for this species; however, in 2017, researchers began identifying biotypes with resistance to paraquat, glyphosate, and chlorimuron, which is the focus of the present study. The goal of this study was to investigate the case of multiple resistance of C. sumatrensis to paraquat, glyphosate, and chlorimuron and to monitor the resistant biotypes in the departments of Canindeyú and Alto Paraná. Seeds were collected from sites where plants survived after herbicide application in the 2017/18 and 2018/19 seasons. After screening, biotypes were selected for the construction of dose–response curves. A resistance factor (RF) of 6.79 was observed for 50% control (C50) and 3.92 for 50% growth reduction (GR50) for the application of paraquat. An RF of 12.32 was found for C50 and 4.15 for GR50 for the application of glyphosate. For the application of chlorimuron, an RF of 11.32 was found for C50 and 10.96 for GR50. This confirms the multiple resistance of the C. sumatrensis biotype to paraquat, glyphosate, and chlorimuron. Population monitoring indicated the presence of C. sumatrensis with multiple resistance in departments of Canindeyú and Alto Paraná, Paraguay. [ABSTRACT FROM AUTHOR]