Lolium rigidum (annual or rigid ryegrass) is a widespread annual weed in cropping systems of southern Australia, and herbicide resistance in L. rigidum is a common problem in this region. In 2010, a random survey was conducted across the grain belt of Western Australia to determine the frequency of herbicide-resistant L. rigidum populations and to compare this with the results of previous surveys in 1998 and 2003. During the survey, 466 cropping fields were visited, with a total of 362 L. rigidum populations collected. Screening of these populations with the herbicides commonly used for control of L. rigidum revealed that resistance to the ACCase-and ALS-inhibiting herbicides was common, with 96% of populations having plants resistant to the ACCase herbicide diclofop-methyl and 98% having plants resistant to the ALS herbicide sulfometuron. Resistance to another ACCase herbicide, clethodim, is increasing, with 65% of populations now containing resistant plants. Resistance to other herbicide modes of action was significantly lower, with 27% of populations containing plants with resistance to the pre-emergent herbicide trifluralin, and glyphosate, atrazine and paraquat providing good control of most of the populations screened in this survey. Ninety five per cent of L. rigidum populations contained plants with resistance to at least two herbicide modes of action. These results demonstrate that resistance levels have increased dramatically for the ACCase-and ALS-inhibiting herbicides since the last survey in 2003 (>95% vs. 70-90%); therefore, the use of a wide range of integrated weed management options are required to sustain these cropping systems in the future.

Raphanus raphanistrum is a problematic weed, which has become increasingly difficult to control in Australian cropping regions. In 2010, a random survey was conducted across 14 million ha of the Western Australian grain belt to establish the frequency of herbicide resistance in R. raphanistrum and to monitor the change in resistance levels by comparing results with a previous survey in 2003. Screening R. raphanistrum populations with herbicides commonly used to control this weed revealed that most populations (84%) contained individual plants resistant to the acetolactate synthase-inhibiting herbicide chlorsulfuron, whereas 49% of populations also had plants resistant to the imidazolinone herbicides. Resistance to other mode of action herbicides (2,4-D (76%) and diflufenican (49%)) was also common. Glyphosate, atrazine and pyrasulfotole + bromoxynil remained effective on most R. raphanistrum populations. These results demonstrate that resistance to some herbicides has increased significantly since 2003 when the values were 54% for chlorsulfuron and 60% for 2,4-D; therefore, a wide range of weed management options that target all phases of the cropping program are needed to sustain these cropping systems in the future.

Random surveys conducted in the Western Australian (WA) grain belt have shown that herbicide-resistant Lolium rigidum and Raphanus raphanistrum are a widespread problem across the cropping region. In 2010, a random survey was conducted to establish the levels of herbicide resistance for common weed species in crop fields, including the minor but emerging weeds Bromus and Hordeum spp. This is the first random survey in WA to establish the frequency of herbicide resistance in these species. For the annual grass weed Bromus, 91 populations were collected, indicating that this species was present in >20% of fields. Nearly all populations were susceptible to the commonly used herbicides tested in this study; however, a small number of populations (13%) displayed resistance to the acetolactate synthase-inhibiting sulfonylurea herbicides. Only one population displayed resistance to the acetyl-coenzyme A carboxylase-inhibiting herbicides. Forty-seven Hordeum populations were collected from 10% of fields, with most populations being susceptible to all herbicides tested. Of the Hordeum populations, 8% were resistant to the sulfonylurea herbicide sulfosulfuron, some with cross-resistance to the imidazolinone herbicides. No resistance was found to glyphosate or paraquat, although resistance to these herbicides has been documented elsewhere in Australia for Hordeum spp. (Victoria) and Bromus spp. (Victoria, South Australia and WA).