Frequency of glyphosate resistance in herbicide resistant annual ryegrass populations.

There has been several reports of glyphosate resistance in ryegrass in southern Australia. Widespread resistance in annual ryegrass in winter cropping systems has yet to occur. Several populations of ryegrass resistant to the group A (acetyl CoA carboxylase inhibitors) and B (acetolactate synthase inhibitors) herbicides were treated with glyphosate and low levels of resistance are reported. The mean level of resistance in 20 populations was 3% when treated with 1.5 L ha of 540 g a.i. L of glyphosate product. Eighty percent of affected farmers reported annual usage of glyphosate.