Winter grass resistance to glyphosate makes it a weed to watch

Bent grass on a golf green infested with a mixture of herbicide resistant (green) and susceptible (yellow) winter grass – is it ready to leap the fence into crops Image: Jyri Kaapro, Bayer Crop Science

Winter grass (Poa annua), once considered a low-key weed, is coming under increasing scrutiny, with glyphosate-resistant populations being confirmed in Victoria. Dr Peter Boutsalis, from the University of Adelaide says the weed has emerged as a force to be reckoned with.

Testing has shown at least 20 populations of winter grass exhibiting resistant 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. As such, it has the potential to challenge annual ryegrass as the most herbicide resistant.

“While all the resistant populations have developed on golf courses, it is a warning that any weed can develop resistance,” says Dr Boutsalis who reports to the Australian Glyphosate Sustainability Working Group (AGSWG).

He said grain growers should remember that present day problem weeds fleabane and sowthistle were not on the radar 20 years ago.

Bayer Crop Science research manager Jyri Kaapro, describes herbicide resistant populations as “very concerning”, with many golf courses now having lost most of their post-emergent herbicides for controlling winter grass and now rely on pre-emergent herbicides.

“It is causing some turf managers to consider using winter grass as a turf species,” Mr Kaapro says.

While some managers try to control winter grass, others have decided to 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.

Winter grass can set seed under intense mowing regimes, and the intensive use of a range of herbicide modes of action has led to a selection of resistant populations.

For information on herbicide sustainability, visit the WeedSmart information hub at: