Weed control strategies that will ensure growers are successful when implementing a stubble-retaining strip and disc system in wheat have been identified for Western Australia’s northern agricultural region.

Weed control in high standing residue systems in the northern agricultural region is a Grains Research and Development (GRDC) investment aiming to provide herbicide-use advice for a stubble-retaining strip and disc system for wheat, canola and lupin crops.

Trials in Mullewa and Binnu compared 18 different herbicide treatments in wheat planted with a disc seeder. Trials in Yuna, Ogilvie and Mingenew compared multiple herbicide treatments in both canola and lupins.

Crop Circle Consulting and Research director Grant Thompson says the first-year trial results from these sites are encouraging, with a few options to choose from.

"A mix of Boxer Gold® with Quadrant® applied after the three-leaf stage of wheat achieved good weed control for the best yield results at both sites," Mr Thompson says.

Another suitable post-emergent is the new broad-spectrum product Mateno®, while Overwatch® and Sakura® provided good options at sowing. Growers should note, however, that Overwatch should not be used with disc seeders when sowing canola or barley.

A ten-inch spacing disc seeder was used to sow plots at 30 and 10 millimetres deep. These plots were compared to the standard grower practice of a 12-inch knife point and press wheel system sowing to a depth of 30mm.

"We tried the shallow seed depth to demonstrate the importance of correct seeder setup and just how much damage can be done if errors happen at sowing,” Mr Thompson says.

Crop safety is typically achieved through separation of the seed from the herbicide treated band, with most products requiring seed to be planted a minimum of 30mm below the treated band.

"Overwatch® in particular showed considerable early crop bleaching when seeded at 10mm compared to the standard 30mm depth.

“Luximax® on the other hand was not suitable for disc seeding and should only be used in the knife point and press wheel system,” he says.

At Ajana, results from plots with different weed control and nitrogen nutrition in canola showed that using the disc seeder into high-standing stripper front stubble with careful nitrogen application could improve water use efficiency by 30 per cent compared to standard practices.

Mr Thompson says strip and disc systems were excellent for keeping stubble intact, and the long residue reduced soil temperatures, evaporation and protected young canola seedlings from harsh wind and sunburn.

"We used the disc seeder again when the crop was at four to six-leaf stage to mid-row band liquid urea-ammonium nitrate below the stubble.

"The discs damaged the crop a little, but the plants quickly recovered, and this treatment achieved the best yields and water use efficiency results.

The 30 per cent improvement provided roughly half a tonne of additional yield compared to standard knife point press wheel with the same nitrogen rates applied.

Mr Thompson says awareness of all competing elements is key to achieving desired results.

"It is a balancing act when it comes to when and where to apply nitrogen fertiliser so that it is not tied up by the soil microbes breaking down the residue.

"When the first application of nitrogen was applied using a streamer, it soaked into the thick stubble where it was bound up early.

"In this situation, the fertiliser effectively just feeds the bugs, and little gets to the crop. 
"However, placing the liquid under the soil or using urea that bounced through the stubble and onto the soil achieved much better results," he says.

The canola herbicides trial in disc seeding will be repeated this coming season, along with an investigation into herbicide droplet deposition using different nozzles, ground speed and droplet sizes.

"I am confident the results will show the benefits strip and disc systems provide in protecting the northern ag region from heat stress are worth the effort to adjust seeding chemical and fertiliser applications," Mr Thompson says.

Further information and results from the project will be available at 2023 spring field walks in the northern agricultural region, as well as in GRDC’s GroundCover magazine.