Ten key lessons from the ’Stubble Initiative’ every agronomist should know

Take home messages

• Be flexible and pro-actively manage stubble for your seeding system.
• Diversify your crops and practices as the platform for farming profitably with stubble.
• A critical threshold of 2-3t/ha provides most of the benefit but read on for 10 more key lessons to retain more stubble profitably – to save soil, water and $50-$100/ha.

Background

The ’Stubble Initiative’ was a five-year program of research (2014-2018) initiated by GRDC following a comprehensive gap analysis (Scott et al.2013) which aimed to provide management guidelines for profitable farming with retained stubble. Farming systems groups (Birchip Cropping Group (BCG), Central West Farming Systems (CWFS), Eyre Peninsula Agricultural Research Foundation (EPARF), FarmLink Research, Hart Field Site group, Irrigated Cropping Council (ICC), Southern Farming Systems (SFS), Lower Eyre Agricultural Development Association (LEADA), MacKillop Farm Management Group (MFMG), Mallee Sustainable Farming (MSF), Riverine Plains, Upper North Farming Systems (UNFS), YP Alkaline Soils Group (YPASG), Vic No Till Farmers Association Inc (VNTFA), Mid North High Rainfall Zone Group (MNHRZ) and Yeruga Crop Research) worked on locally relevant issues and contributed to co-ordinated research (CSIRO) and extension (SARDI) to develop regional guidelines to maintain profit in stubble retained systems. These guidelines and a range of other detailed extension material have been produced and communicated throughout the Initiative (see reading and reference list for links). Here I have selected 10 of the key lessons from the initiative to highlight those which are novel and have the most potential to influence the enduring profit of growers with stubble retained systems.

Lesson 1. Be flexible – no strategy is perfect for every farm or paddock

Stubble is important for soil protection, water conservation and soil organic matter. But it must be managed without compromising the big drivers of profits - timeliness of sowing, target plant populations, effective weed, pest and disease control and crop nutrition. There is no perfect stubble management strategy for every paddock – crop sequence, weed spectrum, stubble loads and available machinery all dictate how stubble can be best managed. Start planning before harvest, and continue through the fallow to sowing. Fortunately, the first few t/ha of stubble provide most of the benefits with few issues, and numerous options exist to manage heavier stubbles well (Swan et al. 2018)

Lesson 2. Diverse systems and practices are the platform for success

Diversity in crops and practices will provide a platform for success and allow effective management of weeds, diseases and nitrogen (N) nutrition in stubble retained systems. In the low rainfall zone, economic analysis of 5-year sequences conducted by EPARF with Ed Hunt showed that a two-year pasture break with wheat was the lowest risk option in dry years but did not capture all of the economic benefits of continuous wheat in above average seasons. A two-year break with well adapted legume and canola had lower risk than continuous wheat and also captured the upside in an average season (GRDC Groundcover Supplement 135, pp 7). In the medium rainfall zone at Temora, FarmLink Research and CSIRO demonstrated that in a paddock with high levels of resistant ryegrass, a diverse sequence of vetch (hay)-canola-wheat-barley allowed all stubble to be retained, while generating the highest profit at lowest risk and driving the annual ryegrass (ARG) weed seedbank to very low levels (Table 1). This performed better than the traditional canola-wheat-wheat systems managed either conservatively (triazine tolerant (TT) canola, lower N) or aggressively (Roundup Ready® (RR) canola, higher N, expensive pre-emergent program), and when scaled up to the whole-farm was less volatile (Swan et al. 2018).

Table 1. Economic performanceand final ryegrass seedbank for different management strategies in a 4-year field experiment at Temora (FarmLink and CSIRO).

Lesson 3. Pro-actively manage stubble to suit your crop, seeding system and farm structure

Managing stubble effectively begins at harvest, and continues through the fallow period to seeding, where the priority must be timely and effective crop establishment and good weed management. Harvesting high or with stripper fronts is fast and efficient (10% reduction in speed per 10cm reduction in harvest height) and can save about $7.00 to $9.00 per 10cm increase in harvest height. Tall standing stubble is more suited to disc-seeding operations, or where grazing, mulching, baling or burning are planned later. The choice of row spacing at sowing will be important depending on the seeding system and its stubble clearance capability (disc or tyne), and the stubble load. Increasing rows above 7 inches (18cm) for inter-row sowing or to avoid blockages can result in yield penalties in cereals, and also reduces crop competition with weeds (GRDC Groundcover Supplement 135, pp 11). These decisions regarding appropriate optimal seeding strategies for stubble management can be very enterprise dependent. For example, whole farm economic analysis derived from experiments on disc and tyne systems in different crop sequence strategies suggested that where farm structure is highly efficient, a tyne opener was more profitable than a disc opener on 300mm row spacing. However, higher returns on assets managed were achieved with a disc opener on 190mm spacing compared to a parallelogram tyne seeder on 300mm row spacing (Swan et al. 2018).

Lesson 4. Water management – maintain a critical threshold – and control fallow weeds

Soil protection and water conservation over the summer-autumn fallow period are the major benefits of stubble retention, and the good news is that the first few t/ha of stubble do most of the work.

Maintain a threshold

About 2t/ha to 3t/ha of cereal residue will achieve 70% cover to minimise erosion and maximise infiltration.

Strict summer weed control

Weeds growing in stubble (even if grazed) use precious water and N. Strict summer weed control can save 40mm water and 40kg N, returning $5.60 on every dollar spent. Retaining stubble but growing weeds makes no sense.

Will more stubble help?

Heavier stubble loads can slow evaporation and increase the duration of soil water storage near the surface in autumn, allowing successful early establishment in some seasons. Benefits will only be obvious in some seasons (e.g. 2017 and 2018), as they depend significantly on the soil type and rainfall timing (Table 2). In that example, stubble could save a maximum of 4mm of water in January, but 14mm in June when 45mm rain fell in two events. But a good policy is to retain as much stubble as possible that allows timely, successful establishment and effective weed control with your seeding equipment. Otherwise, plan to reduce heavy stubble loads with a suitable approach (Swan et al. 2017).

Table 2. Extra water saved from evaporation by 4t/ha stubble (compared to 0.4t/ha) over 30 days with different rainfall distribution under high (January) and low (June) evaporative demand (from Verburg et al. 2012).

Will grazing my stubble reduce water storage and yield?

Several studies have shown that light grazing of stubble had little impact on water storage or yield provided the threshold 2-3t/ha was retained. Grazing heavy stubbles increased N availability to crops and yield in some seasons. Overgrazing and baring out the soil is the real problem – ’sheep do more damage with their mouths than their hooves’.

Lessons 5 and 6. Weed management in stubble retained systems

Long term pro-active weed management using a range of different strategies is required to reduce weed seedbanks and reduce the reliance on selective herbicides (see also Lesson 2).

Lesson 5. At harvest and during the fallow

Harvest weed seed control (HWSC) is influenced by harvest height – dropping from 40cm height to 10cm height increased ryegrass seed capture from 48% to 88% (Groundcover Supplement 135, pp 4). For brome grass, a substantive effect of HWSC relies on timely removal, due to the significant shedding of seed over the time during which harvest generally occurs (Figure 1). The HWSC treatments reduced the subsequent in-crop brome population by 15% more at the earliest time of harvest.

Figure 1. Percentage of brome grass seed remaining at harvestable height over the duration of the harvest period

As a standalone tactic, a 25% reduction in the subsequent weed population was achieved. However, HSWC may still be useful in combination with other more effective control strategies, such as herbicide and sequence-based strategies.

Lesson 6. In-crop weed management

Herbicide efficacy in stubble retained systems

Stubble can prevent pre-emergent herbicides from reaching the soil surface, leading to uneven coverage and reduced efficacy. The taller the standing stubble, the more difficult it was for spray to penetrate to the soil surface. Slowing the speed of the spray rig (<16km/hr), increasing the water rate (80L to 100L) and spray pressure and spraying in the direction of the rows maximised the herbicide hitting the target. Match nozzle spacing to row spacing (when inter-row sowing) to position nozzles between stubble rows ideally on no wider than 25cm nozzle spacing (Groundcover Supplement 135, pp 13).

Crop competition

Increased crop competition through closer row spacing, high vigour varieties, soil wetter agents and edge-row sowing (in non-wetting soils) can increase the relative advantage of the crop against weeds. The newly developed tool Brome RIM (BRIM) found that for brome grass, the net benefit of increasing crop competition was $24/ha and $16/ha for brome and barley grass, respectively.

Alternative herbicides

A range of alternative pre-emergent herbicides that can lead to better brome grass control under no-till seeding conditions were tested in collaboration with the University of Adelaide Weeds research group. Substantially better brome control and seed set reduction were possible using pre-emergence options other than trifluralin. Treatments that included Sakura® resulted in significantly better control than using trifluralin alone with the costly Sakura® + Avadex®  herbicide mixture the most effective (Table 3). However, consistency of performance of all pre-emergent herbicides for brome remains a problem. The additional herbicide cost needs to be evaluated against the better level of control, including the longer-term seedbank benefits.

Table 3. Brome panicle density as affected by different herbicide treatments at Karoonda in 2015.

The Australian Herbicide Resistance Initiative’s (AHRI’s) WeedSmart program provides a range of useful information to manage weeds effectively.

Lessons 7 and 8. Stubble retained system may need more early N

Stubble retained systems need more N than traditional burnt systems, because stubble can tie up N in soil (20-30kg/ha), and because the N in stubble is not a good direct source of N for crops, providing only 1%-6% of subsequent crop requirement.

Lesson 7. Avoid yield penalties from N immobilisation

Immobilisation

Surface retained cereal stubble in modern no-till systems can immobilise N, constrain young crops and reduce yields (by 0.3-0.5t/ha). Adding more N (up to 5kg N per t/ha of cereal residue), adding it early and deep banding if feasible can avoid the problem. Reducing the stubble load to an acceptable threshold also reduced yield loss related to immobilisation.

Timing of N supply

Optimum timing of N depends on up-front fertility - 40-60kg N/ha in top 60cm is required at sowing to allow crop progression to tillering - and the opportunity to apply N in-season.

Managing N on low fertility soils

Low fertility soils have a high demand for N in intensive cropping systems, as demonstrated by N responsiveness in wheat and canola following grain legumes. Profit-Risk analysis has shown that soil-specific N management offers some risk management by increasing inputs on responsive, low fertility soils and reducing inputs on non-responsive soils.

Increasing fertiliser N use efficiency (NUE).

There is scope to increase fertiliser NUE using different N sources, multi-nutrient (N x phosphorus (P) x zinc (Zn)) and fertiliser placement.

Lesson 8. Stubble is not a good source of N for crops

Significant N is known to be lost when stubble is burnt (up to 70%, Scott et al. 2013), but that does not mean that the N in retained stubble is readily available to crops. Studies at three sites (Temora, Horsham, Karoonda) measured the fate of the N contained in wheat residue over two years and found that only 1%-6% of the N was recovered by the following two crops. Stubble carbon (C) and N is a slow and long-term source of N via microbial pools and makes a minor direct contribution to subsequent crops (Kirkegaard et al. 2018).

Lesson 9. A lack of nutrients (NPS) can limit soil C sequestration from stubble under continuous cropping

Conservation cropping systems with no-till and stubble retention can certainly maintain cover, protect soil structure and reduce erosion, but at best can generally maintain, rather than build (sequester) stable soil organic matter. Stable soil organic matter (which is how C is sequestered in soil) is not just C – it is made up of dead and live microbial material with a known ratio of C, N, P and sulphur (S). Crop stubbles are rich in C but nutrient poor, and it may be a lack of nutrients rather than too little C (i.e. stubble) that limits C-sequestration. We have demonstrated that the addition of supplementary nutrients (NPS) could increase the rate of stubble decomposition and the C sequestered in an experiment where 9t/ha of crop residue was retained annually (see Swan et al. 2017 for worked examples). The practical advice from this work is to be conscious that stubble alone will not build soil organic matter, nor arrest its decline, and that maintaining balanced fertility (especially NPS) is essential to maintain crop production and soil fertility.

Lesson 10. Pest management

Conventional practices such as the use of baits and insecticides can be optimised further in retained stubble systems with improved information on their timing, rate and distribution in relation to reproductive and feeding behaviour of the pest.

Snails

Snails are a major pest associated with stubbles, and increasingly threaten market access in China for wheat and barley (AU$1 billion per annum) as vineyard and pointed snails are biosecurity threats. The snail work by LEADA showed that mechanical snail control in late February when hot and dry (light tillage, heavy ribbed rolling) was effective to reduce snail numbers initially, and improved the access to bait applied later. Delaying baits until cooler weather when snails were more active and feeding proved more effective.  Decreasing the number of ineffective bait applications, even by one (Luke and Scott Clark, Jamestown SA), would save growers $10-$12/ha (Meta® @ the label rate). This saving would see a reduction in the overall enterprise cost of snail management, down from $33.10 ± 7.61/ha (GRDC report 2014 DAS00134), whilst maintaining access to premium wheat and barley markets.

’Snails need to be monitored regularly to establish numbers, types, activity and success of controls’

(http://www.leada.org.au/wp-content/uploads/2018/04/Managing-Snails-in-Stubble-Retained-Systems-on-Lower-Eyre-Peninsula-Output-5.pdf).

Earwigs and other seedling pests

For earwigs, detection early in the growing season (late autumn and early winter) is desirable to allow treatment before the juveniles hatch in spring. Understanding when females lay eggs in soil and better monitoring techniques are required.  Canola is especially vulnerable to attack by establishment pests and more information is required to optimise current control advice. https://doi.org/10.25919/5bb385ea01c28

Conclusion

A large collection of regional guidelines on numerous aspects of improving profit in retained stubble systems were developed during the Stubble Initiative in the periods from 2014-2019 and are available on the websites of the grower groups involved, and in recent GRDC Updates and Groundcover articles. We have highlighted some key lessons in this paper, but urge readers to seek more detailed information from the resource and reference list below.

Useful resources

https://grdc.com.au/research/trials,-programs-and-initiatives/stubble-initiative

References

GRDC GROUNDCOVER Supplement Issue 135: July-August 2018.

Kirkegaard et al. (2018). The effects of stubble on nitrogen tie-up and supply. GRDC Updates.https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2018/02/the-effect-of-stubble-on-nitrogen-tie-up-and-supply

Scott et al. (2013). Developments in Stubble Retention in Cropping Systems in southern Australia (2013). (Report to the GRDC by NSW DPI). https://grdc.com.au/resources-and-publications/all-publications/publications/2013/12/developments-in-stubble-retention

https://www.csu.edu.au/research/grahamcentre/publications/monograph-series

Swan et al. (2017).Maintaining profitable farming systems with retained stubble across various rainfall environments in SA, Victoria and central and southern NSW. GRDC Updates in Adelaide, Wagga Wagga and Bendigo.

Swan et al. (2018). Flexible stubble management – how to reap returns to the bottom line. GRDC Business Updates Wagga Wagga. https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2018/08/flexible-stubble-management

Verburg K, Bond WJ, Hunt JR (2012). Fallow management in dryland agriculture: Explaining soil

water accumulation using a pulse paradigm. Field Crops Research 130, 68-79.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of numerous growers and grower groups (BCG, CWFS, EPARF, FarmLink Research, Hart Field Site group, ICC, SFS, LEADA, MFMG, MSF, Riverine Plains, UNFS, YPASG, VNTFA, MNHRZ, Yeruga Crop Research) through both trial cooperation, and the support of the GRDC. CSIRO (research component) and SARDI (extension) are also acknowledged. The author thanks all those participants who provided the material presented on their behalf, and their efforts to develop regional guidelines to maintain profit in stubble retained systems.

Contact details

John Kirkegaard
CSIRO Agriculture and Food, GPO Box 1700, Canberra ACT 2601
0458354630
john.kirkegaard@csiro.au
@AgroJAK