WA Central Regional Summary

The region

The WA Central agroecological zone extends from south of Coorow south to Ongerup. Rainfall in the zone ranges from > 1100 mm annum^-1 on the south coast of WA to < 400 mm annum^-1 for the inland areas of the zone. Rainfall seasonality ranges from winter-dominant in the south to seasonally neutral in the north. Boundaries for the zone are shown in Figure 1.

Modelling regional practices

Producing models that reflect all farming practices throughout a region is difficult so “common practice” models for wheat, canola and lupins for the region were produced. A wheat crop with a 1.9 t ha^-1 yield and 43 kg of fertiliser N applied ha ^-1, a canola crop with a yield of 1.1 t ha^-1 with 45 kg fertiliser N ha ^-1 applied and a lupins crop with a yield of 1.0 t ha^-1 and 5 kg of fertiliser N ha ^-1 applied were modelled. The key assumptions of these models were that;

• Only one pass was made at sowing
• Split fertiliser applications were used for wheat and canola crops
• It was assumed that wheat and canola had three fallow herbicide applications and lupins had three, including a pre-emergent. All crops had one in-crop herbicide application and canola and lupins also had a desiccation spray. Wheat and lupins had one pesticide application and canola had two.
• Stubble was retained
• Lime was applied at the equivalent rate of 4 t ha^-1 every decade and incorporated through scarification

Impact indicators

Hydrogen ion impacts estimate the release of hydrogen ions to the soil associated with crop production. A negative value indicates a reduction in soil acidity whereas a positive value indicates an increase in soil acidity. Soil erosion is an estimate of soil loss that occurs during the production of the crop. The depth of soil lost will depend on many things but an approximate conversion is that 1.5 t of soil loss equals 1 mm of soil. Global warming impacts are the release of greenhouse gases to the atmosphere expressed as carbon dioxide equivalents (CO₂-e), Eutrophication impacts are the release of phosphorous to the environment and are expressed as phosphate equivalents and Particulate Matter impacts are the release of fine particles less than 2.3 micrometres in diameter.

Benchmark results

Results below in Table 1 show the environmental impacts of producing a t of wheat, canola or lupins in the region. Lime use in the region resulted in a reduction in free hydrogen ions in the soil for all crops and soil erosion was estimated at between 3.3 and 6.3 t soil loss t product^-1. Analysis indicates that Global warming impacts associated with the production of these crops ranged from between 143 and 713 kg CO₂-e, Eutrophication impacts ranged between 1.9 and 5.3 kg PO₄-e and Particulate Matter impacts ranged between 0.21 and 0.7 kg of < 2.3 µm particulate matter t product^-1.

Table 1: Hydrogen ion changes, soil erosion and Global Warming, Eutrophication and Particulate matter impacts associated with the production of wheat, canola and lupins in the WA Central agroecological zone.

Greenhouse gas emissions

The emissions profile of all three crops are shown below in figure 2. For wheat and canola, the greatest sources of emissions were the production and use of fertiliser. Emissions associated with the breakdown of residues also made a considerable contribution to total emissions. The emissions profile of lupins was dominated by emissions assocaited with residue breakdown due to the low rate of fertiliser used on the crop.

Greenhouse gas mitigation strategies

Mitigation strategies tested for the region were;

• Sustainable intensification
• Additional applications of lime
• Implementation of variable rate fertiliser technology
• Changing a wheat-wheat rotation to a legume-wheat rotation

More information on the assumptions used to test these strategies and how they might reflect individual enterprises are available on the Mitigation strategies page.

Results (figure 3 below) indicate that emissions of a t of wheat can be reduced by;

• 80% through the implementation of sustainable intensification
• 17% by additional lime applications
• 11% by implementation of variable rate fertiliser technology

Results also indicate the replacing a wheat crop with a legume crop in a two-crop rotation can increase greenhouse gas emissions intensity by 125%.