Resistance of Euphorbia heterophylla l. Biotypes to ALS enzyme inhibitor herbicides used in soybean cropHerbicides are the main tool for weed control in soybean crop, but the selection pressure attributed to the repeated application of the same herbicides and the same mechanism of action can induce selection of resistant biotypes, such as occurred to Euphorbia heterophylla L. in relation to ALS enzyme inhibitors, in areas of the Paraná and Rio Grande do Sul States, in Brazil. In order to verify possible new cases of resistant populations and to test alternative herbicide treatments to manage this population, seeds of E. heterophylla were collected in the Assis region, State of São Paulo, Brazil, in areas where plants of this species had survived to continuous herbicides application. The trial was carried out in glasshouse conditions where biotypes with a history of suspected resistance were compared with a known susceptible biotype. The study used several post-emergence herbicides sprayed at zero, one, two, four and eight times the recommended field application rates of these products. Twenty days after application, the plants were harvested and the percentage of control and the fresh weight were determined to establish the dose-response curves, to get the resistance factor using data of DL50 and GR50 and to verify if there was a multiple resistance. The resistant biotype showed different resistance levels to chlorimuron-ethyl and imazethapyr, showing cross-resistance to sulfonylurea and imidazolinone groups. Nevertheless, this biotype was efficiently controlled by fomesafen (250 g.ha-1), lactofen (120 g.ha-1), flumiclorac-pentil (40 g.ha-1), ammonium-gluphosinate (150 g.ha-1) and glyphosate (360 g.ha-1).
resistant biotype; susceptible biotype; cross-resistance; imidazolinone; sulfonylurea; Euphorbiaceae; weed
