In the soybean crop, the weeds Bidens pilosa/Bidens subalternans and Amaranthus quitensis are controlled by several herbicides, with the acetolactate synthase (ALS) inhibitors being the ones most sprayed by growers. The intensive and repetitive use of these herbicides in soybean areas of São Gabriel do Oeste (Mato Grosso do Sul - Brazil) and in the provinces of Córdoba and Tucumã (Argentina) have imposed a great selection pressure on the resistant population of these weeds. In order to determine the cross resistance of these biotypes to ALS - inhibiting herbicides (sulfonylureas and imidazolinones), an experiment was conducted under greenhouse conditions at the Escola Superior de Agricultura "Luiz de Queiroz", Universidade de São Paulo, Piracicaba, São Paulo, Brazil. The weeds Bidens pilosa/Bidens subalternans and Amaranthus quitensis, whose seeds were collected from suspected sites of herbicide resistance, were treated by the herbicides chlorimuron-ethyl, imazethapyr and nicosulfuron at several rates, and compared to plants whose seeds were obtained from susceptible populations. The treatments were established considering rates giving, at least, 50% of control (GR50), using multiples of the recommended rates. Chlorimuron-ethyl was sprayed at 1500, 750, 150, 75, 15, 7.5, 1.5 g a.i. ha-1, imazethapyr at 8000, 4000, 800, 400, 80, 40, 8 g a.i. ha-1 and nicosulfuron at 1.200, 600, 120, 60, 12, 6, 1.2 g a.i. ha-1. The resistant biotype showed different levels of cross-resistance to the herbicides used. The resistant biotype of Bidens pilosa/Bidens subalternans showed a GR50 for chlorimuron-ethyl, imazethapyr and nicosulfuron of 1.49, 1.27 and 20.08 g a.i. ha-1, and for Amaranthus quitensis 6.8, 2.45, and 23.54 g a.i. ha-1, respectively. The dose-response curves of the weed control, at 14 days after treatments of the resistant biotypes consistently showed lower values than the susceptible biotypes, even at extremely high rates. The resistance levels, measured by the relation GR50(resistant)/GR50 (susceptible) for Bidens pilosa/Bidens subalternans were 9.9 for chlorimuron-ethyl; 9.07 for nicosulfuron and 27.03 for imazethapyr, whereas for Amaranthus quitensis the values were 45.03 for chlorimuron-ethyl, 181 for nicosulfuron and 24.55 for imazethapyr.
Bidens pilosa; Bidens subalternans; imidazolinone; sulfonylurea
