Resistência cruzada da losna-branca (Parthenium hysterophorus) aos herbicidas inibidores da enzima acetolactato sintase

Weed control using herbicide application is a common agricultural practice. However, the application of the same herbicide or herbicides with the same mechanism of action, for consecutive years, in the same area, can result in the selection of herbicide resistant biotypes. The aim of this work was to confirm the resistance of a ragweed (Parthenium hysterophorus) biotype to acetolactate synthase (ALS) inhibiting herbicides. The plants were collected on a farm in Mandaguari, north of Parana State, Brazil. Plants with suspicious resistance were treated with several herbicides and rates and compared with those of a susceptible population. The herbicide treatments were established considering the recommended rates, double and four times higher than the recommended rate as follows: cloransulam-methyl 0.0, 33.6, 67.2 and 134.4 g a.i. ha-1 plus adjuvant 0.2% v/v, chlorimuron-ethyl 0.0, 20.0, 40.0 and 80.0 g a.i., imazethapyr 0.0, 100.0, 200.0 and 400.0 g a.i. ha-1, iodosulfuron-methyl-sodium plus foramsulfuron 0.0, 3.0 + 45.0 ga.i. ha-1 (150.0 g c.p. ha-1), 6.0 + 90.0 g a.i. ha-1 (300.0 g c.p. ha-1) and 12.0 + 180.0 g a.i. ha1 (600.0 g c.p. ha-1). In addition, a treatment with 2,4-D (536.0 g a.e. ha1) was applied. Resistant plant dose-response curves presented lower values when compared to the susceptible population, in all rates and herbicides studied. The ragweed biotype was confirmed as resistant to the ALS inhibiting herbicides. Cross-resistance was observed with herbicides belonging to the chemical groups of imidazolinones (imazethapyr), triazolopyrimidines (cloransulam-methyl), sulfonylureas (chlorimuron-ethyl and iodosulfuron-methyl-sodium plus foramsulfuron). 2,4-D has a different mechanism of action, presenting high values of control, and thus being a management alternative in areas with ragweed resistant population.