Transtasman lessons in glyphosate resistance management?

A new study released this week by the Australian Glyphosate Sustainability Working Group shows 23 of Australia's weed species are at high risk of developing resistance.

The first glyphosate resistant ryegrass population was identified in Australia in 1996. Over the following decade 53 more cases were confirmed and earlier this year the tally was 347. There have also been over 100 situations involving five other weed species that have developed resistance to the herbicide.

The new study shows resistance could become a problem in any weed management situation. Of 200 weed species analysed to determine their innate likelihood to evolve and change in response to continued use of a range of herbicides, one in 10 were found to have a high risk of developing glyphosate resistance.

"Where a species is at high risk of developing glyphosate resistance, it is vital not to rely on a single herbicide," says David Thornby, of Queensland Department of Agriculture, Fisheries and Forestry, one of several bodies involved in the Australian work.

"From a resistance management perspective, it makes sense to devote more time to planning and monitoring, and increasing the range of management options, to species that appear to be at the highest risk of evolving herbicide resistance."

Thornby says it is important to remember there is no relationship between resistance risk and weediness, invasiveness, or ease of control.

"What this study shows is that we need to be thinking about how we use herbicides in every situation, both agricultural and non-agricultural weed control."

The top five highest risk weeds were needle burr, sweet summer grass, Vulpia species, flaxleaf fleabane and liverseed grass. The latter two have already evolved resistance to glyphosate in Australia.

"Luckily almost half of all species assessed are at low risk of resistance, and the remainder at moderate risk," says Thornby.

Australian Glyphosate Sustainability Working Group executive officer Andrew Storrie says the study found many land managers ill-prepared to deal with the loss of herbicides through resistance.

The AGSWG is supported by Government body the Grains Research and Development Corporation and key R&D based crop protection companies with an interest in the sustainability of glyphosate.

New Zealand's first confirmed case of a weed that's developed resistance to glyphosate was announced by the Foundation of Arable Research last week. Poor control of annual ryegrass in a vineyard was reported by a chemical company in 2011. Seed from that population was collected, grown and then tested with varying rates of the herbicide as part of the MPI-funded, Foundation of Arable Research organised Avoiding Glyphosate Resistance project.

FAR CEO Nick Pyke says while the Marlborough find is, to date, an isolated case, it is a warning to users of glyphosate that they need to be aware of the danger of resistance developing and be careful how they use it.

"On-farm the impacts [of resistance] would include reduced income, due to increased chemical costs and reduced crop yields. Removing glyphosate from the suite of available chemicals would also increase the resistance pressure on other herbicides."

Glyphosate is also one of the most environmentally friendly herbicides on the market.

"Environmental repercussions would include the increased use of, and dependence on, less environmentally friendly herbicide options; greater dependence on more intensive cultivation leading to greater degradation of soil structure and soil health; and the risk of some weeds spreading as the cost of controlling them would increase."