Stubble still a challenge

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Most growers would prefer not to wait that long for the necessary experience and expertise, and the University of South Australia's Agricultural Machinery Research and Design Centre (AMRDC) has been working for years to find answers to stubble handling problems.

However, the centre's Dr Jack Desbiolles says the ability to reliably establish sensitive crops in heavy stubble is still a major issue for most farmers in no-till cropping. Particularly following bumper crops, many paddocks are likely to present residuehandling problems at seeding which will significantly influence crop establishment without adequate planning.

"Research trials in tine systems operating in stubble, and incorporating herbicides at sowing, have repeatedly measured high crop establishment losses, linked to issues such as soil throw, stubble clumping, seed placement and soil disturbance," Dr Desbiolles says.

"But limitations also exist with disc systems operating in stubble, due to hair-pinning issues, where stubble is not cut but is pushed into the seeding slot, compromising soil/seed contact, while crop losses increase further when soil applied herbicides are used.

"Research trials have also shown that the severity of soil throw problems in tine systems can often be minimised by operating at sufficiently low speed, or using wider row spacing, which is particularly suitable for rotation crops such as oilseeds and pulses."

Dr Desbiolles says harvest is the first opportunity to start managing potential residue-handling problems at seeding.

He says stubble density and length are the most important field parameters influencing residue handling with tine seeders. Reducing both of these through strategies of slashing, harrowing or partial removal by grazing or baling can effectively minimise machinery plugging at seeding.

However, any stubble removal should be avoided, as crop residue is the lifeblood of microbiological activity and long-term soil health, including soil erosion control.

Residue management should start at harvest, aiming to uniformly spread residue across the paddock and achieve a short uniform length, commensurate with optimum harvesting efficiency.

For instance, it is not uncommon to find twice the paddock residue levels on the header trail, depending on cutting height and straw spreader abilities, and these areas often present the greater challenges for establishing crops.

Wheat stubble levels can amount to 1.3 to 2.8 times the grain yield, and may create handling problems, depending upon row spacing in particular, when paddock levels reach three to four tonnes per hectare.

"Wetter stubble increases friction and adhesion properties, which affect the ease of flow across a tine layout, and shows significantly higher resistance to cutting and lower resistance to bending than dry stubble," Dr Desbiolles says. "This condition promotes hair-pinning with disc coulters as well as with narrow edge-on shanks working in long stubble.

"Stubble on the header trail remains wetter for longer, due to concentrated levels including chaff, so sowing into heavier stubble should be delayed until sufficient drying has occurred."

Dr Desbiolles says stubble decomposition decreases its mass and reduces the proportion of cellulose in the internodes, which in turn decreases shear strength and facilitates residue cutting.

And while rolled or trampled stubble is often wetter, and can be more difficult to handle than standing, over time it decomposes more quickly. Residue cutting is most effective after wet summers and in hard soil conditions

On the other hand, poor stubble decomposition over the summer/autumn can result in greater nitrogen tie-up, significant toxin release and more disease in an emerging, susceptible crop.

Tine trials conducted by the AMRDC generally point to greater residue-handling difficulties in standing stubble than in rolled stubble, but the availability of Differential Global Positioning Systems (DGPS) guidance systems, on up/back working patterns to facilitate inter-row sowing, would reverse this assessment.

In recent AMRDC trials, residue management technologies which move stubble away from the path of the seeding tine or disc, reduce stubble clumping with tine systems or residue hair-pinning with disc systems, showed they could significantly optimise crop establishment in heavy stubble.

Dr Desbioilles says key issues for growers to remember when upgrading or modifying tine equipment include:

  • stubble height should be kept below 60 to 65 per cent of the effective vertical clearance of the tine - the distance from the ground surface to the first major obstruction on the tine shank or mounting head - so as not to restrict residue handling;
  • using narrow points, especially lower fitting knock-on designs, and working at shallower depths can help maximise existing tine vertical clearance values, where needed;
  • stubble length should also remain less than half the lowest value of inter-tine spacing - the narrowest clearance between components of any two tines or between tine and wheel, in any direction;
  • bulky tine seeding attachments, such as parallelogram fitted seed boots, require significantly greater rank spacing in their layout to maintain adequate residue handling capacity;
  • stubble clumps tend to increase in size each time they interact with a tine, while passing through the implement layout; this implies greater clearance requirements towards the back of the seeder;
  • wetter stubble reduces the ease of flow across a tine layout and increases hairpinning tendencies; field observations of the stubble handling performance of seeding tines showed increased clumping tendencies when working behind hair-pinning residue coulters.

More information: Jack Desbiolles, 08 8302 3946,

[Photo (left): Stubble hair-pinning in heavy residue conditions results in patchy canola establishment.]






Photo showing disc seeder[Photo (left): The success of disc seeders in zero-till applications is linked to their residue-cutting ability.]






Photo of disc sower[Photo (left): Residue management can maximise the performance of disc sowing systems in heavy residue conditions.]






Photo showing stubble clumping with tine systems in heavy residue conditions affecting the uniformity of wheat establishment[Photo (left): Stubble clumping with tine systems in heavy residue conditions affects the uniformity of wheat establishment.]




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