Stubble and straw
Evaluating the economicsThe delivered cost of cereal straw biomass (with moisture content of 10–14%) is estimated to be within a range of $53 to $138 per tonne (see Chapter 11 of Bioenergy in Australia: Status and opportunities, published by Bioenergy Australia in 2012. This includes the cost of collecting, baling and delivering the biomass. This analysis is for cereal crop straw grown in the 450–600 mm annual rainfall zone of broadacre agricultural regions of southern Australia. Estimating yields is covered in Section 11.2.3. The delivered cost of straw bales is described in Section 11.2.6. For a full list of references, refer to Section 11.5. If straw harvesting becomes a regular practice, to maintain soil fertility you will need to replace the nutrients removed from the field in the straw. The cost of replacing nutrients is described in Section 11.2.4. Being aware of the status of nutrients in your soil is essential so that you can anticipate and address fertility problems. How much fertility are you removing when you export hay and stubble from your farm? [PDF 95 kb] is the subject of the DAFWA Farmnote no. 97/2001. It describes:
- how to determine amounts of nutrients and lime removed in stubble
- when to replace nutrients
- how to estimate the cost of replacing nutrients
- how to estimate the grain yield cost of not replacing nutrients.
Harvesting stubble and strawThe harvesting process One-pass grain and stubble harvest systems have been developed which deliver grain and large square bales of straw and chaff as separate streams of product. Both one-pass and two-pass scenarios are described in Chapter 11 of Bioenergy in Australia: Status and opportunities, published by Bioenergy Australia in 2012. How much stubble do you leave on the paddock? CSIRO’s assessment of the availability of crop stubble as a potential biofuel resource states the following:
- There is little published information about the implications of harvesting stubble.
- Not all of the non-grain above-ground biomass of a crop will be available for collection.
- Cereal stubble consists of chaff (the material in the head that is separated from the grain at harvest), leaf matter and straw.
- Wheat stems (straw) are preferred over leaves and chaff for bioenergy production.
- The most common method of estimating stubble production is from grain yield data and knowledge about the ratio of grain to total above-ground biomass (known as ‘harvest index’, or HI).
- The amount of straw kept on the paddock will vary depending on soil type, topography and rainfall conditions. In southern areas, protection from wind erosion over the summer fallow is a priority and growers may keep about 1 t/ha of stubble. Where protection from intense rainfall is a priority, growers are likely to keep larger amounts of straw.
- Removing significant amounts of biomass is likely to lead to soil carbon depletion.
- Retaining loose chaff, leaf and stubble to protect the soil will help maintain soil carbon but little is known about how much stubble needs to be retained to maintain soil carbon in the long term.
Supplying stubble and strawSupplying and delivering cereal crop residues and grass crops, including stubble and straw, is described in detail in Chapter 11 of Bioenergy in Australia: Status and opportunities, published by Bioenergy Australia in 2012. Baling, cartage, storage and delivery to processor are covered in Section 11.2.5. In Western Australia, about 10 million tonnes of cereal straw are produced every year, on average. DAFWA is mapping potential biomass collection points in the WA wheatbelt, and this information can be made available to investors.
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