The extent of herbicide resistance in Lolium rigidum Gaud. (annual ryegrass) across south-eastern Australia as determined from random surveys.

Context: Annual ryegrass (Lolium rigidum) is a major weed of crop production in southern Australia that readily develops resistance to herbicides. Resistance increases both yield losses and control costs associated with this species. Aims: This study aimed to gauge the extent and distribution of resistance to herbicides in L. rigidum across south-eastern Australian grain production systems by collecting seed from randomly selected fields. Methods: A total of 1441 weed populations were collected through random surveys conducted over 5 years across 13 agricultural regions of four states with these samples then tested for resistance to eight herbicides from six modes of action. Key results: Resistance to diclofop-methyl and sulfometuron-methyl was most common, being present in 64% and 63% of populations respectively. Glyphosate resistance was present in 4% of populations collected. Only 15% of populations collected were susceptible to all herbicides tested. Large differences in resistance occurred between the 13 regions surveyed with resistance to diclofop-methyl ranging from 15% to 86% of populations and sulfometuron-methyl from 12% to 96%. Resistance to post-emergent herbicides tended to be higher than pre-emergent herbicides. Multiple resistance was common with 60% of populations collected having resistance to two or more herbicide modes of action. Conclusions: There were significant differences in the extent of multiple resistance in L. rigidum populations collected from individual regions suggesting that the rates of resistance evolution have differed between regions. Implications : The high incidence of herbicide resistance in L. rigidum populations randomly collected from south-eastern Australian cropping fields highlights the need for the adoption of additional weed control practices to mitigate the impact of this species on grain production systems. Herbicide resistant weeds are a major threat to crop production, with annual ryegrass being the major species of concern in Australia. Surveys of south-eastern Australia conducted over a 5 year period found over 60% of populations were resistant to two or more herbicide groups and only 15% of ryegrass populations were susceptible to all tested herbicides. The high level of resistance found across south-eastern Australia highlights the need for farmers to adopt non-herbicide practices, in conjunction with herbicides, to control this species. [ABSTRACT FROM AUTHOR]