Non-target-site-based resistance to ALS-inhibiting herbicides in six Bromus rigidus populations from Western Australian cropping fields.

Background: Bromus rigidus is a common weed species that has increased in cropping fields owing to limited control options. During a random field survey in Western Australia, six B. rigidus populations that had survived in-crop weed control programmes were collected. The study aimed to determine the resistance profile of these six populations.

Results: Based on dose-response studies, all six B. rigidus populations had a low-level resistance to sulfosulfuron and sulfometuron (both sulfonylurea herbicides) while remaining susceptible to herbicides with other modes of action. ALS in vitro activity assays revealed no differences in enzyme sensitivity between susceptible and resistant populations, while the use of malathion (a cytochrome P450 inhibitor) in combination with sulfosulfuron caused the resistant populations to behave like the susceptible population.

Conclusion: This study established that these six B. rigidus populations have a low-level resistance to the ALS-inhibiting sulfonylurea herbicides, but are able to be controlled by other herbicide modes of action. The low-level, malathion-reversible resistance, together with a sensitive ALS, strongly suggest that a non-target-site enhanced metabolism is the mechanism of resistance.