Australia - Ryegrass resistance continues to plague New South West growers

Ryegrass control is continuing to prove problematic for New South Wales grain growers with a recent survey revealing that more than 70 per cent of annual ryegrass populations are resistant to at least one selective herbicide group.

Manager of the Charles Sturt University (CSU) Herbicide Resistance Testing Service, Dr John Broster (pictured), will present the survey results during the Grains Research and Development Corporation (GRDC) Grains Research Update at Dubbo on February 27 and 28.

 Dr Broster said the survey found that resistance was highest to the group A fop and group B herbicides.

 “Of the 597 annual ryegrass populations collected across NSW in the most recent surveys for each region, 497 of these had enough seed to be tested to five selective herbicide groups; A fop, A dim, B SU, B Imi and D,” Dr Broster said.

 “Nearly a quarter of these populations (121) were susceptible to all these herbicides, however not surprisingly, 105 of these were from the regions with the lowest resistance levels.

 “Overall, nearly half of these samples were resistant to three or more herbicide groups, while eight of the populations were resistant to all five herbicides which limits the ryegrass control options for growers."

 According to Dr Broster, there are marked variances in the extent of resistance across the different survey regions in NSW, reflecting differences in rainfall pattern, crop types, rotation and intensity and the prevalence and importance of the various weed species in each region. 

 “The greatest percentage of ryegrass populations showing resistance to the Group A (fop and dim) and B (SU and Imi) herbicides were in the slopes (approximately between Newell and Olympic Highways north of the Sturt Highway), higher rainfall (approximately east of the Olympic Highway) and southern (south of Sturt Highway and east of Newell Highway) regions,” he said.

 “The extent of Group D resistance was highest in the slopes and southern regions, with the other regions having few populations resistant to trifluralin.

 “Among the other regions, the extent of Group A fop and B resistance was only slightly lower in the Plains (approximately east of Newell Highway, and north of Irrigation Way and Kidman Way) region but Group A dim and D resistance was markedly lower.

 “The regions with the lowest levels of resistance to the selective herbicides, the Western (approximately west of Newell Highway, Irrigation Way and Kidman Way) and Northern (north of Peak Hill, Dubbo and Wellington), had the greatest percentage of populations resistant to glyphosate.”

 For the past 10 years, CSU through the Graham Centre for Agricultural Innovation has been conducting annual surveys as part of several GRDC investments, to determine the level of herbicide resistance across the cropping regions of NSW.

 Each year a specific region of NSW is surveyed with the aim of re-surveying each region on a five year cycle.

 Samples of annual ryegrass, wild oats, barley grass, brome grass, wild radish, Indian hedge mustard and wild radish are collected wherever present which enables researchers to assess the differences between regions as to the extent of herbicide resistance and the rate of increase between surveys.

 At the same time, the densities of the collected species and any other species in the paddock are estimated and are classified into five categories - very low (occasional plant), low (<1 plant/m2), medium (1-10 plants/m2), high (>10 plant/m2) and very high (>10 plant/m2 and dominating crop).

 Although the greatest levels of herbicide resistance across NSW are found in ryegrass, test results from some of the other species are also significant.

 The extent of herbicide resistance in wild oats is much lower than in ryegrass with 37% of populations resistant to group A fops across NSW. However, over time the survey work has detected developing resistance to dim herbicides and incidences of resistance to clethodim.

 Eight percent of the 109 barley grass populations screened were resistant to group B SU herbicides with populations also resistant to group A fop and paraquat.

 Forty nine wild radish populations were collected for resistance screening with low levels of resistance found to group B SU (13%), Imi (5%), C (4%) and F (4%).

 Of the 212 sowthistle populations collected, 51% were resistant to group B SU and 2% to group I herbicides. Resistance was highest in the plains region (69%) and lowest in the slopes (43%) with the other regions having similar resistance levels.

 Resistance has been found in 44 Indian hedge mustard populations that have been collected during the resistance surveys with 16% resistant to groups B and 2% to group I herbicides. No populations were resistant to group B Imi, C, F or M.

 “Despite the presence of herbicide resistance in all species, and for some species in the majority of populations, many of the weeds are present only in low densities,” Dr Broster said.

 The two-day Dubbo GRDC Grains Research Update (February 27 and 28) will be held at the Dubbo RSL, Brisbane St, Dubbo.