Recent research: Herbicide-resistant ryegrass in southeastern Australia

Herbicide resistance in annual ryegrass (Lolium rigidum) is a growing problem in grain-cropping fields of southeastern Australia because of increased herbicide use as the main weed control method in minimum tillage systems. With thousands of fields infested with herbicide-resistant annual ryegrass, it is the most important weed species of winter grain crops in Australia. This study focused on identifying the scale of the problem in South Australia and Victoria, using annual ryegrass collected during weed surveys between 1998 and 2009.

Outdoor pot studies were conducted during the normal winter growing season for annual ryegrass with PRE-applied trifluralin and POST-applied diclofop-methyl, chlorsulfuron, tralkoxydim, pinoxaden, and clethodim.
•    Trifluralin-resistant annual ryegrass was found in one-third of the fields surveyed from South Australia and in less than 5% of fields in Victoria.
•    Chlorsulfuron-resistance was detected in at least half of the cropped fields across southeastern Australia.
•    Resistance to the cereal-selective aryloxyphenoxypropionate-inhibiting herbicides diclofop-methyl, tralkoxydim, and pinoxaden ranged between 30 and 60% in most regions, but in marginal cropping areas it was less than 12%.
•    Resistance to clethodim varied between 0 and 61%. Higher levels of resistance to clethodim were found in the more intensively cropped, higher-rainfall districts where pulse and canola crops are common.

Fields in the survey fell into 2 groups: those with ryegrass resistant to 0-1 herbicides, which tended to be more common in areas where a lot of pasture was still grown, and those with resistance to 4-6 herbicides, which tended to be in areas of continuous cropping. These weed surveys demonstrate that a high incidence of resistance to most tested herbicides is present in annual ryegrass from cropped fields in southeastern Australia and presents a major challenge for crop producers.

Corresponding author: Dr Peter Boutsalis
Organisations: School of Agriculture, Food and Wine, University of Adelaide
Publication: Boutsalis, P., Gill, G.S. and Preston, C. (2012) Incidence of herbicide resistance in rigid ryegrass (Lolium rigidum) across southeastern Australia. Weed Technology, 26(3): 391-398.
Link: doi: 10.1614/WT-D-11-00150.1

“Recent research” is a series of short, regular posts highlighting recent research papers from the Waite Campus.

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