Resistance of barnyardgrass (Echinochloa crus-galli) to atrazine and quinclorac (original) (raw)

Two populations of Echinochloa crus-galli (R and I) exhibited resistance to quinclorac. Another population (X) exhibited resistance to quinclorac and atrazine. The R and I populations were collected from monocultures of rice in southern Spain. The X population was collected from maize Ðelds subjected to the application of atrazine over several years. The susceptible (S) population of the same genus was collected from locations which had never been treated with herbicides. The quinclorac value (dose causing 50% reduction in shoot ED 50 fresh weight) for the R and I biotypes were 26-and 6-fold greater than for the S biotype. The X biotype was 10 times more tolerant to quinclorac than the S biotype and also showed cross-resistance to atrazine, being 82-fold more resistant to atrazine than the R, I and S biotypes. Chlorophyll Ñuorescence and Hill reaction analysis supported the view that the mechanism of resistance to atrazine in the X biotype was modiÐcation of the target site, the DI protein. Quinclorac at 20 mg litre~1 did not inhibit photosynthetic electron transport in any of the test biotypes. The quinclorac values (herbicide dose needed for 50% Hill I 50 reaction reduction) of the S population was over 50 000-fold higher than the atrazine value for the same S population, indicating that quinclorac is not a I 50 PS II inhibiting herbicide. Propanil at doses greater than 0É5 kg ha~1 controlled all the biotypes.