Herbicide Resistance Levels in Annual Ryegrass ('Lolium rigidum' Gaud.) in Southern New South Wales

In 2007 a random survey was conducted across the cereal cropping zone of southern New South Wales to determine the level of herbicide resistance in annual ryegrass populations. In total, 181 paddocks were visited resulting in 137 samples of annual ryegrass seed collected for testing. These samples were then screened to the commonly used herbicide groups (A, B, C, D and M) for annual ryegrass control in Australia. The majority of samples were resistant to Group A 'fop' (81%) and Group B 'SU' (70%) and 'Imi' (65%) herbicides. These represented increases from the 10-14% experienced in the previous survey in 1991. Seventy six percent of the 117 samples tested to five herbicide Groups (A 'fop, A 'dim, B, C or D) were resistant to two or more of the herbicide groups tested and only 9% were susceptible to all herbicides. Of particular interest is the minimal increase in resistance to simazine and trifluralin in the 16 years since the last survey, and only low incidence of glyphosate resistance was identified in this survey. The rapid increase in the incidence of herbicide resistance in annual ryegrass, particularly in Group A and Group B herbicides, highlights the importance of adopting an integrated approach in weed management. This integrated approach is also necessary for maintaining the low level of resistance in Groups C, D and M, and extending the commercial life of these effective chemicals.