Herbicide resistance frequencies in ryegrass (Lolium spp.) and other grass species in Tasmania.

In January 2010 a random survey was conducted to determine the frequency of herbicide resistance in populations of Lolium spp. (ryegrass), Avena spp. (wild oat), Bromus diandrus Roth. (brome grass) and Hordeum lepinorum Link (barley grass) across the cropping region of Tasmania, Australia. A total of 84 paddocks were surveyed with 80 containing the aforementioned weed species, resulting in the collection of 76 ryegrass, 16 wild oat, seven brome grass and five barley grass samples. These samples were then screened against the most commonly utilised herbicide groups for annual grass control in Australia. Ryegrass resistance frequencies were highest to aryloxyphenoxypropionate (18%) and sulfonylurea (24%) herbicides with lower incidences of resistance to cyclohexanedione (1%), imidazolinone (7%) and dinitroaniline (1%) herbicides. Sixty four percent of ryegrass samples tested against five herbicide groups (aryloxyphenoxypropionate, cyclohexanedione, sulfonylurea, triazine and dinitroaniline) were susceptible to all herbicides, 27% were resistant to one herbicide only, 7% to two herbicide groups and one sample was resistant to three herbicide groups. In the other species collected, resistance was found only to aryloxyphenoxypropionate herbicides with two wild oat and one barley grass sample exhibiting resistance to herbicides in this group. The frequency of resistance observed in screened populations in this survey is much lower than that found in recent surveys of southern Australian cropping regions. A lower frequency of observed herbicide resistance in Tasmania, combined with our ability to use the knowledge gained from 20 years of herbicide resistance management, should result in a predictably slower spread of herbicide resistant grass weeds than experienced in many other regions of Australia.