Different stubble management techniques will influence nitrogen tie-up and nitrogen mineralisation.

Trials conducted in southern New South Wales as part of GRDC’s Maintaining profitable farming systems with retained stubble initiative with CSIRO and FarmLink Research have shown that grazing wheat stubbles can increase the yield of subsequent wheat crops due to less immobilisation and greater availability of mineral nitrogen to following crops.

One of the key messages to come out of the research is that growers wishing to retain all stubble should avoid growing wheat after wheat, but if doing so should reduce residue loads by grazing or late burning, or they should apply supplementary nitrogen to offset that immobilised by the residue.  Deep placement of the nitrogen also appeared to be an effective strategy in 2016 and 2017.

CSIRO chief research scientist – farming systems Dr John Kirkegaard says retaining stubble over summer makes good sense to store soil water and protect soil, but the decision to retain some or all of the stubble should take into account the capacity to manage issues such as timely sowing, effective herbicide application for weed and pest control, diseases and nitrogen availability.

The experiment, which took place on a farm near Temora, NSW, monitored a canola-wheat-wheat sequence in a no-till, controlled traffic system.

“In the eight-year experiment, stubbles were either retained, grazed or burnt just before sowing to investigate impacts on soil water, nitrogen and crop productivity,” Dr Kirkegaard says.

“Grazing cereal stubble - but always retaining at least 70 per cent cover (or about 2.5 tonnes per hectare) - never decreased crop yield, but sometimes increased it due to higher levels of soil mineral nitrogen.

“Late burning of stubble did not reduce the yield in any crop, but increased the yield of the second wheat crop by at least 0.5t/ha, with an average increase of 0.8t/ha, mostly due to reduced nitrogen immobilisation by the retained stubble.

“Beyond the nitrogen effects, burning can also further increase yield of the second wheat crop by reducing frost damage in some years.”

The trials were established on a red chromosol following a lucerne pasture, with the lucerne either being removed in spring 2008 (Phase 1) or in spring 2009 (Phase 2). Between April 2009 until 2016, crops were sown in a canola-wheat-wheat rotation and harvested at about 25 centimetres high.

Dr Kirkegaard says both heavy grazing and burning increased yield in the second wheat crop after canola, while across all years grazing and burning applied together increased yield of the second wheat crop by 1t/ha (Table 1).

Grazing stubble increased soil mineral nitrogen by 13 kilograms per hectare in the first wheat crop and by 33kg/ha in the second wheat crop with no interaction between grazing and stubble treatments.

Similarly, burning stubble had no significant effect on soil mineral N in the 1^st wheat crop, but increased soil mineral N by an average of 13 kg/ha in the 2^nd wheat crop (Table 2).

Dr Kirkegaard says nitrogen dynamics were primarily responsible for the differences in yield.

“Grazing and burning stubbles increased soil mineral nitrogen accumulation during the summer fallow to a much greater extent in the second wheat crop compared to the first wheat crop,” Dr Kirkegaard says.

“This was presumably due to both higher amounts and higher carbon-nitrogen ratio of wheat stubble compared to canola stubble which, based on previous research, would lead to more nitrogen immobilisation.

“The average increase in mineral nitrogen due to grazing in the second wheat treatment was 33 kg/ha of nitrogen. Previous research suggests that grazing either removed carbon from the system or neutralised carbon with potential immobilising power of 52 kg/ha nitrogen.”

Dr Kirkegaard says the closer the contact is between the cereal residue and the soil, the higher chances there are of immobilisation. This effect was evident again in trials run in 2017.

“Stubble that is in contact with or on the ground can immobilise nitrogen,” he says. “It depends quite a bit on rainfall as to how much that actually occurs.

“Where we have added extra nitrogen as fertiliser – such as an extra 50 kg/ha of nitrogen at sowing – we have been able to overcome issues of immobilisation where large cereal stubble loads were retained.

“We have also been examining the effect of banded nitrogen deep below the seed at sowing to reduce the potential immobilisation by microorganisms, which we have found has improved nitrogen availability and uptake by the crop.”

Dr Kirkegaard says the greater effect of grazing on mineral nitrogen compared to burning stubble in the trials was likely due to differences in the timing of the two treatments with respect to soil measurement.

“The grazing treatment was applied immediately after harvest, giving four to five to months between removal of stubble by grazing and measurement of soil nitrogen,” he says.

“In contrast, the late burn treatment was applied only about two to four weeks before measurement of soil mineral nitrogen, giving less time for differences in nitrogen immobilisation to act before the pre-sowing soil nitrogen tests.

“Both treatments influenced grain yield as they both would have presumably altered in-season net N mineralisation.”

Dr Kirkegaard says burning is something most growers try to avoid so they do not increase their risk of erosion.

However, some seasons which produce large stubble loads may warrant a late strategic burn.

“Burning is not something growers want to do every year due to the loss of nutrients, regulations, physical problems of burning and the potential loss from erosion,” Dr Kirkegaard says.

“However, in years where there are heavy stubble loads and high weed burdens, a late strategic burn can assist timely sowing with any seeder, effective herbicide and pesticide application and less physical problems for the emerging seedlings.

“Another option to reduce the stubble load in place of burning could be a shallow cultivation using an implement such as a Speedtiller®. If this is the case, it should be undertaken as soon as possible after harvest following good rainfall so decomposition can start and the stubble has time to sufficiently break down for ease at sowing.

“This allows the soil to return to re-mineralising nitrogen prior to the establishment of the following crop.

“High amounts of retained residue are potentially a source of nitrogen tie-up, so we are continually trying to find out ways to find a compromise of keeping stubble there for the purposes of maintaining long-term soil organic matter,  but also preventing it from tying up nitrogen and reducing crop yield in the short-term.”

The decision on how to manage stubble will also come down to the capability of seeding equipment and efficiency at harvest.

Cutting high at harvest will increase speed and efficiency and leave more stubble standing and less on the ground. Other research conducted with FarmLink Research as part of the GRDC’s stubble imitative has shown that there is a 10 percent reduction in harvester efficiency for every 10-centimetre reduction in harvest height below 60cm.

Dr Kirkegaard says the two strategies investigated in 2016 and 2017 to avoid immobilisation were:

• To provide additional nitrogen, at least 5kg of additional nitrogen per tonne of retained cereal residue
• To deep-band the nitrogen below the seed so that the crop gets an advantage over the microbes (which are mostly in the surface) in accessing the nitrogen.

“We recommend that growers wishing to retain all stubble should avoid growing wheat after wheat, but if not, that residue loads are reduced by grazing and/or burning, or that supplementary nitrogen is applied to offset that immobilised by the residue,” Dr Kirkegaard says.

GRDC research codes: CSP00174

More information

Dr John Kirkegaard, 02 6246 5080, john.kirkegaard@csiro.au

Useful resources

International Nitrogen Initiative Conference - Sheep grazing on crop residues increase soil mineral N and grain N uptake in subsequent wheat crops 

Australian Agronomy Conference - The effect of grazing and burning stubbles on wheat yield and soil mineral nitrogen in a canola-wheat-wheat crop sequence in SNSW

Ground Cover Supplement - Stubble