A five-year survey of the entire Australian cropping belt has revealed a sobering picture of growing herbicide resistance in key cropping weeds.

Dr John Broster from the Graham Centre for Agricultural Innovation heads up a joint herbicide resistance monitoring project between Charles Sturt University, Queensland Department of Agriculture and Fisheries and the Universities of Adelaide and Western Australia.

In a 5-year cycle the group visited 3000 randomly selected paddocks and took 2000 samples of the nation’s number one weed, ryegrass, which is followed by wild radish in WA, wild oats in Queensland and NSW, and sow thistle in fourth place.

The survey revealed that nationally, about 75 per cent of the populations of ryegrass are resistant to Group A ‘fop’ herbicides, ranging from 99 per cent in WA and 90 per cent in southern NSW around Wagga to much lower levels in western NSW where cropping intensity is a bit lower, and in northern NSW where cropping systems are different.

In the last five-year survey, the level of resistance in ryegrass around Wagga was 70-75 per cent, but John says the numbers can vary as the survey monitors different paddocks.

But the most concerning discovery was the increasing resistance to the most important herbicide, glyphosate. Across Australia glyphosate resistance is 5 per cent in the populations of ryegrass, but in a region like the Liverpool Plains in NSW it was in 29 per cent of the populations that were surveyed.

John says if growers lose glyphosate, they’ll have to use more complex, more expensive methods of controlling weeds.

He says the key to remember is that if you use herbicides, you will get resistance.

By using other methods to control plants that survive herbicide applications and prevent them from setting seed, then growers will slow – maybe not stop, but slow – the development of resistance to enable the use of herbicides for longer.

Paddocks are selected at random for the survey to provide an indication of what herbicides are still working and where, and what are at risk in the future. John says farmers generally are responsible in their management of weeds.

Even though there are high levels of resistance in the ryegrass to selective herbicide groups, and even glyphosate, at harvest time when researchers collect the weed seeds, most populations are below one plant per square metre.

So even though it’s more complicated, he says they’re managing populations quite well using alternative herbicides and other methods than herbicides to control the weed populations.

Further information

Dr John Broster
Herbicide Resistance Screening
Charles Sturt University
Wagga Wagga, NSW
02 6933 4001 or 0427 296 641
jbroster@csu.edu.au

• https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2018/03/a-survey-of-herbicide-resistance-in-key-weeds-across-the-northern-grains-region