MONITORING STUBBLES IN FALLOW PERIODS

Assessments and timing of actions to manage stubble loads

STUBBLE IS CRITICAL for protecting Mallee dryland agricultural soils from wind erosion. However careful management is required to ensure that groundcover is maintained above the critical 50% threshold while also not creating issues at seeding time. This guideline discusses options for monitoring and managing stubbles from harvest to seeding.

HOW MUCH STUBBLE DO I HAVE?

Stubble is any above ground plant residue that is remaining in the paddock after harvest, for example, stem, leaf and glumes of cereals. Harvest Index (HI) is the ratio of grain yield to total above ground biomass and can be used to estimate stubble loads. For wheat

HI generally ranges from 0.3 -0.5, therefore you could expect a 2t/ha crop to produce 4-6t/ha of stubble.

HOW MUCH STUBBLE DO I NEED?

The primary benefit of stubble retention in low rainfall farming systems such as the Mallee is to protect the soil surface from wind erosion. To prevent wind erosion during non-crop periods, stubble is needed to develop groundcover levels of at least 50%. Standing stubble (anchored by its roots) of at least 10cm height is twice as effective as loose flat stubble at reducing wind erosion. Cereal stubbles provide better erosion protection than legume stubbles. Legume stubbles are more prone to breakdown than cereal stubbles.

To achieve 50% groundcover, a minimum of 1t/ha of stubble is generally required. Figure 1 shows that where more than 1t/ha of stubble is present, groundcover levels are generally maintained above the critical threshold.

Monitoring stubbles in fallow periods

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However, too much stubble can also be a problem with 3-4t/ha of stubble enough to create stubble handling issues at sowing. There are three opportunities to manage stubbles to prevent future problems:

• At harvest• After harvest• At sowing

MANAGEMENT OF STUBBLES AT HARVEST

The management of stubbles starts at harvest by cutting at the appropriate height and spreading as evenly as possible. The height at which stubble is cut depends on the seeding equipment. Cutting height can be established using the following rules of thumb:

• Stubble height should be kept belowapproximately 60-65% of the effective tyne vertical clearance (distance from the ground surface to the first major obstruction on the tyne shank or mounting head).

• Stubble height should be no more than 65-70%,preferably less than 50%, of the lowest value of inter-tyne spacing (narrowest clearance between components of any two tynes or between tyne and wheel, in any direction).

Header trails can result in greater stubble handling problems as up to twice the paddock reside level can be found in the trail and they can remain wetter for longer. Effective chaff and straw spreaders are essential in stubble retention systems.

Other stubble handling tactics that can be used at harvest include straw choppers, harvesting on the diagonal and secondary cutter bars. Rotary harvesters also smash and break up stubbles more than conventional headers.

MANAGEMENT OF STUBBLES AFTER HARVEST

There have been many machines developed for manipulating stubbles after harvest. These include harrows, prickle chains, disc chains, flail mulches and the stubble cruncher. Summer weed control is also vital in stubble retention systems as weeds can add to the trash levels and the vines produced by weeds such as melons can block seeding equipment.

Care needs to be taken when manipulating stubbles to ensure groundcover levels are maintained above critical thresholds. For example, summer stubble management techniques were demonstrated at three Mallee locations with differing levels of groundcover. Groundcover levels declined following all stubble treatments with rolling or chaining reducing groundcover by 5-40% while cultivation dramatically reduced groundcover relative to the standing stubble treatment with all cultivated treatments falling below the critical threshold of 50%.

Grazing stubbles is another common management strategy. The large size of Mallee paddocks means that without paddock size reduction, the stocking pressure is likely to be too low to graze paddocks quickly, resulting in slower grazing and higher risk of erosion. Smaller paddocks also reduce selective grazing, trampling and camping. Temporary electric fencing systems such as the “Rappa” have been successfully used by Mallee farmers to reduce the size of stubble paddocks. Fencing of high erosion risk or low groundcover areas can be used to extend grazing value of the paddock.

Good design and location of water points can also prevent livestock camping and improve the uniformity of grazing. A good rule of thumb in large paddocks is that stock should not walk any further than 500-700m to access water. Key factors in water point design are to ensure there is always water available, that livestock do

LOCATIONSTUBBLE TREATMENT

Standing Rolled Chained Cultivated

Euston 52% 35% 31% 14%

Lameroo 66% 63% 58% 30%

Kyalite 84% 76% 72% 46%

Table 1. Impact of summer stubble management practices on April groundcover levels at three Mallee demonstration sites

Figure 1. Relationship between stubble load and groundcover measured in five Mallee paddocks.

kg/ha

% G

roun

dcov

erR² = 0.3858

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not have to wait to drink, either because of insufficient flow rate or the trough is too small, and that the water is clean and cool.

Burning is another option to reduce high stubble loads, however this practice has many drawbacks in Mallee farming systems. Stubble burning removes stubble cover and exposures the soil to erosion. It also results in the loss of nutrients from the paddock with 85% of nitrogen, 74% of sulphur and 15% of other nutrients lost during burning. An ongoing burning regime can result in a higher depletion of major nutrients in comparison to stubble retention, and a higher fertiliser cost to farmers in replacing these nutrients.

MANAGEMENT OF STUBBLES AT SOWING

The major challenge of stubble retention systems is the physical management of residues at sowing. One of the first decisions is whether to use a tyned or disc implement:

• Tyne seeding machines are often less expensivebut handle less stubble. They result in more soil disturbance which can be a benefit by reducing Rhizoctonia and incorporating herbicides.

• Disc machines handle heavier stubble loadsand disturb the soil less. Anecdotal evidence inthe Mallee suggests Rhizoctonia and herbicideincorporation are issues for disc seeders. Discseeders can also suffer from ‘hair pinning’.

There are a number of seeding bar setup and operation factors that can influence the stubble handling ability of sowing equipment. These are summarised in Table 2.

The operation of tyned machines can be improved by complementary strategies at seeding including:

• Sow into heavy stubble while dry• Using lower reaching narrow points to maximise

vertical clearance• Slower operating speeds to reduce residue clumping• Fitting trash guards or polymer coil rapping to

improve residue flow• Fitting coulter discs to the front of the seeders bar

Inter-row sowing also allows crops to be sown into standing stubble and relatively high stubble loads. Crops have been successfully sown inter-row on 22.5cm spacing and wider. Inter-row sowing also minimises the need for many other stubble management techniques.

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MORE INFORMATION

A full list of research cited in this Farm Talk is available at www.msfp.org.au

This Farm Talk was published in 2018 and developed by MSF through the GRDC project MSF00003 ‘Maintaining profitable farming systems with retained stubble in the Mallee’.

Any recommendations, suggestions or opinions contained in this publication do not necessarily represent the policy or views of MSF or the Grains Research and Development Corporation (GRDC). No person should act on the basis of the contents of this publication without first obtaining specific, independent professional advice. MSF, GRDC and contributors to these guidelines may identify products by proprietary or trade names to help readers identify particular types of products. We do not endorse or recommend the products of any manufacturer referred to. Other products may perform as well as or better than those specifically referred to. MSF and GRDC will not be liable for any loss, damage, cost or expense incurred or arising by reason of any person using or relying on the information in this publication.

Acknowledgements: Michael Moodie, Moodie Agronomy

SUMMARY

A minimum of 1t/ha of stubble should be retained at all times to ensure Mallee soils are protected from erosion. Managing stubble to ensure that it does not

cause handling issues at sowing starts at harvest. Good seeding bar setup and operation is also required to ensure stubbles are effectively handled as strategies to minimise stubble load often risk groundcover levels falling below critical thresholds.

FACTOR IMPROVES STUBBLE HANDLING RESTRICTS STUBBLE HANDLING

Tyne cross section Circular diameter 40-85mm Rectangular with sharp corners

Tyne shank angle* Vertical, leaning back and some high “C” shapes Low “C” shapes (curves) close to the soil)

Row spacing As wide as practical narrow

Vertical clearance More than 500mm between ground and assembly Less than 500mm

Rank or bar spacing More than 600mm Less than 600mm

Tyne Pattern In-line or continuous Staggered interrupted clumps

Points* Narrow points Wide points or sweeps

Soil* Dry, firm and well structured Wet and loose

Depth* Minimum Deep

Speed* Slow Fast

Harvest treatment Cut low and chaff spread Cut heads and leave chaff windrows

Post-harvest treatment Mulch, slash, harrowFlatten stubble

*In some situations, the reverse may apply. For example, crop varieties with very high or very low straw strength, diseased stubbles and sowing at speeds too slow to maintain an even stubble flow.

Table 2. Factors influencing machinery setup on stubble handling