Sowing time delivers competitive edge on weeds

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Photo of weed expert Chris Preston

Weed expert Chris Preston has been investigating how crop competition and pre-emergent herbicides can be effectively combined to tackle weeds.

PHOTO: Alistair Lawson

New research into pre-emergent herbicides suggests northern New South Wales wheat growers could substantially lift the effectiveness of their weed-control programs by sowing the crop on time – within its optimum sowing window.

 The research focus on pre-emergent herbicides as a tool for controlling hard-to-kill weeds has two main drivers. These are weed populations that are developing resistance to post-emergent herbicides, and the need for an integrated approach to weed management to help extend the lifespan of herbicide chemistries.

Adelaide University’s associate professor of weed management, Dr Chris Preston, is leading the research that aims to improve weed management where pre-emergent herbicides are the only option for grass weed control.

He says the GRDC-supported work has highlighted two key opportunities for reducing weed-seed set. Specifically, early sowing to improve the competitiveness of crops against weeds, and pre-emergent herbicide use. “While it has long been the mantra that weedy paddocks should be sown last to allow an extra knockdown herbicide application, this means wheat grows slower as the soil temperature decreases heading into winter. So it takes more time for crop canopy closure and weeds are given a better chance to use resources,” Dr Preston says.

“Given that use of post-emergent herbicides is often unavailable due to resistance, you have to ask, ‘is this still the best strategy?’ Essentially, our research shows that it is not.”

He says the research has found that early-sowing wheat with a pre-emergent herbicide package can control annual ryegrass as effectively as later-sowing the crop and using an additional knockdown herbicide application.

“For wheat, time of sowing is very important. You’re better off sowing early with a robust pre-emergent herbicide package than wasting time trying to get rid of all weeds prior to sowing and missing the ideal sowing window.”

The research data has shown that early-sown wheat is more competitive against weeds and tends to reduce the number of weed-seed heads produced per plant.

When early sowing of wheat was coupled with effective pre-emergent herbicide packages it allowed for greater competition against grass weeds, as well as increased wheat yields and limited weed-seed set. “Similar trials with hybrid canola varieties again showed this competition method to be very effective, providing an opportunity to reduce annual ryegrass seed-set by about 50 per cent compared with open-pollinated varieties,” Dr Preston says.

He says early sowing has the following advantages:

  • early and vigorous crop growth in warm soil;
  • high plant biomass and a more competitive crop to shade and better compete with any remaining weeds; and
  • higher grain yields in paddocks usually limited by weed numbers or later time of sowing.

However, he emphasised that the trials showed it was not just time of sowing that was critical for effective weed management.

“Timing is a critical part of weed management – time of sowing, time of rain and time of herbicide application,” Dr Preston says.

He advised growers to plan carefully before using pre-emergent herbicides to help ensure the chemicals are applied before weeds emerge, close to weed seed and when moisture conditions can activate the herbicide and prevent potential crop injury.

Incorrect application can fail to control some weeds early in the season and when not managed they can set a large amount of seed.

Dr Preston says in instances where post-emergent herbicides are still effective, they can be used to manage weeds that escape control, and while the value of crop competition would be less obvious, it still adds to the overall level of weed control. “It’s about an integrated weed-management approach; the question should be, not so much do I need to employ crop competition, but how can I do that in a practical way.”

More information:

Dr Christopher Preston, University of Adelaide,
08 8313 7237,

Useful resource

Preemergent Herbicide Use Fact Sheet


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GRDC Project Code UCS00020

Region North