Occurrence of glyphosate resistant annual ryegrass (Lolium rigidum) on fence lines of South Australian cropping fields

Glyphosate resistance in annual ryegrass has been reported at 63 fence line sites in Australia. As growers continue to rely heavily on this herbicide to control annual ryegrass on their fence lines, the number of resistant populations is expected to increase. A survey was undertaken to assess annual ryegrass from fence lines in South Australia that had been treated with glyphosate for five or more years. Plants were collected from fence lines and tested for resistance to glyphosate using the Syngenta Quick Test. Of the fifteen fence lines surveyed, ten had annual ryegrass plants with resistance to glyphosate. A dose response experiment conducted on one putative resistant population collected from a fence line in Clare, South Australia, showed 11-fold greater resistance to glyphosate than the susceptible control.