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Resistant ryegrass found in summer crops and further north than ever

Annual ryegrass, the Australian grain industry’s most costly weed, is making an unwelcome appearance in summer crops and extending its range further north in winter crops.

This ‘winter weed’ is germinating in summer crops and fallows in south-eastern Australia, already carrying genes resistant to many common herbicides.

Before annual ryegrass became widespread in new environments, researchers Bhagirath Chauhan, Professor, Queensland Alliance for Agriculture and Food Innovation and Michael Walsh (former) Director Weed Research, Sydney Institute of Agriculture, University of Sydney, wanted to identify alternative control options.

Seed collected from plants adjacent to and within cotton crops near Griffiths, NSW were sown in a pot trial at the University of Queensland, Gatton. The seedlings were tested for their susceptibility to a range of post-emergent herbicides.

“The two populations of summer-emerging annual ryegrass were poorly controlled by glufosinate, glyphosate, haloxyfop and pinoxaden herbicides,” says Dr Chauhan. “Over 75 per cent of the seedlings survived the field rate of glufosinate. Over 80 per cent of the seedlings survived the field rate of glyphosate, having been exposed to additional glyphosate applications in Roundup Ready cotton production.”

“When put together, this suggests that future use of XtendFlex cotton with tolerance to glyphosate, glufosinate and dicamba, may not provide effective control to ryegrass emerging in summer crops.”

Similarly, over 50 per cent of the summer-emerging ryegrass seedlings survived haloxyfop application, threatening the use of this herbicide for grass weed control in fallow situations and early in the cotton season.

“Surprisingly, these populations were fully susceptible to butroxydim, clethodim, paraquat, and paraquat + amitrole,” he says. “Once again, we see the importance of herbicide resistance testing to determine what chemistries will provide the best control of a specific weed population.”

Dr Chauhan is urging cotton and grain growers to be vigilant in scouting for and destroying annual ryegrass plants growing in or near fields in summer. The ability of this remarkably adaptive weed to germinate in summer and in regions previously considered ‘too far north’ is of considerable concern.

“Increased dormancy in annual ryegrass is extending the seasonal range of this weed and the time required to run down the seed bank,” he says. “Preventative measures can slow the spread of this weed in new environments. Early efforts to keep this invasive weed out of grain and cotton fields will save yield and avoid high control costs.”

This trial also tested seed collected from winter-germinating annual ryegrass in Croppa Creek and Wagga Wagga. These two populations were susceptible to a range of herbicides, but this is of little comfort, knowing how quickly resistance can develop in this weed.

Paul McIntosh, WeedSmart northern extension agronomist says annual ryegrass has been identified in several locations across the Darling Downs in Queensland in the last two years, confirming the extended range of this costly weed. Plants have been found in paddocks and roadsides in the Norwin, Warra, Brigalow and Dulacca areas and more, confirming the ability of transported seed to germinate in winter in the northern region.

“The distribution of this weed is still relatively isolated and there is an opportunity for it to be eradicated before it gains a foothold in the north,” he says. “This is a highly competitive weed that we can’t afford to have here. Every effort needs to be made to identify and destroy any ryegrass plants on roadsides, in paddocks and in areas where hay has been fed out to livestock before the ryegrass sets seed.”

The WeedSmart Big 6 is an integrated weed management program that growers can apply across all cropping systems. A diverse approach that includes several chemical and non-chemical tactics places downward pressure on the weed seed bank and reduces herbicide resistance risk.

Download the research paper.

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