Dose-response assays with rice herbicides to screen Echinochloa within-population variability

This work is part of a study carried out by several research units with the aim of classifying Echinochloa species present in Italian rice fields on the basis of morphological and genetic traits and herbicide sensitivity. Echinochloa species are the main weeds infesting Italian rice fields. These plants show a high variability in morphological and competition-related traits, such as size, tillering ability and germination behaviour. This variability makes field identification of different species difficult and uncertain. Recent studies have pointed-out the presence in Italy of Echinochloa populations showing different sensitivity to propanil. In our study, the within-population variability of sensitivity to propanil and other rice herbicides in Echinochloa species has been investigated with greenhouse experiments. Whole-plant bioassays were carried out starting from seeds of 38 accessions collected in paddy fields during summer 2001. The seeds were scarified with sulphuric acid for 30’ and then directly placed on 36 cm2 pots filled with commercial potting substrate. The seedlings were sprayed at 3-4 leaf-stage at four rates (0, 0.5x, 1x, and 2x recommended field rate) of either azimsulfuron, bensulfuron-methyl, bispyribac-sodium, cyhalofop-butyl, molinate, propanil or quinclorac. Aboveground fresh weight per pot was determined 7 or 15 days after spraying, according to the herbicide. The accessions showed a high variability of response to the tested herbicides, especially for azimsulfuron, bispyribac-sodium and molinate. The treatment with bensulfuron-methyl, bispyribac-sodium and molinate at 2x resulted in less than 50% fresh weight reduction in about 10% of the tested accessions. A lower amount of sensitivity variability was found with propanil and quinclorac. These results are in keeping with the variability found on both morphological and molecular traits of Italian Echinochloa populations.