HERBICIDE SENSITIVITY OF ECHINOCHLOA CRUS-GALLI POPULATIONS: A COMPARISON BETWEEN CROPPING SYSTEMS.Echinochloa crus-galli populations exhibit high morphological variability and their response to herbicides varies from field to field.  Differential response to herbicides could reflect differences in selection pressure, caused by years of cropping system related herbicide usage.  This study investigates the relation between herbicide sensitivity of Echinochloa crus-galli populations and the cropping system to which they were subjected.  The herbicide sensitivity of Echinochloa crus-galli was evaluated for populations collected on 18 fields, representing three cropping systems, namely (1) a long-term organic cropping system, (2) a conventional cropping system with corn in crop rotation or (3) a conventional cropping system with long-term monoculture of corn.  Each cropping system was represented by 6 E. crus-galli populations.  All fields were located on sandy soils.  Dose-response pot experiments were conducted in the greenhouse to assess the effectiveness of three foliar-applied corn herbicides: nicosulfuron (ALS-inhibitor), cycloxydim (ACCase-inhibitor) and topramezone (HPPD-inhibitor), and two soil-applied corn herbicides: S-metolachlor and dimethenamid-P (both VLCFA-inhibitors).  Foliar-applied herbicides were tested at a quarter, half and full recommended doses.  Soil-applied herbicides were tested within a dose range of 0-22.5 g a.i. ha(-1) for S-metolachlor and 0-45 g a.i. ha(-1) for dimethenamid-P.  Foliar-applied herbicides were applied at the three true leaves stage.  Soil-applied herbicides were treated immediately after sowing the radicle-emerged seeds.  All experiments were performed twice.  Plant responses to herbicides were expressed as biomass reduction (%, relative to the untreated control).  Sensitivity to foliar-applied herbicides varied among cropping systems.  Compared to populations from monoculture corn fields, populations originating from organic fields were significantly more sensitive to cycloxydim, topramezone and nicosulfuron (resp. 5.3%, 5.9% and 12.3%).  Populations from the conventional crop rotation system showed intermediate sensitivity levels.  Contrary to foliar-applied herbicides, the effectiveness of soil-applied herbicides was not affected by cropping system.  Integrated weed management may be necessary to preserve herbicide efficacy on the long term.
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