The Grains Research and Development Corp. of Australia has found widespread multiple herbicide resistant annual ryegrass (Lolium rigidum) in the western part of that country and concludes there are severe management and sustainability issues for graingrowers.

In less than 25 years, ryegrass across the western Australian wheatbelt has evolved from being susceptible to being resistant to many herbicides.

Glyphosate-resistant ryegrass also has been confirmed in both Southern and Northern California.

Researchers collected ryegrass seed from 452 fields within western Australia's 14 million hectares of wheat.

Of those tested, 68 percent were resistant to the Group A herbicide diclofop-methyl and 88 percent were resistant to Group B herbicide sulfometuron.

This was an increase of 20 percent in frequency of resistance, compared with resistance levels surveyed in the same agronomic zone five years earlier.

According to Owen, 64 percent of the ryegrass populations in the current survey displayed multiple resistance to both herbicide groups (A and B).

A concern is that 24 percent of ryegrass populations were developing resistance to trifluralin and 8 percent to clethodim (Select). Both of these herbicides are relied on to control Group A- and B-resistant ryegrass.

On a more optimistic note, most ryegrass populations are still susceptible to the wheatbelt's most popular knockdown herbicide, glyphosate, with less than 1 percent showing resistance, Owen says.

Paraquat, triazines (e.g. atrazine, simazine) and trifluralin also were effective on most ryegrass populations.