Conyza spp. is among the main weeds reported worldwide. Due to its aggressiveness, such as high seed production and dispersion, and the growing reports of biotypes resistant to glyphosate, paraquat, and other herbicides, different control practices are required. Thus, this study aimed to evaluate the efficacy of herbicides with sequential application of glufosinate in soybean pre-sowing for control of Conyza spp. with indicative of resistant to paraquat. The study was carried out in the field, at Assis Chateaubriand and Palotina, state of Paraná, Brazil, in the 2018/19 season. The experiments were conducted in a randomized block design with four replications. The treatments consisted in application of glyphosate, 2,4-D, saflufenacil, glufosinate, saflufenacil/imazethapyr, diclosulam, paraquat/diuron, paraquat and imazethapyr/flumioxazin, at different combinations, in soybean pre-sowing. Control of Conyza spp., crop injury to soybean plants and variables related to agronomic performance (plant height and yield) were evaluated. All treatments were selective for soybean, which showed stronger crop injury in the presence of diclosulam herbicide, but this did not compromise soybean agronomic performance. In general, control levels were high for the treatments used. Except for paraquat treatments, in the area with the highest frequency of Conyza spp. with indicative of resistant to paraquat, and imazethapyr/flumioxazin treatment in both areas. These control results emphasized the importance of glufosinate in this management system and showed promising results for saflufenacil/imazethapyr.

No abstract

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.

No abstract

Sourgrass (Digitaria insularis) is one of the main weeds in the soybean crop. In order to control its growth, an increase of herbicide rates is required to simplify its management as it a plant with high vegetative capacity and seed production. It implies to select the herbicide-resistent Digitaria insularis biotypes. Nevertheless, some information is still contrasting the antagonist of synthetic auxinic herbicides, associated with glyphosate and ACCase inhibitors mixtures, for the control of weeds resistant or tolerant to herbicides. This study aimed to evaluate the D. insularis control, with a mixture of herbicides applied in soybean pre-emergence, with sequential application in soybean post-emergence, and to check possible antagonism between ACCase inhibitors herbicides with synthetic auxins and other latifolicides. The experiment was conducted in Palotina, Paraná (Brazil) and Corpus Christi, Canindeyú, (Paraguay.) The treatments consisted of associations of glyphosate, ACCase inhibitors (clethodim, haloxyfop), and latifolicides (2,4-D, triclopyr, dicamba, carfentrazone, saflufenacil, chlorimuron). A randomized block design was used. Only in Palotina, the weed control was satisfactory after sequential application in post-emergence. An antagonism for all synthetic auxins was observed with glyphosate+clethodim or haloxyfop mixtures, in both locations. As a result, in Palotina an efficacious control of perennial D. insularis was found in pre-emergence burndown for some mixtures such as glyphosate+ACCase inhibitor added to carfentrazone, saflufenacil, or chlorimuron. Antagonism was observed for all synthetic auxins, in both locations. In Corpus Christi, the herbicide associations were not effective, even with the postemergence application in soybean of glyphosate+clethodim. With ineffective control for treatments composed with synthetic auxins, the post-emergence application in soybean increased the weed control with satisfactory final controls for all treatments.