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 Western Australia were screened for herbicide resistance in 1992 and 1993. There was a strong relationship between the number of applications of a herbicide group and development of resistance in ryegrass populations. Resistance was detected in all populations that received >7 applications of aryloxyphenoxypropionate (AOPP) and cyclohexanedione (CHD) herbicides or >4 applications of sulfonylurea (SU) herbicides. Some AOPP-resistant populations had also developed crossresistance to SU herbicides, a group with a different mode of action. Inclusion of pasture in the rotation had little 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-resistant populations to sulfometuron, a nonselective SU herbicide which has been shown to be effective against metabolic-type resistance. All triasulfuron-resistant populations were found to be resistant to sulfometuron, possibly due to insensitive acetolactate synthase (ALS) in these ryegrass populations. Some of these SU-resistant populations were also resistant to the imidazolinone herbicide imazethapyr, another ALS inhibitor. However, there were several populations with a high level of SU resistance that were still susceptible to imazethapyr.