A PREVIOUSLY low-key weed, winter grass (Poa annua) is challenging the herbicide resistance crown held by annual ryegrass, with glyphosate-resistant populations being confirmed in Victoria.
Perhaps considered as being more like Clark Kent than Superman, herbicide resistance testing has shown otherwise with at least 20 populations of winter grass exhibiting resistance to simazine (group C), propyzamide (group D), group B and group Z herbicides.
Another population is resistant to the five herbicide modes of action B, C, D, M and Z.
Plant Science Consulting director and the University of Adelaide researcher Dr Peter Boutsalis, along with reports to the Australian Glyphosate Sustainability Working Group (AGSWG), said winter grass resistance was causing increasing concern.
“While winter grass might look like a soft plant, it has shown it is a force to be reckoned with,” Dr Boutsalis said.
“The levels and extent of resistance have surprised me, and while all the resistant populations have developed on golf courses, it is a warning that any weed can develop resistance.”
“This discovery serves as a timely reminder that any previously insignificant weed can become a huge problem with changes to management or environment.
“Grain growers should remember that present day problem weeds fleabane and sowthistle were not on the radar 20 years ago.”
In the United States, winter grass from golf courses and sports turf has become resistant to groups B, C, D and M herbicides, while in Britain there is at least one population resistant to paraquat (L) from a vineyard.
Winter grass is a genetically diverse winter annual species that also has perennial populations.
A native of Europe, it has now spread around the world, predominantly in temperate countries, but is also found infesting the sub Antarctic Islands of Macquarie and Heard.
It is predominantly a primary colonising weed of disturbed areas and is highly adaptable to heavy grazing and close mowing.
Bayer Crop Science research manager Jyri Kaapro, said “these herbicide resistant populations are very concerning”.
“Many golf courses have now lost most of their post-emergent herbicides for controlling winter grass and now rely on pre-emergent herbicides,” Mr Kaapro said.
“It is causing some turf managers to consider using winter grass as a turf species.”
Winter grass, also known as annual bluegrass in North America, maintains a Jekyll and Hyde relationship with the managers of golf courses.
While some managers try to control winter grass, others have decided they will live with it and make the most of its useful characteristics.
A number of golf courses in Australia and New Zealand have replaced Bent grass greens with winter grass.
In the US, several universities have golf green winter grass breeding programs for trying to select more perennial lines that have reasonable seed production.
Golf greens are intensively managed and this places enormous selection pressure on the plant species present.
Winter grass can set seed under intense mowing regimes and the intensive use of a range of herbicide modes of action has led to this selection of resistant populations.
While more perennial lines tend to develop in cooler climates, there are populations in Adelaide that are becoming perennial and heat-tolerant - a problem with more perennial lines is they produce fewer seeds than annual lines.
The AGSWG is supported by the Grains Research and Development Corporation (GRDC) and key R&D-based crop protection companies with an interest in the sustainability of glyphosate.
The group’s website has a range of information about glyphosate resistance, including a register of glyphosate-resistant weed populations, guides and links for management of glyphosate resistance in different crops and management situations, go to glyphosateresistance.org.au for more information.
For information on herbicide sustainability, visit the WeedSmart information hub weedsmart.org.au.