Take home messages
- Retaining cereal stubble increases soil microbial biomass and thus nitrogen (N) tie up.
- Rule of thumb – 1t/ha stubble ties up 5kg/ha N.
- Add N at sowing if soil N levels are below 40kg N/ha, nil if above.
- 7.5t/ha stubble contains N 56/kg/ha, P 5kg/ha, K 17kg/ha, S 1kg/ha.
- Calculate N requirements for each crop – the more relevant information you have available the more accurate your calculated rate will be
|KEY WORDS: Stubble, retained, nutrition, nitrogen, HRZ, top dressing, urea|
Stubble residues from cereal crops in the high rainfall zone (HRZ) contain a range of different nutrients. If residues are removed a high percentage of the nutrients are removed as well (Table 1).
Retaining cereal stubble increases microbial biomass (Chowdhury et-al 2015, CSIRO unpublished) which initially ties up N when breaking down the stubble to release the nutrients. CSIRO, FarmLink Research, Mallee Sustainable Farming (MSF) and Birchip Cropping group (BCG) have implemented trials tracing 15N (N labelled with nuclear isotopes) from 2014 to 2016. There was very little information available on the cycling of N in stubble retained systems prior to this research. The work has revealed some interesting insights into what happens to N in a stubble retained farming system (not published as not completed).
What N strategies can be used to maximise retained stubble crop Nutrition in the HRZ?
As in any agricultural system there are many variables to consider when managing nutrition for stubble retained systems. Rotations, weed burdens, disease pressures, pest burdens and enterprise mix all impact nutrition decisions. Below is a step-by-step approach to managing macro nutrients in retained stubble situations, with a strong focus on N management.
These provide valuable data on existing levels of nutrients in the soil. Advisors are well trained in assessing soil test results and providing recommendations specifically for our region. Stubbles can be tested for nutrient content and early research indicates 2-10kg N/ha from retained stubble is made available to the next crop depending on seasonal and nutritional conditions the previous season.
Rules of thumb
- If deep N’s are below 40kg N/ha add N at sowing. This can be applied separately, pre or post sowing, or with the seed at sowing in granular or liquid form. Additional N at sowing helps to provide the nutrition required to break down the previous stubble but also makes sure the germinating crop has sufficient nitrogen to establish, which ultimately helps maintain yield potential. If soil nitrogen is above 40kg N/ha it can be applied later when seasonal conditions can be more accurately assessed, this can help to minimise the risk of loss through denitrification or leaching. Having sufficient early nitrogen in the soil, in an available form, is vital to help maintain season long yield potential. Over winter, opportunities to spread N can often be limited in the HRZ, due to trafficability or waterlogging and a well developed plant will be better able to forage for all available nitrogen until late winter when the first top dressing is usually applied
Trial work by CSIRO and FarmLink Research in 2016 showed deep banding N at sowing is another approach that’s available to make nitrogen available to the crop later in the season. They achieved significant (p<0.01) increases plant dry matter at anthesis and increased yield compared to surface spreading at sowing. This is only one years information, from a high yielding season, and so should be treated with caution. If sowing into dry soil, care should be taken to separate seed from fertiliser to avoid damage to seed which may reduce germination percentages. Reference tables to help growers determine safe rates of a wide range of fertilisers with different sowing systems are available on the website of the International Plant Nutrition Institute, http://anz.ipni.net/articles/ANZ0044-EN
Estimate yield potential
This is a complex process and can be easily under or over estimated. Factors such as time of sowing, water logging damage, spring rainfall, heat stress damage, frost damage and disease incidence are unpredictable and difficult to manage. Use available tools to assist your N decision making process. Short term weather models are increasingly accurate, historical yield data from similar seasons, crop modelling tools such as Yield Prophet and moisture probe readings showing soil moisture levels can all help increase the accuracy of yield estimates.
Calculate N requirements for each crop
In order to try and calculate N requirements the more relevant information you have available the more accurate your calculated rate will be. The nitrogen supply for the crop will come from two areas:
- Soil N supply
- Soil N residues from the previous crop
- For soil N residues use deep N soil test data and split it up into zones in the soil (0-30cm, 30-60cm)
- Mineralisable N
- Potential nitrogen mineralisation, often based on soil organic matter levels
- Soil N residues from the previous crop
- Applied nitrogen
- Animal manures
For crop N requirement= Crop N demand (Yield potential + intended protein %) – Soil N supply
Rules of thumb
- Target application rates to yields. For nitrogen (N) apply 40kg N/ha for every t/ha of estimated wheat or barley yield and apply 80kg N/ha for every t/ha of estimated canola yield. Both these figures assume approx 50% nitrogen utilised from the total amount applied. Legumes may still require N in the early growth stages if it is at low levels.
- N mineralisation in stubble retained systems. N mineralisation in stubble retained systems is severely reduced due to microbial tie up reducing N mobilisation (5kg N/ha per t/ha of stubble). Mineralisation is very difficult to quantify accurately and is dependent on many factors.
- Calculation – A wheat crop paddock with 25kg N/ha available in the 0-60cm profile pre sowing, soil organic C =1%, 40kg N/ha applied at sowing and a yield estimated at 6t/ha is calculated thus;
Estimated yield x 40 – (N on soil test + N applied at sowing)
used to determine the soil’s nitrogen supplying
Rough guide to Nitrogen mineralised from soil (kg N/ha) = Organic C (%) x 50
6t/ha x 40kg/ha – (25kg/ha N in soil test + 40kg N/ha applied at sowing + 50 kg N/ha)
240 – (115) = 125kg/ha applied N or 272kg/ha urea
It may soon be possible to include the nutrient content of stubble in the calculation, but further studies are required to accurately estimate its contribution.
Urea (46%N) would need to be applied at 272kg/ha. Most growers will not apply such a large quantity in one single application and they will split the amounts to limit the potential losses from denitrification, run off or volatilisation.
Grain and fertiliser prices at the time of application will strongly influence what growers decide to apply. The decision on what rates to apply should be strongly influenced by each growers cost to benefit relationship of N and yield increases needed to pay for it and less so by agronomic ones.
- Monitor crop responses to applications. Tissue testing, visual inspection, tiller counts, dry matter (DM) production, NDVI, head and grain counts are all good indicators of potential N response of crops. Post-harvest grain protein levels are a good retrospective indicator of the success of your N management. Proteins above 11% indicate N management that maximised yield potential.
Retaining stubble in the HRZ provides some different challenges in comparison to traditional remove or burn systems. Rotations including a legume phase are an important component of a profitable stubble retention system for a range of reasons.
In the SW Victorian HRZ, faba beans have filled the legume role in continuous cropping systems and pastures play a pivotal role in mixed farming operations. SW Victorian HRZ is unsuited to many of the crop legume options available to other regions and finding other reliable legume species is seen as an important step forward in increasing profitability and reducing risk. This region can provide seasonal opportunities to sow summer legumes for grazing or N fixation but suitable species are again limited.
In-crop applications of N require a large financial investment to maximise yields in the HRZ. Limiting risk is also a priority that needs careful consideration. Using all available tools and knowledge can assist greatly in the decision making process.
Author: GRDC, SFS, Precision Agriculture, Vaderstad
Title: Managing Stubble
Author: S. CHOWDHURY1, M. FARRELL2, G. BUTLER3 & N. BOLAN4,5
1 Department of Civil and Environmental Engineering, Hannam University, 133, Ojeong Dong, Daedeok-gu, Daejeon, South Korea,
2 CSIRO Agriculture
3 South Australian No-till Farmers Association (SANTFA)
4 Global Institute for Environmental Research (GIER), University of Newcastle
5 Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC CARE)
Title: Assessing the effect of crop residue removal on soil organic
Carbon storage and microbial activity in a no-till cropping system
Author: Swan et al
Title: Maintaining profitable farming systems with retained stubble across various rainfall environments in SA, Victoria and central and southern NSW.
This research is being conducted by SFS 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’).
Disclaimer: Any recommendations, suggestions or opinions contained in this publication do not necessarily represent the policy or views of Southern Farming Systems (SFS) or the Grains Research and Development Corporation (GRDC). No person should act on the basis of the contents of this publication without first obtaining specific, independent professional advice. SFS and GRDC and contributors to these guidelines may identify products by proprietary or trade names to help readers identify particular types of products. We do not endorse or recommend the products of any manufacturer referred to. Other products may perform as well as or better than those specifically referred to. SFS and GRDC will not be liable for any loss, damage, cost or expense incurred or arising by reason of any person using or relying on the information in this publication.
Stubble project overview: This guideline has been developed for SFS Farming Systems Group as part of the Maintaining Profitable Farming Systems with Retained Stubble initiative, funded by the Grains Research and Development Corporation (GRDC). The initiative involves farming systems groups in Victoria, South Australia, southern and central New South Wales and Tasmania collaborating with research organisation’s and agribusiness to explore and address issues for growers that impact the profitability of cropping systems with stubble, including pests, diseases, weeds, nutrition and the physical aspects of sowing and establishing crops in heavy residues.