Weed herbicide resistance is a phenomenon defined as a response reduction of a population to a chemical product sprayed at a recommended rate, as a result of its continuous application. The incidence of herbicide-resistant weeds in Brazil has increased lately, but few scientific studies have been conducted to elucidate the phenomenon. In this regard, experiments aiming to build rate-response curves comparing a resistant (R) and a susceptible (S) biotype of the weed Bidens pilosa L., to ALS inhibitor herbicides, were set up. At the stage of three to four leaves, biotypes R and S of B. pilosa were sprayed with the herbicides chlorymuron-ethyl, metsulfuron-methyl, nicosulfuron and imazethapyr, at multiples of 0.0; 0.001, 0.01, 0.1; 1.0; 10; 100 and 1,000 of the recommended field rates. Results lead to the building of percentage control rate-response curves. The ratio between the C50 of the resistant biotype, divided by the C50 of the susceptible biotype, were 40.92; 173.84; 57.47 and 57.16 for the herbicides chlorymuron-ethyl, nicosulfuron, metsulfuron-methyl and imazethapyr, respectively. The R biotype of B. pilosa had a high degree of resistance in relation to all ALS inhibitor herbicides studied, being also cross resistant to imidazolinone and sulfonylurea.
weed; herbicide resistance; sulfonylurea; imidazolinone
