Like any weed management system, the use of harvesters, chaff managing implements and other methods to control weed seed collected during grain harvesting has drawbacks in terms of developing weed resistance.

Research agronomist Mike Ashworth said there was evidence that long-term, consistent use of harvest weed seed control could result in patches of weeds that possess an evasion trait, such as the ability to shed their seeds early, before harvest.

“It takes a long time, but all weed control tactics apply selection pressure to weed populations for any trait that allows individuals to evade or survive the control measure, allowing them to proliferate,” Dr Ashworth said.

“We have studied a population of wild radish plants that sheds its seed pods before wheat harvest and distinctly earlier than other wild radish populations in the area, in what seems like a response to 20 years of consistent and effective harvest weed seed control.”

Harvest weed seed control provides excellent control – up to 90 per cent – of many key weeds in Australian grain systems, including annual rye-grass, wild radish, wild oats and brome grass.

Although these species are all well suited to HWSC, variability in seed dormancy, early flowering and early seed or pod shedding can enable a portion of the population to avoid being collected by the header at harvest.

These seeds are then able to carry this trait into the next generation.

“Diversity is always the answer,” Dr Ashworth said.

“If patches of weeds are flowering early, chances are they are also germinating early. This means early weed control and strong crop competition will help keep weed numbers low.

“There may also be an opportunity to choose crops or varieties that mature early, allowing the harvest weed seed control tool to do its job on these early populations.”

A study in WA found that 86 per cent of wild radish pods on one property had fallen to the ground before harvest, compared to zero to three per cent on neighbouring farms and along roadsides.

The selection pressure had enabled a shift in the population towards the early shedding trait.

WeedSmart is an industry voice delivering science-backed weed control solutions to enhance on-farm practices and promote the long-term, sustainable use of herbicides in Australian agriculture and has support from GRDC, major herbicide, machinery and seed companies, and university and government research partners.

WeedSmart recommends doing ‘everything you can’ as early as possible to drive down the weed seed bank when weeds first emerge, including spraying small weeds twice.

Farmers are also urged to look for and record changes in weed growth and maturity.

For further information about weeds evading control methods, visit: https://www.facebook.com/WeedSmartAU/ or X at @WeedSmartAU

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Wednesday, 26th March 2025

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