Ryegrass resistance continues to spread

Author: | Date: 25 Sep 2017

A profile picture of John Broster

Caption: Dr John Broster of the Charles Sturt University Graham Centre for Agricultural Innovation said each type of resistance that develops on-farm means there is one less tool available to the grower to manage weeds.

The most recent herbicide resistance surveys of New South Wales have shown very high levels of A ‘fop’ and B resistance in annual ryegrass across northern areas of the state and west of the Newell Highway to Narrandera.

The surveys, an investment by the Grains Research and Development Corporation (GRDC) and undertaken by Charles Sturt University (CSU), through the Graham Centre for Agricultural Innovation, aim to identify and map the herbicide resistance status of key weed species in the northern agriculture region.

Manager of the CSU Herbicide Resistance Testing Service, Dr John Broster said herbicide resistant weed populations are now found throughout all cropping areas of Australia, with at least 40 weed species in Australia with resistance to one or more herbicide modes-of-action (MOA).

“The survey work targeted common sowthistle, fleabane, wild turnip, African turnip weed, wild oats, feathertop Rhodes grass, awnless barnyard grass, sweet summer grass, liverseed grass, windmill grass, annual ryegrass, wild oats and other Brassica species,” he said.

“All commonly used herbicide MOA were screened, and we took over 530 samples for the 2016 surveys.

“Across all surveys in NSW since 2013, ryegrass resistance was the highest to the Group A ‘fop’ herbicides at 64% of samples, followed by the Group B’s, both the ‘SUs’ with 56% and ‘Imis’ with 48% of samples resistant.”

At a more regional level, the surveys showed in northern NSW the incidence of resistance is lower with 44% of ryegrass samples resistant to the Group A ‘fops’, 29% to the ‘SU’ and 33% to the ‘Imi’ herbicides.

However, in the same region there was a higher incidence of resistance to the Group M herbicide glyphosate (Roundup®) with 10% of samples resistant compared with 2% for the rest of NSW.

For the area west of the Newell Hwy 71% of samples were resistant to the Group A ‘fops’, 47% to the ‘SU’ and 44% to the ‘Imi’ herbicides. This was an increase from 49%, 42% and 32% respectively for these three herbicide groups in the 2010 survey of the same region.

“Since 2013, 497 samples have been tested for multiple or cross resistance to ‘fop’, ‘dim’ ‘SU’ ‘Imi’ and D herbicides.  Of these 31% are resistant to three of these groups, 11% to four and 2% to all five.  Less than one quarter of these samples were susceptible to all of the five groups,” Dr Broster said.

“This means that many growers no longer have the full range of herbicides available for use against annual ryegrass and this may be impacting their cropping systems by limiting rotational options.

“As well as the high levels of resistance in annual ryegrass in the surveys conducted since 2013 high levels of ‘fop’ resistance in wild oats, and ‘SU’ resistance in sow thistle were also found.”

Testing to determine the resistance status of the weed species, other than ryegrass, collected in the 2016 surveys is still underway.

“Essentially we have discovered some resistance was found in every species we have collected, which really emphasises that the number of weed species with resistance and the areas affected are increasing,” he said.

General survey data is used to prepare large scale maps, without identifying individual samples or properties, to show the distribution of resistant weed populations across Queensland and NSW to help highlight priority issues and ensure they are addressed.

The GRDC will continue to invest in ongoing herbicide resistance surveys and testing to ensure affected areas and weed species data remains up to date.

Resistance develops when repeated applications of the same MOA herbicide kill susceptible weeds, and resistant weeds survive and set seed.

Dr Broster said each type of resistance that develops on-farm means there is one less tool available to the grower to manage weeds.

“Where multiple resistance exists, effective herbicide options become severely limited, and we can see through the surveys that this issue continues to spread and increase,” he said.

“It’s important that growers help stop resistance. The best way to stop resistance developing is to prevent any survivors of a herbicide application from setting seed and germinating.

“This is rarely achievable with a single weed management tactic, so growers need to use integrated weed management (IWM) - a range of chemical, non-chemical and agronomy tactics - for optimal weed control.”

More information on the Herbicide Resistance Surveys undertaken by CSU via the CSU website.

Growers can read more about IWM and Herbicide Resistance via the website.

Contact Details

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