Riverina Independent Agronomy’s Neil Durning told the GRDC Northern Region Update that disc seeding had a range of advantages and disadvantages, depending largely on whether it was a dry or wet season.

“They (disc seeders) seem to improve the ability to harvest a crop in dry conditions,” he said. “But in a wet year, there can be too much water and disease in disc-sown systems.”

Mr Durning said disc seeding minimised moisture loss during sowing due to reduced disturbance but led to reduced seedling vigour. This meant less biomass is produced during winter, which he says slows water use, leaving more moisture in the soil if spring conditions are dry.

Disc advantages

“One of the major benefits of disc seeding is the capacity to plant through stubbles that have been retained over many years,” Mr Durning said.

“This improves rainfall infiltration, particularly in heavy summer storms, and reduces stored water loss in dry winters. We also see less wind and water erosion.”

He said the push into disc seeding recently had come about through people keen to sow on narrow row spacings to increase crop competition with weeds, and only a disc machine could reliably sow into big stubbles on narrow row spacings.

“All tynes struggle to establish crops into big stubbles at row spacings of less than 254 millimetres (10 inches),” he said.

“If you’re using a stripper front that produces large, tall stubbles, tynes don’t allow enough trash clearance.”

Mr Durning said a mulch cover improved soil stability and he had seen a lot more earthworms in disc-sown paddocks.

“In very low rainfall conditions, Birchip Cropping Group data demonstrated that no stubble cover produced a grain yield of 0.24 tonnes per hectare, whereas standing stubble produced a 0.5t/ha grain yield.”

Sowing canola into wheat residue left from a stripper front using a disc seeder. Photo: Nicole Baxter

Other benefits also apply once soil organic matter levels have been built up. For example, modelling of nitrogen mineralisation data from Melbourne University’s Dr James Hunt found stubble-retained systems had much higher levels of nitrogen mineralisation across four sites than the stubble removed system.

“When we burn a 5t/ha stubble we need to be conscious we could be losing $50/ha of nitrogen and $60/ha of phosphorus.”

Mr Durning said one of the major advantages of disc seeding was the capacity to establish crops on less rainfall.

“In conditions that could be considered traditionally as a false break, disc seeding will enable a crop to be reliably planted and established on time, whereas tyne sowing needs to be deeper to ensure crop establishment,” he said.

“Also, when there are large areas to sow, discs can travel faster and cover more hectares quicker because the ground is not having to be fractured.”

For mixed farming systems, he said disc seeding can be used to drill additional pasture species into established pastures, without destroying the existing plant cover.

Disc downsides

Mr Durning said a heavy mulch layer facilitates annual ryegrass survival because seeds are protected in retained residue.

“You cannot use trifluralin in disc-seeding systems and, when we are forced into using products that penetrate the stubble layer, crop damage can result if heavy rain falls after application when seedlings have not fully emerged,” he said.

“We need to be careful in hard-setting and light-textured soils, which tend to result in more herbicide damage to seedlings.”

He said the early post-emergent herbicide Mateno® Complete (a mixture of aclonifen, pyroxasulfone and diflufenican) potentially offers an improvement where it can be applied at the two to three-leaf stage for grass control in wheat and barley.

Mr Durning trialled bixlozone (Overwatch® Herbicide) for grass control before it was released, using disc seeder-sown barley.

“It has the potential to cause damage, which was always said, so when you are using mobile herbicide products and rainfall is forecast, aim to plant seed deeper.”

The inclusion of double breaks (legume/canola) and harvest weed seed management was important to help manage annual ryegrass in disc-sown systems, he said. This called for pre-harvest glyphosate use in canola and barley, and hay cutting to deal with weed escapes.

Another disadvantage of high stubble retention farming systems, he said, was that slaters and Portuguese millipedes build up and damage emerging crops.

“If you try to burn the stubble to remove them, they just become cranky and attack the emerging crop.

“If you have a heavy mulch layer and spring moisture, slugs build up numbers and attack subsequent canola and pulse crops. This applies to any high stubble retention system that keeps soil conditions wet in spring, not just discs. However, due to the amount of stubble retained over multiple years, slugs are more prolific in disc systems.”

He referred to a client with a property west of Wagga Wagga, New South Wales, where patches of crop were damaged.

“The leaf symptoms looked like slug damage, but we couldn’t find them during the day. We came back at night and identified black keeled slugs eating the crop. Once you find slugs in your system they hang around for several years and need to be baited.”

He had also observed reliability issues when sowing canola into heavy stubble loads. “Hair pinning, which is when discs don’t cut through the stubble, can be a problem when stubbles are lying down.

“Worn discs, worn boots, wear in pins and bushes creates wobble that causes canola seed to ‘spray’ into the stubble and not make it into the slot, which lowers establishment.”

In terms of diseases, he said they were manageable in disc farming systems, but because residues are retained, this creates a humid environment and a massive spore load.

“You have to be careful with variety selection and set up an effective fungicide program because you do see more disease in disc-sown crops,” he said. “Yellow leaf spot was an issue where stripper fronts had been used.”

Another disadvantage of disc seeding systems is nitrogen tie-up when first building organic matter levels.

“Melbourne University’s Dr James Hunt showed there was much higher nitrogen immobilisation in a retained stubble system,” Mr Durning said.

“Interestingly, the research indicated nitrogen immobilisation reduced when nitrogen inputs matched nitrogen outputs.”

Another disc seeding factor to be aware of is extra maintenance. He said he had one client who spent about $18,000 a year – $4.50/ha – to maintain his disc seeder for use across a 4000ha program.

“Discs have a lot of moving parts and to maintain consistency and accuracy you may have to spend up to $30,000 a year, depending on the machine and how particular you are,” he said.

Some final points made were that the absence of tynes meant wheel tracks were not being levelled and could become trenches after a wet harvest, and if using discs in hard-setting soils they needed higher downforce pressure to ensure enough soil penetration.

More information: Neil Durning, neil@riagronomy.com