Drive down weed seed banks with three-pronged attack

Chris Preston encourages growers to adopt and implement a three-pronged attack to drive down weed seed banks. Photo: C Preston

Southern region grain growers are being encouraged to adopt and implement a three-pronged attack to drive down weed seed banks and curb the development of herbicide resistance within their farming systems.

Herbicide resistance expert Dr Chris Preston urges growers to introduce three effective control practices in every crop in the rotation.

“Three weed control tactics per crop, per year, every year is what we need to aim for – if we don’t do this, herbicide resistance will beat us,” said Dr Preston, when speaking at recent Grains Research and Development Corporation (GRDC) Grains Research Updates.

Dr Preston, Associate Professor of weed management at the University of Adelaide, says strategic use of herbicides, non-chemical tactics such as crop competition and harvest weed seed destruction, and choosing the right crop rotations, are among the tools growers can implement as part of a tailored three-point plan for weed control.

“To start with, there are a few strategies involving herbicide management that can be used to reduce herbicide resistance,” Dr Preston said.

“Rotation of herbicides is one strategy, and although it does not stop resistance from occurring, it can delay resistance.

“Resistance is more likely to some modes of action than to others, so resistance can be delayed by using high risk modes of action less often. And sometimes, resistance does not occur to all herbicides from a mode of action, allowing use of one or more herbicides from that group to control resistant weeds.”

In terms of herbicide rates, Dr Preston said both low and high rates of herbicides select for resistance in weeds. High rates will typically select for resistance faster (especially in self-pollinated weeds) because the selection pressure is stronger, while low rates can select for weak resistance mechanisms and can result in resistant populations with more complex mixtures of resistance mechanisms.

“My advice is to use robust rates that provide high levels of weed control.”

Non-chemical tactics, while on their own are generally not as effective or as easy to use as herbicides, can help to reduce the pressure on herbicides and delay resistance. They are likely to be most useful where they help to reduce weed seed set, according to Dr Preston.

“Crop competition is one of these tactics. Increased crop competition can be obtained in numerous ways – by sowing a more competitive crop, sowing a more competitive variety, increasing crop seeding rates, reducing crop row spacing, grading for larger seed for sowing, and east-west sowing of cereals in some regions.”

Dr Preston said recent research in the southern region had shown that early sowing of wheat and sowing hybrid canola instead of open-pollinated canola could both reduce annual ryegrass seed production by up to 50 percent.

“Another set of non-chemical tactics is harvest weed seed control. These tactics – such as chaff carts, narrow windrow burning, chaff lining and mechanical seed destruction – rely on collecting and destroying weed seeds that enter the harvester, and can reduce weed emergence by about 50% in the next season.”

Dr Preston said choosing the right crop rotation, another tactic, allowed growers to “bring in tools for managing troublesome weeds that they wouldn’t otherwise be using”.

“Cereal crops are often the best option for managing herbicide-resistant broadleaf weeds, due to the range of tactics that can be included. Likewise, break crops are often the best option for managing herbicide resistant grass weeds.

“Rotations that have too many cereal crops often become infested with herbicide-resistant grass weeds, and rotations with a high density of pulse crops will have problems with herbicide-resistant broadleaf weeds.”

An important insight from herbicide resistance surveys led by Dr Preston is that having pasture in rotations reduces the risk of herbicide resistance, because most growers use few herbicides in the pasture phase.

“This shows that the major risk factor for developing herbicide resistance is using herbicides,” Dr Preston said. “People with pastures in the rotation are taking the opportunity to not use herbicides, so resistance in those situations is occurring more slowly.”

Data from weed resistance surveys undertaken with investment from the GRDC, University of Adelaide and Charles Sturt University, suggests that most, if not all, grain growers have some herbicide resistance on their farms.

“Australia is better at herbicide resistance than winter Olympic sports – in fact, we’re silver medallists, sitting just behind the United States in terms of the number of weed species with herbicide resistance,” Dr Preston said.

In Australia, there are now 50 weed species with resistance to herbicides from 11 modes of action. Resistance is known in all six States, but South Australia, Victoria and Western Australia tend to have more resistance than other states.

“Herbicide resistance has occurred in all situations where herbicides are used, but is most common and widespread in grain cropping. In fact, herbicide resistance in grass species is typically the most problematic issue in grain production due to the dominance of cereals in cropping rotations.

“In the future we can expect to see more resistance occurring because we’ll continue to use herbicides. But by limiting weed seed production we can better manage herbicide-resistant weeds.

“There are a range of techniques available to growers to achieve this – it’s just a matter of selecting and employing the best tactical recipe for each crop within each farming system.”

Source: GRDC

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