Weed-free summer fallows set up crops

Tips for summer herbicide application

  • Always follow label recommendations.
  • Water rates should be high (at least 60 litres per hectare).
  • Add a surfactant and/or spraying oil to post-emergent mixes (unless otherwise directed on the label).
  • Do not spray stressed plants.
  • Spray grazing can be effective at high stocking rates.

Source: DAFWA

The benefits of capturing summer rainfall when it occurs and the long-term advantages of keeping the weed seedbank low are driving the need to use double-knock herbicide applications to manage weeds in the summer fallow.

This summer weed control benefits crop yields when summer rainfall is captured and stored to augment in-crop rainfall.

In recent years, rain during harvest in many eastern areas has kickstarted summer weed-control programs just after headers are locked away.

This has been to control both winter weeds, such as wild radish (Raphanus raphanistrum L), that have been cut off at harvesting and newly germinated summer weeds.

It is a costly nuisance, but has major benefits in killing weeds before they use moisture that could be stored for the next crop or set viable seed to add to the seedbank – and/or grow big enough to cause problems at the subsequent seeding.

Harvest and early summer fallow weed control have been shown to conserve moisture and nutrients for the next year’s crop – with corresponding increases in crop yield and grain quality.

A harvest knockdown trial at nine eastern wheatbelt sites in 2012, funded by the GRDC’s Kwinana East Regional Cropping Solutions Network (RCSN) group, found a significant moisture conservation response in the root zone (to a depth of 40 centimetres) of sprayed plots compared with unsprayed plots.

The response ranged from about nine millimetres at Doodlakine up to about 60mm at Southern Cross.

Spraying out summer weeds at harvest also improved early crop vigour.

Kellerberrin grain growers Scott and Ann Dixon see a lot of value in controlling weeds and storing moisture during summer as the long-term trend is a decline in growing-season rainfall in their area.

Image of a man

Kelllerberrin grower Scott Dixon's summer weed control system is featured in the new GRDC Summer Fallow Weed Management manual.

PHOTO: Cox Inall Communications

The couple, who crop 4850 hectares, received about 150mm of summer rain between November and January in 2011-12 and again in 2012-13.

Control of the major summer weeds on their property – afghan melons (Citrullus lanatus), caltrop (Tribulus terrestris) and some prickly paddy melon (Cucumis myriocarpus) – in these fallow periods increased wheat and canola yields in 2012 and 2013 by about 0.6 to 0.8 tonnes/ha.

Increased stored soil moisture in those seasons also allowed Scott and Ann to sow canola on the recommended date and achieve good plant densities, despite dry autumn and early winter conditions.

The Dixons only received 20mm of summer rainfall in 2013-14 and did not need to control summer weeds in that period.

Scott says that during seasons with high rainfall in late spring and summer, improved herbicide application technology allows him to spray his whole operation in less than 10 days.

He uses a 36-metre Miller Nitro 4000 self-propelled boom with two nozzles mounted behind each wheel, autosteer and GPS for improved night spraying and high water rates. In wetter harvests, the boomspray follows the header.

Scott says a double knock of glyphosate followed by paraquat is highly effective at controlling summer weeds, while reducing selection for resistance to glyphosate.

“Our biggest limitation is suitable spray conditions and our main time of spraying is between midnight and 8am, when temperatures and humidity are low,” he says. “Our daily spray target is 300ha and our application volume is 70 to 90 litres/ha – depending on temperature and humidity.  

“When spraying at night, the risk of a temperature inversion is a lot higher than in daytime, with an associated increase in drift risk.

“Special attention is paid to weather conditions and spraying ceases if a temperature inversion develops.” 

Scott and Ann’s summer weed control system is featured in a new GRDC manual, Summer Fallow Weed Management, developed for the western and southern grain production regions.

It outlines a range of case studies, strategies, tips and useful tools for summer fallow weed control and is available on the GRDC website.

More information:

Scott Dixon,
0427 773 606,

John Cameron, ICAN Rural,
0417 109 709,

The Summer Fallow Weed Management manual is available at:

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Ground Cover Issue 112 – Managed environment facilities


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GRDC Project Code ICN00012

Region West