Penoxsulam-resistant barnyardgrass (Echinochloa crus-galli) in rice fields in China

Barnyardgrass (Echinochloa crus-galli) proliferation seriously threatens rice production worldwide. Whole-plant bioassays were conducted in order to test the sensitivity to penoxsulam of 52 barnyardgrass populations and the resistance of six penoxsulam-resistant populations to 12 other herbicides that are commonly used in rice fields. Among the 48 populations that had escaped penoxsulam control in the rice fields, 8.3% showed a very high level of resistance, 58.3% showed a high level of resistance and 10.4% showed a moderate level of resistance. Multiple resistance was confirmed in all six penoxsulam-resistant populations that were tested further. They exhibited at least a moderate level of resistance; that is, to 6-10 of the total of 13 herbicides that was tested. Most of the six penoxsulam-resistant populations showed at least a moderate level of resistance to bispyribac-sodium, quinclorac, metamifop, cyhalofop-butyl and oxadiazon, three populations held at least a moderate level of resistance to oxyfluorfen and pretilachlor, two populations also held at least a moderate level of resistance to pyrazosulfuron-ethyl, pyribenzoxim and fenoxaprop-P-ethyl, but the resistance indices of the six populations to pendimethalin were all low. This study has confirmed resistance to pretilachlor and oxadiazon in weeds for the first time.