Improving soil moisture and minimising erosion with retained stubble
- Retained stubble will minimise wind erosion. It can also provide infiltration and moisture conservation benefits, depending on stubble load throughout the year.
- Leaving the stubble standing, with adequate residue spread, is the best way to minimise wind erosion and to capture any moisture conservation benefits.
- More than 8t/ha of stubble is needed to reduce evaporation and conserve moisture during summer. This may not always be achievable in the Mallee in the short-term, but it can be achieved through an accumulation over time in a stubble retained system.
- Loosely anchored crops (such as field peas) require more stubble to be retained to protect the soil from erosion, especially if harvested low.
Throughout the year, crop residues and stubble will play a fundamental role in moisture conservation and erosion prevention.
The most visual benefit of stubble retention has been in the protection of the topsoil from wind erosion during the summer months. The Wimmera Mallee region unfortunately is not immune to the violent northerly winds that occur during the peak of summer when daily temperatures can exceed 38oC. It is under these conditions that significant losses of topsoil, nutrients and moisture through surface evaporation will occur where there is a lack of groundcover.
While there is no escaping hot and windy days in summer, achieving adequate groundcover during this period is an essential management practice. Traditionally, practices such as cultivation and deep ripping were used to prevent erosion and continuous workings were common. These practices are unsuitable on sandy soils because not enough surface aggregates are produced by the cultivation or ripping to significantly reduce the wind speed and effectively protect the soil from erosion.
Maintaining the crop residue, or stubble, from the previous crop/s is the cheapest and most effective practice to reduce the soils exposure to wind erosion during the summer months. This is because the stubble will act as a physical barrier to the wind, subsequently slowing the movement of air closer to the soil surface and reducing the amount of soil blown. Similarly, structure and roots from stubble hold soil together and make it less prone to wind erosion. Residue laying on the surface (such as chaff) will provide similar benefits however, the material needs to be anchored either by weight or still attached to the ground.
What stubble load is required to reduce wind erosion?
Even low quantities of standing stubble (2t/ha) can reduce soil erosion, although there is a greater importance then placed on management of remaining stubble (Farmlink 2013). However, for surface evaporation to be significantly reduced over summer, a stubble load of greater than 8t/ha is required (figure 1.). While there may be a delay in the evaporation due to the reduction of wind at the surface, there will be little difference at the end of summer unless there is a substantial stubble load (>8t/ha) (Scott, BJ. 2010).
Can stubbles still be grazed?
Livestock are an integral part of many Wimmera Mallee farms and are vital for reducing whole farm risk. While grazing stubbles is not discouraged, growers need to monitor stubbles closely to ensure they don’t fall below 70% stubble cover on erosion prone land and at least 50% cover in April. These are thresholds required for optimum soil protection (McClelland T. 2012). Maintaining soil cover and ensuring at least 1.5t/ha of stubble remains is critical. Grazing can flatten and dislodge crop residue from the soil. If overgrazed, the soil will also become loose and friable which in turn will increase the risk of erosion.
Avoid grazing those less anchored stubbles such as field pea stubbles as more stubble from these crop types is needed to protect the soil.
Spatial grazing of sheep is another consideration as it is common for sheep to camp on the sides of hills. Research undertaken in Nandaly of the Victorian Mallee has shown that sheep tend to graze 75% of a paddock and leave 25% ungrazed, reducing carrying capacity by uneven grazing habits (Llewellyn et al. 2017). Portable electric fencing can be used to prevent them from camping on fragile areas in the paddock. This is not always a practical solution.
Crop stubbles provide a valuable food source to livestock in the Mallee over the Summer period. Research has shown that stubble grazing does not reduce the following crop yield or the amount of moisture stored in the soil. Stubble loads in this region are rarely great enough to have any impact on reducing evaporation so growers should make use of this feed source (McClelland T. 2012).
Is it better to leave the stubble standing?
There is very little evidence to indicate that standing stubble is more beneficial than slashed stubble in terms of protecting the soil.
Research undertaken in Colorado found that with a greater proportion of standing stubble, less moisture is lost from the soil during a fallow period with 50:50 standing and flat stubble retaining 52% of rainfall and bare soil only retaining 18.6% (Smika, 1983). This result shows some correlation to wind speed. Suggesting that the standing stubble could be acting as a small wind break between rows to reduce the speed of drying out of soils (figure 4).
That said, there are many other benefits of leaving the stubble standing as described in guidelines (inter-row sowing, establishing break crops, herbicide safety and efficacy, weed control, disease management).
It has been found that to minimise the wind erosion risk, 70 per cent ground cover (Figure 3) for laying stubble and 20-30 per cent for standing stubble (figure 5) is required (van Rees, H 2014).
What amount of stubble is required to conserve moisture in summer, winter and spring?
In general, the hardest period to conserve moisture by preventing evaporation is during summer when temperatures and evaporation rates are high. The extreme temperatures over a prolonged period of time can mean the net result is insignificant. However, differences in moisture loss through evaporation have been found when stubble loads exceed 8t/ha on the soil surface (Figure 1) (Scott, BJ. 2010). For many growers in the Wimmera and Mallee, this amount is mostly unrealistic, but over a number of years there are cumulative benefits. In this region, stubble loads are not usually great enough to reduce evaporation over the Summer months. Research undertaken by BCG in 2009 at Hopetoun in the Mallee saw no significant difference in plant available water at sowing following the summer period between stubble retained and bare ground treatments, rather the removal of weeds had the greatest impact on soil moisture when stubble loads were that typical of the region (Table 1).
This trial received Summer rainfall over relatively small events (no more than 27mm in one event) that would not have been able to infiltrate deeply enough into the soil to be protected from evaporation (Browne et al. 2009). However, it is known that retaining stubbles improves infiltration of water by reducing the flow rate over the soil surface (Hunt et al. 2013). Larger rainfall events therefore, may have seen different results between stubble retained and bare ground treatments.
Greater gains in moisture conservation under stubble retention have been observed closer to sowing. Seed bed moisture can be maintained for a longer period (two to three days) following a significant rainfall event when stubble is retained. This may allow for more crops to establish earlier or closer to the preferred date rather than having to wait until the next rainfall event, which is particularly important in dry seasons or those with late breaks (Radford and Nielsen, 1983).
During spring the value of the standing stubble may become less important as the crop has usually produced sufficient biomass and canopy closure to reduce wind erosion, often growing above retained stubble height. At this time, water use and transpiration from the crop can be the main reason moisture is removed from the soil. Wider row spacing or shorter crop types would leave a greater area exposed to solar radiation and evaporation and therefore benefits would be observed.
What effect does Stubble type have on wind erosion and soil moisture conservation?
The majority of data presented in this article, and research that has been carried out has been focused on cereal stubbles. The impact of stubble type will also come in to play when looking at stubble management for moisture conservation and controlling wind erosion.
BCG carried out a field trial in 2015 at Kalkee in the Wimmera looking at the effect of stubble type and stubble load on soil moisture and nutrient content following a summer fallow period. The lowest soil moisture at sowing following the summer where stubbles were retained, was observed in the low canola stubble load treatment although there was no statistical difference found between any treatments (Figure 6). However, soil analysis results showed a significantly higher nitrogen content under all stubble treatments when compared to samples from the fallow, an added benefit for the following crop from retaining stubbles.
Neither crop type or stubble loads achievable in the Wimmera Mallee for these 3 crop types will have a significant effect on the stored soil moisture at sowing. The presence of stubble on the soil surface will however, aid in water infiltration through slowing rainfall runoff speed (Hunt et al. 2013).
Stubble loads on the soil surface of 2.5t/ha and 5.t/ha have been found to reduce soil surface temperatures by up to 10˚C and 12˚C during the day and increase temperatures at night by 3˚C and 5˚C (Kitonyo et al. 2018). Reducing soil temperatures during the daytime will help to reduce moisture loss through evaporation from the soil surface.
Wind erosion will be a greater risk for those stubble types that are more loosely anchored to the ground, such as field peas and where a greater portion of ground is exposed. This may be an issue in canola stubbles where stubble is more sparsely distributed.
Research undertaken in the Mallee in 2006/2007 following drought conditions on 12 paddocks to assess soil erosion risk on tilled vs non tilled paddocks using the method of Leys (2002) looking at vegetative cover and soil aggregate size. Soil erosion was then measured using a dust sampler with measurements taken at multiple heights above the soil surface. It was found that soil loss from stubble retained paddocks was often below detectable levels (Moody and Chapman 2007). That, in fact only relatively low levels of stubble cover during January to March will provide effective protection against wind erosion of soil in the Victorian Mallee (Moody and Chapman 2007).
Browne C, Hunt J, Walsh M, (2009) Conserving Moisture During the Summer, BCG 2009 Research Results, p34. Available at: https://www.farmtrials.com.au/trial/13986
Hunt JR, Browne C, McBeath TM, Verburg K, Craig S, Whitbread AM (2013) Summer fallow weed control and residue management impacts on winter crop yield through soil water and N accumulation in a winter-dominant, low rainfall region of southern Australia, Crop and Pasture Science, 54, 922-934
Kitonyo O, Sadras V, Zhou Y, Denton M (2018) Improving wheat yields in no-till systems of southern Australia, Proceedings of the Adelaide GRDC Research Updates 2018
Llewellyn R, Moodie M, Monjardino M, Trotter M, Marini D, Lee C (2017). Potential for Virtual Fencing and Spatial grazing in Mixed Farming Systems, available at: https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2017/08/potential-for-virtual-fencing-and-spatial-grazing-in-mixed-farming-systems
Moody M and Chapman K (2007) Soil Erosion Assessment and Management, BCG 2007 Season Research Results pp. 204-208
Radford BJ and Nielsen, RGH (1983). Extension of crop sowing time during dry weather by means of stubble mulching and water injection. Australian Journal of Experimental Agriculture and Animal Husbandry 23, 302-308.
Scott BJ, Eberbach PL, Evans J and Wade LJ (2010). EH Graham Centre Monograph No. 1: Stubble Retention in Cropping Systems in Southern Australia: Benefits and Challenges. Ed by EH Clayton and HM Burns. Industry & Investment NSW, Orange. Available at: http://www.csu.edu.au/research/grahamcentre/
Smika DE (1983) Soil water change as related to position of wheat straw mulch on the soil surface. Soil Science Society of America Journal 47, 988-991. Available at: https://www.ars.usda.gov/ARSUserFiles/30100000/1980-1989Documents/1980-1989/146%201983%20Smika%20SSSAJ.pdf
Author: Simon Craig (Consultant)
This research is being conducted by BCG as part of the GRDC Maintaining profitable farming systems with retained stubble initiative (project BWD00024 ‘Maintaining profitable farming systems with retained stubble in Victoria and Tasmania’).