Geographical distribution and resistance level to chlorimuron of Amaranthus spp. populations in the main soybeans producing regions of Brazil

: Background: Species of Amaranthus genus are common in agricultural areas of Brazil. Such weeds are problematic, and they bring complexity to the management mainly due to herbicide resistance. Thus, the monitoring and mapping of chlorimuron-resistant Amaranthus spp. is necessary to detect resistance in different locations of Brazil. Objective : Elaborate discriminating dose of distinction between Amaranthus spp. populations that are susceptible and resistant to chlorimuron and monitor the resistance dispersal throughout five crops. Methods : 33 pigweed populations from the main grain producing properties in Brazil by means of dose-response curves were evaluated. For the D dose, chlorimuron dose of 20 g ha -1 ai was considered. Once the discriminating dose was identified, a monitoring screening of the dispersal of resistance of Amaranthus spp. to chlorimuron was conducted with 226 samples between the 2016 and 2020 crops. Results : The discriminating dose (“base line”) considered ideal to control susceptible plants was 20 g ha -1 of chlorimuron. Among 226 pigweed samples evaluated in the five years of monitoring, 74% of populations were considered susceptible (S), while those classified as resistant (R) and segregating (r) did not exceed 26.0%. Conclusions : By comparing susceptible biotypes of Amaranthus spp. with international scientific literature standards and leaflet averages, it could be safely concluded that the discriminating dose of chlorimuron is 20 g ha -1 . Resistance of Amaranthus spp. to chlorimuron in Brazil is present in the main soybean producing regions evaluated, with a frequency of 26% of the total samples evaluated.