DEVELOPMENT OF HERBICIDE RESISTANCE IN ANNUAL RYEGRASS POPULATIONS (LOLIUM-RIGIDUM GAUD.) IN THE CROPPING BELT OF WESTERN-AUSTRALIA

Annual ryegrass (Lolium rigidum) samples from the cropping belt of Wes tern Australia were screened for herbicide resistance in 1992 and 1993 . There was a strong relationship between the number of applications o f a herbicide group and development of resistance in ryegrass populati ons. Resistance was detected in all populations that received greater than or equal to 7 applications of aryloxyphenoxypropionate (AOPP) and cyclohexanedione (CHD) herbicides or greater than or equal to 4 appli cations of sulfonylurea (SU) herbicides. Some AOPP-resistant populatio ns had also developed cross-resistance to SU herbicides, a group with a different mode of action. Inclusion of pasture in the rotation had l ittle effect on the relationship between the number of applications of the AOPP and SU herbicides and development of resistance. A subset of 33 populations was chosen to determine the response of triasulfuron-r esistant populations to sulfometuron, a nonselective SU herbicide whic h has been shown to be effective against metabolic-type resistance. Ai l triasulfuron-resistant populations were found to be resistant to sul fometuron, possibly due to insensitive acetolactate synthase (ALS) in these ryegrass populations. Some of these SU-resistant populations wer e also resistant to the imidazolinone herbicide imazethapyr, another A LS inhibitor. However, there were several populations with a high leve l of SU resistance that were still susceptible to imazethapyr.