Investigating the Probability of Barnyardgrass (Echinochloa spp.) Resistance to Some Common Herbicides in Rice Fields of Iran

The probability of Barnyardgrass resistance to some common rice herbicides used in Gilan, Mazandaran and Fars rice fields was investigated by conducting a series of greenhouse and Petri dish bioassay tests on 37 biotypes collected in 2005-2006. Greenhouse experiments implied screening tests with Petri dish tests implementing dose response assays to determine doses of herbicides inhibiting root and stem length growth by 50% (ID50) as well to determine the sensitivity of biotype to herbicides. Screening tests were carried out for each province separately for a given herbicide using a randomized complete blocks design with four replications. Minimum recommended doses of Butachlor, Molinate, and Thiobencarb added to pots of 8cm water height before 2-leaf growth stage of Barnyard grass. Propanil were sprayed at 3-4 leaf stage. Measurement of dry weight, number of surviving plants and visual rating (based on EWRC rating scale) were made 4 weeks after treatments. A completely randomized design with four replications also used in Petri dish tests. Sensitive biotype was subjected to a range of herbicides doses resulting in 50% inhibition of root and stem growth and the response of biotypes to the ID50 of sensitive biotype was further studied. Root (for Propanil) and stem length (for Butachlor, Molinate, and Thiobencarb) was measured 7 days after treatments. According to results obtained from greenhouse and Petri dish assays, it was revealed that Barnyardgrass biotypes collected from Gilan, Mazandaran and Fars were not resistance to testes herbicides (Propanil, Butachlor, Molinate, and Thiobencarb). The results from Petri dish tests were correlated to those of greenhouse experiments.