Four Australian populations of the increasingly widespread annual sub-tropical weed feathertop Rhodes grass (Chloris virgata) have been confirmed resistant to the key herbicide glyphosate.
Feathertop Rhodes grass is yet another species that has increased its abundance during the past 10 years, largely due to widespread adoption of no-till cropping and the shift to glyphosate-based weed control on road verges.
‘‘We have now confirmed that two populations from cropping land in NSW and Queensland and two from roadsides in South Australia are not controlled with glyphosate at the seedling stage and therefore are classified as resistant,’’ Australian Glyphosate Sustainability Working Group (AGSWG) chair Dr Chris Preston said.
‘‘Glyphosate is normally effective on actively growing seedlings, however, once feathertop Rhodes grass begins to tiller it is tolerant of very high rates. Again this is another unwanted world first for Australia,’’ Dr Preston said.
While the weed is not listed on any glyphosate herbicide labels, glyphosate has been widely used in Queensland and northern NSW to control seedlings.
Feathertop Rhodes grass has been found across Australia for decades as a weed of roadsides, fence lines and unmanaged land, especially in summer rainfall areas and irrigated agriculture.
During the past 15 years it has become a major cropping weed in Queensland and northern NSW as well as horticultural plantings such as vineyards.
It is also dominating many roadsides across southern Australia.
The success of feathertop Rhodes grass is due to the rapid production of large numbers of seed that are easily shed from the heads. Seed germinates if left on the soil surface, with sufficient moisture and temperatures above 25°C.
Seed banks appear to be short-lived at around 12 months and burial of seed at any depth prevents germination.
Management strategies need to involve a range a tactics aimed at stopping the production of any fertile seed.
‘‘This poses significant challenges on roadsides where most road managers have opted for glyphosate as the main strategy,’’ Dr Preston said.
‘‘A shift to grass-selective Group A herbicides without a robust second knock will lead to the rapid development of Group A resistance in this species.’’
For information on herbicide sustainability visit www.weedsmart.org.au