Managing Stubble After a Wet Harvest
Throughout multiple regions of Australia’s east coast, a continual downpour of rain has brought the prospect of a wet harvest to the forefront of farmers’ minds. As was the case in 2021, combines will be running late into the year and into early 2023, leaving a shorter space of time to deal with stubble residues. While a wet harvest may present a broad range of issues, most notably bogged machinery and soil compaction issues, managing stubble loads after machines leave the paddock presents a variety of additional problems.
Common Methods of Residue Control
Burning, incorporation, grazing and mulching: these methods are the most commonly used practices to control residue. In wetter seasons, these methods are even more popular, when full stubble retention creates dilemmas for the following season’s sowing.
Incorporating residue into the ground is still a popular and useful means of dealing with residue where required. By mixing residues with microorganisms in the soil, a quicker decomposition of residues is achieved. The incorporation of residue can also provide benefits in eradicating mice and, depending on the type of machine used, can be key in providing an even seedbed preparation. However, flooded soil can often create blockage issues in seeding and create wheel ruts.
For generations, burning off stubble residue has been a popular and effective method of control. This tried and tested method provides a variety of benefits, from the clean-up of trash to the control of mice infestations and herbicide-resistant weeds. Despite the longevity of the practice, burning off can also dry out the soil by losing the carbon matter that retains moisture. Due to increasing environmental awareness and calls for sustainability, other methods of stubble residue control have begun to take its place.
Grazing stubbles is yet another popular method of residue control when looking to clean a paddock. While this provides supplements for stock, it can take longer than other mechanical processes and also increases the threat of erosion due to too much soil being exposed. Another issue with grazing as a method of control is that there is limited control of how the stubble is cut, often resulting in a patchy breakdown of residue.
Retaining stubble after harvest has many benefits, including:
Reducing the risk of erosion over summer,
Maintaining and improving soil structure and health,
Aiding in nutrient retention,
Providing a source of feed for livestock,
Improving water infiltration and soil moisture retention.
The old-fashioned slasher has evolved over the years into a variety of powerful mulching machines that can easily clean up thick stubbles and residue. Whether it be a flex-wing slasher with a mulching kit fitted to it or a flail mulcher, these new machines can be used to finely mulch residue for a rapid breakdown process. This method of dealing with stubble first came to prominence when the chaff rows created by combines showed substantially more growth over other parts of a paddock in years with a below average yield. Slashing and mulching leave the root system in the soil, preventing the stubble from being blown away and the soil structure from being destroyed. However, flail mulchers bare the soil and can limit the ability to control cut height (the ideal cut height is 75-100mm).
The Process of Residue Breakdown
Residue decomposition includes the processes of nitrogen immobilization and mineralization, both of which involve soil microbes.
During decomposition, soil microbes feed upon the carbon in crop residue and require nitrogen for the process.
Immobilization occurs when nitrogen is consumed by soil microbes and is not available to plants.
Once the residue is decomposed, nitrogen is released back into the soil where it can be accessed by the new crop.
The quantity and quality of the stubble, as well as the handling of it, influence the supply of nitrogen. For example, legume residues generally contain more nitrogen, and, as a consequence, require less soil-derived nitrogen and break down more quickly than cereal stubbles. Poor quality cereal stubbles, (where the ratio of carbon to nitrogen is greater than 80:1) immobilize nitrogen during their initial composition, largely within the first 8 weeks, thus limiting the nitrogen available to the new crop.
A worst-case scenario for nitrogen tie-up is when large amounts of cereal residue are incorporated into the soil close to the time of sowing, as the competition for nitrogen between microbes and the crop is intense, and early nitrogen deficiency can limit crop yield potential in some circumstances.
The incorporation of smaller stubble pieces will potentially result in a more rapid, but shorter-lived immobilisation, due to a faster breakdown of organic matter and release of nitrogen. This shorter immobilisation period will allow the new crop to have quicker access to soil-derived nitrogen.
There are many methods used to manage residue after harvest, each with its individual benefits and problems. Deciding which method to use is always difficult, but, with a limited space of time between the end of harvest and the beginning of the next crop cycle, making the right decision becomes critical to the success of the new crop.
Choosing a residue management method that results in a faster breakdown of residue and a shorter nitrogen immobilisation period, will lower the chances of nitrogen deficiency in the new crop. Longer-term sustainability should also always be considered to ensure that the future of the land is not compromised through ground erosion, poor soil health or other degrading effects.