Glyphosate-resistant smooth-pigweed (Amaranthus hybridus) in Brazil

Background: Amaranthus hybridus, a common agricultural weed in Brazil, has been associated with complications in the management of major crops. Recently, A. hybridus biotypes with suspected resistance to the herbicide glyphosate have been observed in the state of Parana. Objective: To characterize glyphosate-resistant Amaranthus hybridus biotypes. Methods: Three independent experiments were conducted to evaluate four biotypes of A. hybridus. Twelve doses of glyphosate were applied, including 64D, 32D, 16D, 8D, 4D, 2D, D, 1/2D, 1/4D, 1/8D, 1/16D, and a control with no herbicide. The dose D of glyphosate applied was 1080 g ha(-1) ae. Four replicates for each treatment and a randomized block design were used. At 28 (DAA), the control (%) and residual dry mass of each plot were analyzed. Results: The biotypes with suspected resistance (R1, R2, and R3) had a GR(50) in the range of 3019.13 to 3316.45 g ha(-1) ae and LD50 in the range of 4081.52 to 4310.29 g ha(-1) ae whereas the susceptible biotype had a GR(50) of 227.63 g ha(-1) ae and LD50 of 196.59 g ha(-1) ae. Conclusions: Biotypes R1, R2, and R3 are resistant to the herbicide glyphosate.