RESISTANCE OF BARNYARDGRASS (ECHINOCHLOA-CRUS-GALLI) TO ATRAZINE AND QUINCLORAC

Two populations of Echinochloa crus-galli (R and I) exhibited resistan ce to quinclorac. Another population (X) exhibited resistance to quinc lorac and atrazine. The R and I populations were collected from monocu ltures of rice in southern Spain. The X population was collected from maize fields subjected to the application of atrazine over several yea rs. The susceptible (S) population of the same genus was collected fro m locations which had never been treated with herbicides. The quinclor ac ED50 value (dose causing 50% reduction in shoot fresh weight) for t he 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 bi otype and also showed cross-resistance to atrazine, being 82-fold more resistant to atrazine than the R, I and S biotypes. Chlorophyll fluor escence and Hill reaction analysis supported the view that the mechani sm of resistance to atrazine in the X biotype was modification of the target site, the DI protein. Quinclorac at 20 mg litre(-1) did not inh ibit photosynthetic electron transport in any of the test biotypes. Th e quinclorac I-50 values (herbicide dose needed for 50% Hill reaction reduction) of the S population was over 50000-fold higher than the atr azine I-50 value for the same S population, indicating that quinclorac is not a PS II inhibiting herbicide. Propanil at doses greater than 0 .5 kg ha(-1) controlled all the biotypes.