Managing stubble in summer protects soils and profits
GroundCover™ Supplement Issue: 135 July-August 2018 | Author: Dr John Kirkegaard
Weeds, stubble and livestock are the most important factors in protecting soil: aim for a weed-free fallow with at least two tonnes of stubble per hectare retained on the soil surface.
Australian growers are the world’s leading adopters of no-till, stubble-retained farming systems because protecting the soil and capturing water make sense in dryland farming. But just how valuable is stubble cover, and how can we maximise these benefits while minimising the problems posed by heavy stubbles?
Diligent weed control during the summer fallow is the key to maximising the benefits from stubble-retained systems. Summer weeds, even at low density or when grazed, can use precious soil water that may otherwise be stored for subsequent crops. Weeds also take up mineral nitrogen, which is then tied up in the weed residue.
In a series of 26 recent experiments across southern Australia, as part of the GRDC Water Use Efficiency Initiative (2008–13), strict summer weed control increased the amount of stored water by 40 millimetres at sowing, mineral nitrogen by 40 kilograms per hectare, and wheat yield by 0.8 tonnes per hectare, with a return on investment of $5.60 for every dollar spent.
Delayed or missed sprays could halve the return on investment by reducing water and nitrogen available to crops, but were always preferable to not spraying at all. Without this weed control the benefits of the retained stubble to water conservation are largely lost.
How much stubble?
Maintaining stubble cover to protect soil structure and increase infiltration and water storage over summer is accepted practice. The big decision for growers is whether to manage, reduce or remove stubble prior to sowing to ensure effective and timely seeding, and this depends on the seeding equipment, weed management strategy and the type of crop to be sown.
Fortunately, the first few tonnes of stubble do most of the work in terms of soil protection and improved water infiltration. About 2 to 3t/ha of cereal residue achieves the 70 per cent cover required to minimise soil erosion and maximise water capture in the majority of seasons (Figure 1).
This amount of stubble presents few problems for most seeding systems or for weed management, and should be considered a target threshold to maintain where possible.
Heavier stubble loads can increase the duration of soil water storage near the soil surface by slowing evaporation in autumn and late summer, and this benefit can often be seen in windrows of heavy stubble, or when fires or stock remove areas of stubble.
The benefits of retaining surface water in heavier stubbles for early sowing depend on the timing and amount of rainfall prior to sowing and may not occur on all soils or in all seasons. A good policy is to retain stubble whenever possible, but manage it to ensure a timely seeding operation and good weed control.
Several recent studies as part of the GRDC’s Stubble Initiative have shown that light grazing of stubble in summer has little impact on water storage or the yield of subsequent crops, provided sufficient cover (70 per cent) is retained on the soil surface. Increased soil mineral nitrogen after grazing heavy stubbles in some seasons can actually increase the yield of some crops.
Consequently, whole-farm income is generally unaffected or improved by careful stubble grazing. The real problem is overgrazing – sheep do damage with their mouths, not their hooves.
GRDC Research Codes CSP00186, CSP00174, MSF00003
Dr John Kirkegaard, CSIRO
02 6246 5080
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