Population dynamics of Digitaria spp submitted to selection pressure by herbicides in sugarcane crop

The objective of this research was to study population dynamics of the weed crabgrass, genus Digitaria, submitted to selection pressure by herbicides currently applied in sugarcane crops in Brazil. In the first experiment two crabgrass species (Digitaria nuda and Digitaria ciliaris) and eight herbicide treatments applied in preemergence were used, and control percentage was evaluated at 7, 14, and 21 days after herbicide application (DAA). In the second experiment the level of tolerance through dose-response curve was determined for the species D. nuda and D. ciliaris, to the herbicides imazapyr, tebuthiuron, ametryne, and metribuzin. All the herbicides studied were efficient in controlling D. ciliaris, however, for D. nuda the best results were obtained only with ametryne, metribuzin, and isoxaflutole. The relation (T/S) between the rate required to reduce plant dry biomass (GR50) at 21 DAA of D. nuda and D. ciliaris was 16 for imazapyr and 6.3 for tebuthiuron, showing differential susceptibility of species; however for ametryne the rate T/S of 1.1 showed that D. nuda was not tolerant to this herbicide. For metribuzin, at 1.92 kg a.i. ha(-1), reduction of dry biomass was 80 and 90% to D. nuda and D. ciliaris, respectively. Even being controlled by metribuzin, D. nuda presented a higher level of tolerance to this herbicide, what was confirmed by the relationship T/S 14.4. As general conclusion of the research, it can be stated that the species D. nuda is more tolerant to ALS inhibiting herbicides and substituted urea, when compared with D. ciliaris; probably, D. nuda was selected by repetitive use of these herbicides.