Test now for herbicide resistance to inform future weed strategies

Author: | Date: 08 Oct 2018

image of aaron long
GRDC Crop Protection Officer – South, Aaron Long, says results from herbicide resistance testing will help inform growers’ integrated weed management strategies ahead of the 2019 cropping season and beyond Photo: GRDC.

The weeks prior to harvest or cutting of crops are an opportune time for grain growers to collect weed seeds for herbicide resistance testing.

Determining the status of herbicide resistance provides growers with valuable information on the effectiveness of herbicides on target weeds, potentially preventing the wasteful use of ineffective herbicides and reducing the spread of herbicide resistance.

Grains Research and Development Corporation (GRDC) Crop Protection Officer – South, Aaron Long, says while weeds from any paddock where herbicide resistance is suspected can be tested, priority should be given to testing weeds in high risk paddocks where there is a lengthy history of herbicide use and herbicide survivors have been allowed to set seed.

“Results from testing will help inform growers’ integrated weed management strategies ahead of the 2019 cropping season and beyond,” Mr Long says.

Mr Long says while results from extensive herbicide resistance surveys conducted across the southern cropping region through a GRDC investment provide an indication of the resistance status of regional weed populations, it is important for growers to have a good grasp of the status of herbicide resistance within their own paddocks.

Herbicide resistance researcher Peter Boutsalis, from the University of Adelaide and Plant Science Consulting, says while pre-harvest is the perfect time to be collecting samples of weed seeds for testing levels of resistance, collection can also occur during and after harvest or cutting crops for fodder.

Dr Boutsalis says contaminated grain or header screenings can also be sent for testing as commercial testing services can separate weed seeds from other material.

The method of sampling employed will depend on the resistance situation of each paddock.

“If resistance is widespread, seeds should be collected following a ‘W’ shaped area every 10-20 metres across the suspected paddock or problem area,” Dr Boutsalis says. “Alternatively, collect seeds from suspect areas.

“It is important that growers and advisers do not bias the samples by collecting seeds from a small number of plants – they should instead aim to collect a similar number of seeds from each plant.”

If the seeds are not completely dry, they should be sent in paper envelopes to avoid rotting in plastic packaging.

If growers wish to have ryegrass seed tested, about one cup equivalent of clean ryegrass (about 50 seed heads) is required.

“Where there are lots of ryegrass individuals in the paddock don’t collect from only a few, but try to collect one seed head per plant,” Dr Boutsalis advises.

“For species with larger seeds such as wild oats, brome, barley grass and wild radish, an ice-cream container full is sufficient – this is equivalent to an A4 sized envelope full of seeds.”

Dr Boutsalis says it is important to provide sufficient seed to represent the area of interest: “Sending more seed is better than not enough.”

If a paddock contains more than one suspect area, samples from each area should be tested separately because resistance can vary across a paddock.

Testing three or four herbicides of differing modes of action can be helpful in identifying those herbicides that are still effective.

The GRDC recently announced that resistance to pre-emergent herbicides from a total of three mode of action groups has now been confirmed in Australian annual ryegrass populations.

National weed resistance surveys of growers’ paddocks have identified annual ryegrass populations resistant to the Group D, Group J and Group K herbicides and combinations of these herbicides (more information is available at http://bit.ly/2xRK7mY).

Herbicide resistance is a priority issue for southern region grain growers, as identified by the GRDC Southern Region Panel and the GRDC Southern Regional Cropping Solutions Network (RCSN).

Weed seed resistance testing services are available via:

More information on herbicide resistance and weed management is available via the GRDC’s Integrated Weed Management hub at http://www.grdc.com.au/IWMhub and the GRDC-supported WeedSmart resource centre at http://www.weedsmart.org.au.

Contact Details

For Interviews

Aaron Long, GRDC
0438 647 211


Sharon Watt, GRDC
0409 675 100

GRDC Project code: UCS00024