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Fallow tool for the low-rainfall zone

Dr Chao Chen and Dr Andrew Fletcher have been part of a team from CSIRO developing tools to improve the profitability of cropping in low-rainfall Western Australian cropping systems, including a rule of thumb for fallow integration.
Photo: CSIRO

Key points

  • There is a significant opportunity cost when fallow is integrated into farming systems
  • In most scenarios the future benefit does not justify the opportunity cost incurred
  • Areas have been identified in Western Australia where fallow could play a part in crop sequences and farm businesses
  • Fallow could play a role in weed and disease management
  • Strategic fallow may have a role in managing risks

A rule of thumb for the role of fallow in the low-rainfall zone (LRZ) of Western Australia has been established by CSIRO scientists by combining four years of experimental data with 31 years of modelling scenarios.

Although a significant opportunity cost is incurred and not easily recovered, the research has shown that there are production areas in which the practice has the greatest chance of success.

Dr Chao Chen from CSIRO, who conducted the modelling work, says the scientists found that fallow has the best fit where growing-season rainfall is less than 230 millimetres and stored plant-available water at sowing after fallow is greater than 30mm.

The practice of fallow in Australian faming systems refers to taking land out of cropping for an entire growing season, usually 18 months from harvest to sowing the next crop. Before the 1980s, when there were fewer viable break crop options, fallow played an important role in crop sequences to conserve moisture, improve nutrients and deal with weed and disease problems.

“The use of fallow has declined, in part due to an increase in the number of break crop options, improved agronomic practices and increased fertiliser use,” Dr Chen says.

“But there is renewed interest in the practice due to decreasing rainfall and changing climate patterns, an increase in herbicide-resistant weeds and escalating input costs, and a need to update our scientific understanding.”

Tools to improve profitability in the LRZ

This work was part of a larger GRDC-supported project led by CSIRO farming systems scientist Dr Andrew Fletcher that focused on delivering new tools to growers in the LRZ to deal with cropping challenges in these regions.

“Low-rainfall-zone farming systems are characterised by variable and low average yields, high risk and erratic profitability,” Dr Fletcher says.

Terminal drought and heat stress during grain filling are also common and may increase with changing climatic conditions. To cope with this, growers have adopted cereal-dominant, risk-averse approaches with low use of inputs such as fertiliser.

“Profitability improvements for this zone may require a change of risk appetite. To tackle this, we have been evaluating a number of different tools, including new early sowing options, reduced tillering wheat, higher-value break crops such as chickpeas and re-evaluating the role of fallow.”

Four years of experimental work based at Hyden and Merredin has enabled the team to collect data to inform their analysis. The fallow treatment incorporated in these trials was a complete brown-out fallow of 18 months.

“But, notably, we have considered the performance and economics of combinations of these practices over longer-term timeframes to understand how they can be optimised to improve grower returns using the Agricultural Production Systems sIMulator (APSIM) modelling program.”

Fallow as a tool to improve profitability

The field trials reported a 0.6-tonne-per-hectare yield increase in wheat after fallow when compared to the continuous wheat rotation at Merredin. The fallow used was a total brown-out fallow.

At Hyden, there was a 0.45t/ha increase in wheat yield in the fallow-wheat rotation when compared to the continuous wheat rotation. The protein of the following wheat crop also increased after a fallow rotation in these trials.

However, in both trials the yield penalty incurred during the fallow period was not compensated for completely.

The APSIM model, driven by an approximate five kilometre by five kilometre  gridded climate over the dry period of 1990–2020, was used to simulate five rotations (fallow followed by one, two, three or four wheat crops, respectively) and continuous wheat. Simulated wheat yields, plant-available water at sowing after a wheat crop or long fallow, and nitrogen fertiliser requirement were compared and analysed for fallow/wheat and wheat/wheat.

Wheat yields and economic returns were also calculated for each rotation and compared with continuous wheat.

Variables such as (yield response to fallow, economic response to fallow, soil water response, cropping system yield, economic return of cropping system) were then mapped to identify hotspots where fallowing could be an alternative rotation option.

Figure 1 shows the differences in the average gross margin over time during 1990–2020 among the cropping systems using the crop rotations. Each map shows the difference in financial return (dollars/ha) of including an initial year of fallow in each sequence compared to continual wheat rotations.

Figure 1: The differences in temporal average gross margin during 1990–2020 among the cropping systems: FW-WW (a), FWW-WWW (b), FWWW-WWWW (c) and FWWWW-WWWWW

Source: www.sciencedirect.com/science/article/pii/S0308521X22001974

Wheat yield benefits to fallowing showed a clear geographical pattern, which increased from south-west to north-east ranging from 0.3 t/ha loss to 1.1 t/ha gain in the year following fallow, which could be explained by climate and soil conditions. But due to the opportunity cost, production and profitability over time, was less.

“We found that grain yield following fallow could increase by six per cent or more,” Dr Chen says. “This is equivalent to 13 per cent economic increase, in the areas where growing-season rainfall was less than230mm and the difference in plant-available water at sowing between fallowed and continuously cropped soil was greater than30mm.

Fallow resulted in an overall reduction in crop production over the five-year period. However, integrating one fallow into a continuous wheat sequence every three or more years might achieve similar profit to continuous wheat for fields with weed and disease challenges in the north-eastern wheatbelt.

However, with the current fallow management cost, cropping system with fallow integrated was not able to achieve a competitive advantage over continuous wheat across most regions of the wheatbelt when the effects of other factors (weeds, diseases and insects) on economic returns of cropping systems were not considered.

Fallow: a complex decision

“The decision to include fallow in a rotation is not a simple one and is influenced by a number of factors,” Dr Fletcher says.

“The whole-farm operations need to be considered to determine the benefit of including fallow to improve the efficient use of machinery. This may reduce sowing time.”

Then the type of fallow needs to be decided upon: a complete brown-out fallow, a weedy fallow with some plant growth or a volunteer or sown pasture.

“Weed and disease issues need to be assessed, together with any thresholds which may determine the appropriate economic management methods.

Growers may also want to consider the use of a fixed or strategic fallow to respond to early or late breaks.

“Our analysis assumed that fallow was implemented in every season. However, farmers may use factors such as the timing of the break of season or seasonal forecast to increase or decrease the amount of fallow.”

Additional studies are required to determine the influence of these issues as triggers for grower decision-making on incorporating fallow into their farming system.

Fallow benefit ground-truthed by growers

GRDC works with its National Grower Network (NGN), the community of growers and grains industry stakeholders across Australia’s growing regions, to assist in developing locally relevant research development and extension investments.

To ground-truth the agronomic and financial benefits of different management practices in a fallow farming system, six farm-scale trials were established across the Kwinana East port zone with the NGN in 2022. This zone receives highly variable annual rainfall of between 200 to 350 millimetres and the dominant crop produced is wheat.

Benj Graham, one of the growers participating in the GRDC-supported Laconik study, spraying-out various fallow treatments at his Bonnie Rock property. Photo: Evan Collis

Each trial site has been comprehensively soil tested and is being monitored for ground cover and soil moisture over two seasons. Four treatments were established in 2022: wheat produced by grower practice, complete brown-out fallow, seeded vetch pasture and volunteer pasture both brown-manured before seed-set.

Trials were sown using growers’ equipment and overseen by Laconik founder Dr Darren Hughes. In 2023, the trial sites will be sown to wheat, as per grower practice and grain yield collected.

“The project will determine the agronomic and financial benefits of different management practices in a fallow farming system across the Kwinana East port zone,” Dr Hughes says.

The overall aim is to give growers more confidence in making decisions about the role fallow has in their faming systems in this low-rainfall zone.

Using production and financial data collected from the trials, Planfarm will model a ‘typical’ eastern wheatbelt farm with rotations, soil types and financial health metrics, and assess how fallow changes whole farm profitability.

Planfarm’s work will be supported by Dr Yvette Oliver from CSIRO, who has previously worked with Planfarm on fallow farming systems. It is anticipated that an economic analysis for each treatment will be completed and reported on by March 2024.

More information: Dr Andrew Fletcher, 0477 347 449, andrew.fletcher@csiro.au; Dr Chao Chen, 0450 530 863, chao.chen@csiro.au

Integrating long fallow into wheat-based cropping systems in Western Australia: spatial pattern of yield and economic responses

Dr Darren Hughes, 0436 115 462, darren.hughes@laconik.com.au

Read also: Fallow integral to fluid decision-making.

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