Herbicide resistance levels in annual ryegrass (Lolium rigidum Gaud.) and wild oat (Avena spp.) in southwestern New South Wales

In 2010, a random survey was conducted across the cereal cropping zone of southwestern New South Wales to determine the level of herbicide resistance in annual ryegrass (Lolium rigidum Gaud.) and wild oat (Avena spp.) populations. In total, 192 paddocks were visited resulting in 124 annual ryegrass and 104 wild oat seed samples collected for testing. These samples were then screened to the herbicide groups commonly used for annual ryegrass (ACCase, ALS, triazine, dinitroaniline and glycine) and wild oat (ACCase, ALS, thiocarbamate and glycine) control in Australia. The majority of ryegrass samples were resistant to aryloxyphenoxypropionate (56%) and sulfonylurea (53%) herbicides. High levels of resistance were also found for cyclohexanedione (32%) and imidazolinone (38%) herbicides. This represents an increase compared to the 10-14% detected in a 1991 survey. Resistance was found only within ACCase herbicides in the wild oat; 37% were resistant to a aryloxyphenoxypropionate herbicide and 14% were resistant to a cyclohexanedione herbicide. Thirty four percent of the 110 ryegrass samples tested to five herbicide groups (aryloxyphenoxypropionate, cyclohexanedione, ALS, triazine and dinitroaniline) were resistant to two or more herbicide groups, while multiple resistance was only detected in 8% of wild oat samples tested to four herbicide groups (aryloxyphenoxypropionate, cyclohexanedione, ALS and thiocarbamate).Thirty percent of ryegrass and 62% of wild oat samples were susceptible to all herbicides. Resistance levels for both ryegrass and wild oat in this survey were lower than reported for a 2007 survey to the immediate east of this survey. The levels of resistance found for many herbicides, while lower than the adjoining survey, are still significant and highlight the importance of adopting an integrated approach to weed management. This integrated approach is necessary for minimising resistance to commonly used herbicides, and extending their commercial