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Crop competition a key to suppressing weeds in Victoria's HRZs

Sowing into the wheel track using a single disc located behind the wheel of the seeder in a controlled traffic farming system is one way to promote improved crop competition and suppress weed growth.
Photo: GRDC

Growers advised to get sowing strategies right to help crops combat weeds this season.

In a region where more than half of annual ryegrass seed can be shed prior to harvest, Victorias high rainfall zone (HRZ) growers are looking to crop competition as a tactic to halt the spread of this costly weed.

GRDC-supported research by the University of Adelaide and Southern Farming Systems (SFS) shows crop competition may be a viable alternative strategy to harvest weed seed control (HWSC), which can be difficult to implement in the HRZ.

Cam Nicholson, the leader of the HRZ Regional Cropping Systems Network (RCSN), says trials have found: changing crop type and/or variety; reducing row spacing; increasing seeding rates; changing row orientation; or changing planting times can be effective. Although, efficacy can vary greatly from environment to environment.

Early sowing of wheat can reduce annual ryegrass seed production in medium rainfall zones, however, its value in the HRZ may be lower, Mr Nicholson says.

A trial at Lake Bolac, in Victoria (run since 2012), initially examined the value of pre-sowing cultural tactics on annual ryegrass populations. Treatments included:

  • Retained stubble
  • Burning stubbl
  • Incorporating stubble
  • Mouldboard plough operation followed by retained stubble.
  • Each followed by an in-crop treatment of either of three different intensities of herbicide management.

The trial showed the mouldboard plough operation reduced establishment of annual ryegrass by more than 95 per cent in the year it was implemented.

Annual ryegrass seed head numbers increased in all management strategies between 2012 and 2016. The increase was less with the most intensive management than with the other management strategies.

Early sowing of wheat can reduce annual ryegrass seed production in medium rainfall zones, however, its value in the HRZ may be lower - HRZ Regional Cropping Systems Network leader Cam Nicholson

Following crop topping of faba beans for all strategies in 2016, weed numbers were greatly reduced during 2017.

Despite this, annual ryegrass seed head production was still substantially higher under the low intensity management strategy, compared with the other management strategies.

Mr Nicholson says profit and the long-term sustainability of continuous cropping systems are the primary motivations behind trying to find practical solutions to the issue.

Growers prefer to grow wheat every alternate year, as it is frequently their most profitable and reliable crop," he says.

"This is despite the fact that cereals are considered the weak link in annual ryegrass control and that is known that a one-year break, such as canola, will not provide a sufficient reduction in populations.

Double break crops in rotations are effective at reducing annual ryegrass population, due to crop topping.

Moderate populations (less than 100 plants/m2) of annual ryegrass do not greatly reduce crop yield in the HRZ, so strategies that drive annual ryegrass to low levels are not always the most profitable.

Through GRDC investment, five demonstration trial sites across Victoria and South Australia have been established in the HRZ to identify effective and profitable strategies for the management of annual ryegrass in the HRZ.

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