Manage stubbles without compromising the ‘big things’ – weeds, disease, pests and timeliness!

Take home messages

• Don’t let stubble compromise the BIG THINGS - weeds, disease, pests and timeliness.
• Be flexible and pro-actively manage stubble for your seeding system.
• Harvest high = reduced costs and is quicker.
• Cereal stubble should be thought of as a source of carbon (C), not nitrogen (N) (<6% crop N needs obtained from straw).
• Post-harvest management: if necessary reduce stubble load by mulching, incorporation with nutrients, baling or grazing.
• If stubbles are too thick to sow through, consider strategic late burn, especially before the second wheat crop or if sowing canola into large stubbles.
• Managing N at sowing: deep band N and add 5kgN/t cereal stubble.
• Diversify your crop sequence and add legumes to rotation.
• Incorporate AHRI’s ‘Big 6’ for weeds (disc or tine)

Background

Following a GRDC review that identified gaps regarding the impact of stubble retention in southern cropping systems, a five-year program was initiated by GRDC in 2013. Ten projects comprising 16 farming systems groups and research organisations which include BCG, CSIRO, CWFS, EPARF, Farmlink Research, Hart Field Site group, ICC, LEADA, MFMG, MSF, Riverine Plains, SARDI, UNFS, VNTFA, Yeruga Crop Research were involved in exploring the issues that impact on the profitability of retaining stubbles across a range of environments in southern Australia.

Previous studies have highlighted potential negative yield impacts of retained stubble in southern NSW (Kirkegaard 1995; Scott et al. 2013), but strict no-till advocates recommend retaining all of the stubble to enhance water capture and storage, ‘soil health’ and crop yields. Over past decades, growers and scientists have continued to examine methods to flexibly manage stubble in order to improve profitability. These methods include the use of zero till (disc seeders), diversifying management strategies (changing crop sequences) and N and herbicide options.

In this paper, the questions growers and advisers need to ask when managing stubble using a flexible approach are examined. Some of the main issues that growers need to deal with when trying to retain stubble in their farming systems are examined. More in-depth information and explanations on experiments, methodologies and results can be found in the following papers and on the GRDC website (see Reference section of this paper).

The authors would also like to acknowledge the many co-authors who contributed to two previous GRDC 2017 update papers, Swan et al 2017a and Kirkegaard et al 2018, where large sections of those papers have been reproduced here. The authors would like to especially thank Paul Breust, Claire Brown, Amanda Cook, Blake Gontar, Clive Kirkby, Helen McMillian, Michael Nash, Sarah Noack, Trent Potter, Cassandra Schefe, Naomi Scholz and Felicity Turner.

Managing stubble with a flexible approach

There are many benefits to a flexible approach to retaining stubble. There is no perfect stubble management strategy for every year. Crop rotations, weeds, disease, pests, stubble loads, sowing machinery and potential sowing problems will largely dictate how stubble is managed.

A flexible approach means crops can be harvested high or low depending on the season and situation, stubbles can be retained intact, but could also be grazed with considerable economic advantage, or straw baled and sold, or burnt. Consider reducing stubble in paddocks where the stubble may impact the following crop yield e.g. wheat on wheat paddocks.

It has been well documented that to successfully establish a crop into a full stubble-retained system requires an integrated management approach incorporating three stages of stubble management – (i) pre-harvest, (ii) post-harvest/pre-sowing, and (iii) at sowing. During these periods, a series of questions should be considered:

• What is my seeding system – disc or tine – row spacing and accuracy of sowing?
• What crop will be sown into the paddock next year?
• What crop am I harvesting, potential grain yield and estimated crop residue level?
• What is the preferred harvest height and potential harvest speed?
• Is the crop standing or lodged?
• Do I need to harvest very low and spread straw evenly – tine/weeds?
• Do I have a weed problem which requires harvest weed seed control: narrow windrow burning, chaff carts or chutes, integrated Harrington Seed Destruction (iHSD)?
• Will I need any post-harvest stubble management (grazing, baling, mulching, incorporate plus nutrients or to burn)?
• What is the risk of pests and disease in the following crop?
• What herbicide options am I considering for all crop types and stubble loads?
• What is the erosion risk based on soil type and topography?

Prior to harvest, all crops should be assessed to estimate grain yield, potential stubble load and weed issues. As a rule of thumb, the stubble load following harvest will be approximately 1.5 to 2 times the grain yield for wheat and between 2 to 3 times the grain yield for canola.

Stubble height

Using a stripper front or harvesting high is the quickest and most efficient method to produce the least amount of residue that needs to be threshed, chopped and spread by the combine. Harvesting high (40cm) compared to low (15cm) increased grain yield and combine efficiency by reducing bulk material going through the header and reduced harvests costs by 37% (Table 1). As a general rule, there is a 10% reduction in harvest speed for each 10cm reduction in harvest height (Tables 1). Slower harvest speed across a farm also exposes more unharvested crop to the risk of weather losses (sprouting, head/pod loss, lodging) during the harvest period, and the cost of this is not accounted for in Table 1.

Table 1. Harvesting wheat low or high using a Case 8230 combine with a 13 m front in 2015. Ground speed was altered to achieve similar level of rotor losses at both harvest heights. Operating costs determined at $600/hr.

ns = no significant difference

However, there are some negatives to retaining tall wheat stubble. Several farming systems groups in the GRDC stubble initiative found that wheat sown into taller wheat stubble (45cm compared with 15cm) received less radiation and were exposed to cooler temperatures. This can reduce early growth and significantly reduce tiller numbers. For example, in a Riverine Plains experiment in 2014, there was a significant reduction in grain yield (4.98t/ha compared with 5.66t/ha with lsd @ P<0.05 = 0.45t/ha) in tall compared to short stubble. In 2015 the group found no difference in grain yield. In 2016, significantly less tillers were found in several trials in tall stubble, however in all of these trials, this did not result in any difference in grain yield.

Herbicide resistant weeds, especially annual rye grass (ARG) continues to be a problem. Harvest weed seed control (HWSC) which includes narrow windrow burning, chaff carts, chaff lining, direct baling, and mechanical weed seed destruction is an essential component of integrated management to keep weed populations at low levels and thus slow the evolution and spread of herbicide resistance. HWSC requires crops to be harvested low in order for weed seeds to be captured in the chaff fraction from the combine, and if practiced provides an additional reason to harvest low.

The prototype iHSD was tested in Temora, NSW in December 2015, Inverleigh in December 2015 and Furner, SA in January 2016 at a constant speed of 4 km/hr to compare the efficiency and cost with non-weed seed destruction methods (Table 2). Early results indicated that there was an increase in fuel usage by 38% (L/ha) and 47% (L/hr), respectively. The challenge for growers is making sure the weed seeds enter the harvester in the first place. To learn more about SA growers reaping the benefits of HWSC, read the article by Trent Potter in the GRDC Groundcover Supplement, Issue 135.

Table 2. A Case 9120 harvesting wheat conventionally at 30cm, harvesting at 15cm for baling or narrow windrow burning and harvesting at 15cm with a prototype iHSD at Furner, SA in 2016. Data supplied by GRDC project SFS00032.

A pro-active and flexible approach to stubble management is one that recognises and avoids situations in which stubble can reduce productivity or profitability. It must be acknowledged that following harvest, there is often large amounts of retained stubble with a high C:N ratio, especially cereal stubble. This can ‘tie-up’ soil nitrogen leading to N deficiency in the following crop and could reduce yield. The timing, extent and consequences of N tie-up are all driven by variable weather events (rainfall and temperature) as well as soil and stubble type, so quite different outcomes may occur from season to season and in different paddocks (Kirkegaard et al 2018).

Can stubble really reduce yield significantly in no-till systems – and is N-tie up a factor?

From Kirkegaard et al 2018.

Harden long-term site

In a long-term study at Harden in NSW (28 years) the average wheat yield has been reduced by 0.3t/ha in stubble retained versus stubble burnt treatments, but the negative impacts of stubble were greater in wetter seasons (Figure 1). Nitrogen tie-up may be implicated in wetter years, due to higher crop demand for N and increased losses due to leaching or denitrification. But we rarely found significant differences in the starting soil mineral N pre-sowing. For many years, we were not convinced N tie-up was an issue (though we had insufficient measurements to confirm it).

Figure 1. Effect of retained stubble on wheat yieldis worse in wetter seasonsat the Harden(circles) and Wagga (squares) long-term tillage sites. Open symbols where difference between retain and burnt were not significant (NS), solid where significant (S).

The process of ‘N-tie up’ (immobilisation) – put simply

From Kirkegaard et al 2018.

Growers are always growing two crops – the above-ground crop (wheat, canola, lupins etc) is obvious, but the below-ground crop (the microbes) are always growing as well; and like the above-ground crop they need water, warm temperatures and nutrients to grow (there’s as much total nutrient in the microbes/ha as in the mature crop, and two-thirds are in the top 10cm of soil). There are two main differences between these two ‘crops’ – firstly the microbes can’t get energy (carbon) from the sun like the above-ground plants, so they rely on crop residues as the source of energy (carbon). Secondly, they don’t live as long as crops – they can grow, die and decompose again (‘turnover’) much more quickly than the plants – maybe 2-3 cycles in one growing season of the plant. The microbes are thus immobilising and then mineralising N as the energy sources available to them come and go.

In a growing season it is typical for the live microbial biomass to double by consuming carbon (C) in residues and root exudates – but they need mineral nutrients as well. Over the longer-term the dead microbe bodies (containing C, N, phosphorus (P), sulphur (S) become the stable organic matter (humus) that slowly releases fertility to the soil. In the long-term, crop stubble provides a primary C-source to maintain that long-term fertility, but in the short-term the low N content in the cereal stubble means microbes initially need to use the existing soil mineral N (including fertiliser N) to grow, and compete with the plant for the soil N.

Cereal stubble isn’t a good source of N for crops

From Kirkegaard et al 2018.

Studies at three sites in southern Australia (Temora, Horsham and Karoonda) have tracked the fate of the N in stubble to determine how valuable it is for succeeding wheat crops under Australian systems. Stubble labelled with 15N (a stable isotope that can be tracked in the soil) was used to track where the stubble N went. At Temora (Figure 2), of the 55kg/ha of N contained in 7.5t/ha of retained wheat residue retained in 2014, only 6.6kg/ha N (12%) was taken up by the first crop (representing 12% of crop requirement); and 5.6kg/ha N (10%) was taken up by the second wheat crop (4.4% of crop requirement).

The majority of the N after two years remained in the soil organic matter pool (19.1kg N/ha or 35%) and some remained as undecomposed stubble (10% or 5.5kg N/ha). Thus we can account for around 67 % of the original stubble N in crop (22%), soil (35%) and stubble (10%) with 33% unaccounted (lost below 50cm, denitrified). In similar work carried out in the UK which persisted for four years, crop uptake was 6.6%, 3.5%, 2.2% and 2.2% over the four years (total of 14.5%), 55% remained in the soil to 70cm, and 29% was lost from the system (Hart et al., 1993).

The main point is that the N in cereal stubble represented only 6% of crop requirements over two years (7.6% Year 1; 4.4% Year 2) and takes some time to be released through the organic pool into available forms during which losses can occur.

Figure 2. The fate of the N contained in retained wheat stubble over two years in successive wheat crops following the addition of 7.5t/has of wheat stubble containing 55kg/ha N. The successive crops took up 12% (6.6kg N/ha) and 10% (5.6kg N/ha) of the N derived from the original stubblerepresenting only 7.6% and 4.4% of the crops requirements. Most of the stubble N remained in the soil (35%) or was lost (33%).

Post- harvest management options in stubble retained farming systems

Option 1: MULCH and incorporate

Lightly incorporating the stubble into the surface soil using a disc chain or disc machine (i.e. Speed tiller, Grizzly, Amazone Cattross, Vaderstad Topdown or Lemken Heliodor) soon after harvest while the stubble has a higher nutritional value is another option for growers wanting to maintain all of their stubble, especially where a tined seeder is the primary sowing implement, or where lime and stubble needs to be incorporated into the soil in a disc-seeding system. On the lighter sandier soils in SA, the recommendation would be to delay incorporation until 3-4 weeks before seeding as these soils are more prone to wind and water erosion. Mulching and incorporation requires soil moisture, warm soil temperature, soil/stubble contact and nutrients to convert a carbon rich feed source into the humus fraction. Early mulching and incorporation allows time for the stubble to decompose and immobilise N well before sowing, reducing the likelihood of reduced N availability.

When trying to decompose a large quantity of stubble in a short period of time (i.e. to convert stubble into humus), it may be beneficial to add some nutrients to the stubble prior to incorporation. To assist in minimising the amount of fertiliser required to add to the stubble, determining the concentration of the nutrients in the stubble is important. As humus is so nutrient rich and the stubble residues are relatively nutrient poor, only a small proportion of the total carbon in the crop residues can be converted into humus. Dr Clive Kirkby has found that a maximum of 30% of the total carbon from stubble residues could be converted to humus, so recommends lowering the humification rate to 20% rather than 30%.

In our example (Table 3), the quantity of fertiliser (sulphate of ammonia) that would need to be applied to the 10t/ha residual cereal stubble load where the stubble had a nutrient concentration of 0.7%N, 0.1%P and 0.1%S and the grower wanted a humification rate of 20% would be 33.1kg/ha of N and 7kg/ha of S at an estimated cost of $14.90/ha for nutrients only. In contrast, if a grower was trying to build up their organic carbon concentration in the soil from this stubble residue to the maximum possible amount (30% humification rate), the quantity of nutrients required increases to 45.4kgN/ha, 3.8kgP/ha and 7.6kgS/ha, at a cost of $74.40 for nutrients (Table 4). The nutrients applied are not lost, but should form a source of slow release nutrition to the following crop as humus while avoiding ‘nutrient tie-up’ caused by late incorporation of nutrient poor residues. Thus, later inputs could potentially be reduced if costs were of concern.

Table 3. A screenshot of Dr Clive Kirkby’s stubble nutrient humification calculator to estimate the amount of fertiliser (N and S only) as Sulphate of ammonia (kg/ha) that would need to be applied to a cereal stubble load of 10t/ha with a humification rate of 20% to assist in rapid breakdown of the residual stubble.

Financial support provided by NIEI, EH Graham Centre, CSIRO and GRDC project DAN00152.

In an experiment at Harden, NSW between 2008 and 2011, Dr Kirkby incorporated between 8.7 and 10.6t/ha of cereal or canola stubble without nutrients or with nutrients at a humification rate of 30%. In May 2009, following the incorporation of 8.7t/ha wheat stubble in February 2009, they measured the quantity of wheat stubble that had broken down and found that only 24% of the stubble remained where nutrients had been added whereas 88% remained where the stubble had been incorporated only (Kirkby et al. 2016).

Table 4. A screenshot of Dr Clive Kirkby’s stubble nutrient humification calculator to estimate of the amount of fertiliser (N:P:S) as Urea and Single Superphosphate (kg/ha) that would need to be applied to a cereal stubble load of 10t/ha with a humification rate of 30% to assist in more rapid breakdown of the residual stubble.

Financial support provided by NIEI, EH Graham Centre, CSIRO and GRDC project DAN00152.

Option 2: Baling

In many areas across southern Australia, a significant area of stubble was baled in 2016/17 season. Baling allows the grower to harvest high and efficiently (use stripper front if possible), and reduce the stubble load in the paddock to minimise problems at sowing. One of the negatives of baling stubble is the loss of nutrients from the paddock, however, we should continue to think of cereal stubble as a source of C, rather than N.

Option 3: Grazing

For mixed farmers, the option to graze the stubble soon after harvest can be quite profitable. In a long term no-till controlled traffic grazing experiment in Temora between 2010-2017 with a crop rotation of canola-wheat-wheat, four treatments were compared including a full stubble retention system (nil graze, stubble retain) and a post-harvest grazing of the stubble (stubble graze, stubble retain). Each of these were split to accommodate a late burn pre-sowing (i.e. nil graze, stubble burn & stubble graze, stubble burn) (Table 5). In the stubble retain treatment, stubble was left standing through summer, and fallow weeds were strictly controlled. All plots were inter-row sown with deep knife points and machinery operations conducted using controlled traffic. Stubble grazed plots were grazed within 2-3 weeks of harvest at approximately 300 dry sheep equivalent (DSE)/ha for five days ensuring >3t/ha remained for soil protection and water retention. All plots were sown, fertilised and kept weed free such that weeds, disease and nutrients did not limit yield.

Averaged across both phases for the seven years of the experiment, grazing and then retaining the stubble generated the highest gross income with a $55/ha increase in gross income where sheep were used to graze the stubbles compared to nil grazing if no grazing value was assumed. This increase was related to higher yields and grain quality in subsequent crops driven by greater N availability in the grazed stubble. If the grazing was valued assuming one DSE consumed 7.6MJ of energy per day at an agistment rate of $0.4/DSE/week, the grazing value of the stubble was $117/ha/year, combined with the increase $55/ha/year due to yields and N availability resulting in a total increase of $172/ha/year.

Table 5. Gross income per year averaged across two phases where stubble was either grazed post-harvest or not, and either burnt just before sowing or retained, 2010-2016 at Temora, NSW.

At the Temora experiment, grazing stubbles never reduced the yield of any crop at the site, but increased the yield of the second wheat crop by 1.2t/ha in 2013 (Phase 1) and by 1.0t/ha in 2015 (Phase 2) (Table 6). This was unrelated to pre-sowing soil N in 2013 (both had ~85kg N/ha at sowing) where we suspect increased frost effects in the un-grazed stubble – while in 2015, the yield benefit was related to pre-sowing N with an extra 61kg/ha N at sowing in the grazed plots. Overall, grazing increased the pre-sowing N by 13kg/ha in the first wheat crop and by 33kg/ha in the second wheat crop (Table 6). See also Swan et al (2018) “Grazing benefits stubble-retention cropping systems as well as livestock in GRDC Groundcover Supplement Issue 135; July-August 2018.

Table 6. Effect of grazing stubble on grain yields at Temora in Phase 1 and 2. Crops in italics are canola, and bold are the 2nd wheat crops.

* shows where significantly different (P<0.05)

Option 4 - Strategic late burn

Burning is an effective, inexpensive method of removing stubble, assisting in reducing disease carryover, reducing certain seedling pests and weed populations and if using a flexible managament approach should be considered in strategic situations. With careful planning and diverse management, burning can be kept for those occassions where the system needs to be reset which can result in growers retaining stubble for another series of years. A late burn, conducted wisely just prior to sowing to minimise the time the soil is exposed is one option growers may need to consider. Some negatives to burning include loss of nutrients (amount depends on temperature), increased regulation and potential losses of soil from erosion.

At the Temora experiment in un-grazed treatments, retaining stubble, rather than burning had no impact on the yield of canola or the first wheat crop over the nine years, but consistently reduced the yield of the second wheat crop by an average on 0.5t/ha (Table 7). This yield penalty was associated with an overall significant reduction in pre-sowing soil mineral-N of 13kg/ha, while there was no significant difference in pre-sowing N for the first wheat crop (Table 8).

Table 7. Effect of stubble burning on grain yields at Temora in Phase 1 and 2. Crops in italics are canola, and bold are the 2nd wheat crops.

* shows where significantly different (P<0.05)

Table 8. Mean effect of stubble burning or grazing across years and phases on soil mineral N (kg N/ha) to 1.6m depth prior to sowing either 1st or 2nd wheat crops at Temora.LSD for interaction of treatment and rotational position where P<0.05.

How much stubble do we need for erosion control?

Two of the big advantages of retaining stubble are to protect the soil from erosion and maximising the capture of water for the following crop. It has been well recognised that at least 2-3t/ha of stubble is required to protect the soil Figure 3 and Figure 4) and maximise infiltration (Hunt et al 2016b). As an example, in a grazing experiment at Condobolin Research Station (AAR of 450mm) managed by NSW DPI, found that the 70% cover maximised infiltration over summer and there was no effect or a positive effect of ensuring >3.5t/ha of stubble in a dry year (data not shown).

Figure 3. The relationship between the amount of stubble (t/ha) and projected ground cover (%) for wheat stubble from A. Bowman, unpublished, Sallaway et al., 1988, Mannering and Meyer (1963), Anon (1985), Gregory (1982), Early et al. (1997) and Loch and Donnollan (1988).

Aim for cereal stubble load of 2 to 3t/ha to achieve 70 per cent ground cover.

Figure 4. Relationship between cereal stubble load and ground cover (%). Source: Michael Moodie, Mallee Sustainable Farming in Groundcover Supplement Issue, 135 July-Aug 2018.

What is the interaction between stubble cover and soil water evaporation?

Research in the 1970s found that cereal stubble delayed the loss of water from the soil due to evaporation (Figure 5). If there is no follow up rainfall the effect on evaporation is, however, transient. In the laboratory experiment of Figure 5 that used moist soil columns exposed to a relatively high evaporation potential of 7mm/day, about 30 days were required for the cumulative evaporation under a stubble load of 2.2t/ha to ‘catch up’ to that of bare soil and about 50 days where 4.5t/ha of stubble was retained.

Figure 5. The effect of rate of applied wheaten straw on the cumulative evaporation from moist soil columns at an evaporative potential of 7mm/day over 65 days. Sources: Bond and Willis (1970) & B Scott (2013).

Figure 6. Cumulative evaporation under 4t/ha stubble in weighing lysimeters. Source: Dalgliesh (2014).

The typical conditions experienced during summer of high evaporative demand and low frequency of rainfall events mean that the effects of stubble on evaporation are often only transient. This explains why retaining stubble often has little or no effect on soil water storage over summer (Verburg et al 2012; Verburg and Whish 2016) unless there is a significant rainfall event that moves the soil water deeper into the profile. Lysimeter observations and modelling by Verburg et al (2012) indicated, however, that stubble cover did reduce cumulative evaporation losses under conditions of low evaporative demand post-sowing in early winter (average of ~ 1mm/day). During the months of late March to April where the evaporation potential is intermediate (3-4mm/day), 2 to 4t/ha of stubble can, therefore, result in ensuring the soil moisture is maintained at a level suitable for germinating seedlings at sowing.

Compared to the smaller effect of stubble on water loss from evaporation, the effect of weeds on summer water accumulation is often significantly higher (Figure 7). Hunt et al (2013) found that the presence of 2.4 to 5.8t/ha of stubble in a three year experiment at Hopetoun, Victoria had no effect on soil water accumulated over the summer fallow on both sand and clay soils, but there was a slight increase in grain yield on the clay. However, controlling summer weeds, primarily heliotrope and volunteer crop species, increased soil water accumulation by an average of 45mm and soil mineral nitrogen by 45kgN/ha.

Figure 7. GRDC Water Use efficiency Fact Sheet 2009 Southern and Western region. Converting rainfall to grain

Managing nitrogen with full stubble retention

Option 1: Can additional nitrogen at sowing improve crop yield and uptake?

From Kirkegaard et al 2018.

In 2017, two different experiments were implemented in sub-plots at Harden, NSW to investigate the potential role of nitrogen tie-up in the growth and yield penalties associated with stubble. A crop of wheat (cv. ScepterA) was sown on 5 May following a sequence of lupin-canola-wheat in the previous years. In both the stubble-retained and stubble-burnt treatments we compared 50 or 100kg N/ha broadcast as urea at sowing in one experiment, and compared the 100kg N/ha surface applied with 100kg/N deep-banded below the seed. The pre-sowing N to 1.6m was 166kg N/ha in retained and 191kg N/ha in burnt, but was not significantly different.

Plant population, growth and N content at GS 30 did not differ between treatments (data not shown) but by anthesis, the biomass and tiller density were significantly increased by the additional 50kg/ha of surface-applied N in the stubble-retained treatment, while there was no response in the stubble burnt treatment. At harvest, both stubble retention and increased N improved grain yield, but the increase due to N was higher under stubble retention (0.6t/ha) than stubble burnt presumably due to improved water availability. The increase in yield with higher N, and the low protein overall (and with low N) suggests N may have been limiting at the site, but the water-saving benefits of the stubble may have outweighed the earlier effects of immobilisation.

Table 9. Effect of additional surface applied and deep-placed N on wheat response in stubble burnt and retained treatments at Harden in 2017.

Option 2: Can deep banding nitrogen at sowing improve crop yield and uptake?

From Kirkegaard et al 2018.

Deep-banding the N fertiliser had no impact on crop biomass or N% at GS 30, but increased both the biomass and N content of the tissue at anthesis more in the retained-stubble than in burnt stubble (Table 10). Retaining stubble decreased biomass overall but not tissue N. N uptake (kg/ha) at anthesis was significantly increased by deep-banding in both stubble treatments, however the increase was substantially higher in the stubble-retain treatment than in the burn treatment (38kg N/ha compared with15kg N/ha). The overall impact of deep-banding on yield persisted at harvest, but there was no effect, nor interaction with stubble retention, presumably due to other interactions with water availability.

However, the fact that deep-banding N has had a bigger impact in the stubble retained treatment provides evidence of an N-related growth limitation related to retained stubble. It’s appearance at anthesis, and not earlier, presumably reflects the high starting soil N levels which were adequate to support early growth but the cold dry winter generated N deficiencies as the crop entered the rapid stem elongation phase. The increased protein content related to both burning and deep-banding and its independence from yield, suggest on-going N deficiencies generated by those treatments.

Table 10. Effect of surface-applied and deep-banded N on wheat response in stubble-burnt and stubble-retained treatments at Harden in 2017.

The evidence emerging from these studies suggests that even where cereal crop residues are retained on the soil surface (either standing or partially standing) and not incorporated, significant N immobilisation can be detected pre-sowing in some seasons. The extent to which differences emerge are related to seasonal conditions (wet, warm conditions) and to the time period between stubble treatment (burning or grazing) and soil sampling to allow differences to develop. However, even where soil N levels at sowing are similar between retained and burnt treatments (which may result from the fact that burning is done quite late) ongoing N immobilisation POST-SOWING by the microbes growing in-crop is likely to reduce the N available to crops in retained stubble as compared to those in burnt stubble.

What else can growers do to retain stubble and improve crop yields?

Diverse cropping sequence

A diverse cropping sequence provides many benefits for growers wanting to retain all their stubble annually. Diversity allows each crop to be sown into a less antagonistic stubble by reducing physical, disease, pest and weed constraints. i.e. canola sown into a pulse legume stubble emerges in a more suitable environment that following several years of cereal stubble. Whereas a vigorous barley plant is able to emerge from a wheat stubble, likewise, a large seeded pulse from barley stubble. Diverse crop rotations that include double breaks, premium herbicides and some crop competition from crops such as barley have proved to very profitable with lower risk and are very effective at keeping herbicide resistant weed populations low.

A fully phased systems experiment was established in Temora in 2014 at a site with high levels of Group B resistant ARG to examine if a diverse crop rotation (vetch hay-TT canola-wheat-barley) could improve the profitability of stubble retained no-till (Flexi-Coil tine seeder with Stiletto knife points and deep banding & splitting boots) and zero-till (Excel single-disc seeder with Arricks’ wheel) systems. Three cropping strategies (Aggressive, Conservative and Diverse) were compared with the rotations for each as Aggressive (RR canola-wheat-wheat), Conservative (TT canola-wheat-wheat) and Diverse (as above). In the cereal crops in the Aggressive and Diverse strategies, new-generation pre-emergent herbicides (Sakura® and Boxer Gold®) were used for grass weed control in cereals and propzamide in the canola. In the Conservative strategy, trifluralin and diuron were used for grass weed control in the tine system, and diuron alone in the disc system.

Diversity and profitability

The Diverse management strategy is extremely profitable achieving a net margin ($476/ha/year) which is higher than in the Aggressive strategy ($454/ha/year) and at lower cost ($489 compared with $539/ha/year) and thus higher profit:cost ratio ($0.98 compared with $0.83) (Table 11). The reduced costs in the Diverse strategy are driven by lower fertiliser N inputs from the inclusion of vetch hay, which require no fertiliser N and provides residual N for subsequent crops. The average cost for nitrogen fertiliser in the Aggressive strategy was $111/ha/year compared to $72/ha/year in the Diverse strategy, a saving of $41 /ha/year.

Over the four phased years, the barley crop in the Diverse strategy was also more profitable than the second wheat crop in either the Aggressive or Conservative strategies (Table 11), despite record low barley prices in this 2016/17 season. There were no significant differences in crop yield between the disc and tine seeders where weeds were successfully managed in the Diverse and Aggressive strategies. This equated to similar net margins in the Diverse ($484/ha – tine versus $469/ha in disc) and in the Aggressive strategy ($467/ha – tine versus $440/ha – disc). In the conservative strategy with low-cost pre-emergent herbicides, crop yields were higher in the tine system where ryegrass could be managed with the addition of trifluralin.

Table 11. Average net margins (EBIT) – effect of crop strategy at Temora, NSW, 2014-2017 averaged across opener types (disc and tine seeders).

The Riverine Plains group compared a wheat-faba bean-wheat rotation against a wheat-wheat-wheat (+/- burning) and found there was no significant difference in wheat yield following wheat stubble that was retained or burnt (average 3.42t/ha), but there was a 2t/ha increase in wheat yield following faba bean.

The wheat stubble also acted as a trellis assisting to keep the beans off the ground and improve airflow and the higher nitrogen concentration following the bean crop combined with the increased decomposition of the wheat stubble resulted in the bean crop ‘resetting’ the system and burning was not required. Similar findings have been observed by the Hart Field Site group (SA) in relation to lentils using the wheat stubble as a trellis.

Earlier maturing varieties such as BlitzA were found to be taller with increasing stubble height (30 and 60cm stubble height compared with 15cm or baled). They also found that the type of stubble was important for the following crop, with wheat maintaining its supportive structure better than barley.

Diversity and weed management

Implementing a Diverse management strategy that includes a double break was not only profitable and less risky (lower profit:cost ratio), but was the most effective system at reducing annual ryegrass (ARG) weed populations. The ARG seedbank was reduced by 70% in three years in the Diverse strategy compared the Aggressive strategy (Table 12). Where low cost herbicides were used in the Conservative sequence, the ARG numbers in the wheat-wheat-canola sequence by 800%. In the wet year of 2016, the pre-emergent herbicides had become in-effective by August, and the competitive barley was extremely effective at continuing to reduce any late emerging ARG weeds.

Table 12. The average annual ryegrass seedbank population for each management strategy across opener types from 2015-2018 at Temora, NSW.

*No lsd – data analysed by square root and back transformed. Letters indicate significant difference.

#No lsd – data analysed by log e and back transformed. Letters indicate significant difference.

There was a significant effect of opener type on ryegrass control with higher ARG populations found when sown with a disc opener compared to a tine opener (Table 13). The ARG populations increased significantly in the Conservative strategy sown with a disc seeder where low-cost herbicides were used, but this was no surprise as there are limited low cost herbicide on label for a disc opener. Therefore, it is recommended that where high ARG populations are suspected, more expensive herbicide options and greater crop competition from narrower row spacing and crop types should be used where a disc opener is preferred.

It should also be acknowledged that irrespective of opener type, all AHRI’s ‘Big 6’ strategies should be implemented to keep the weed seedbank low. However, in this experiment, only a maximum of four were able to be implemented. The addition of crop topping and harvest weed seed destruction could have further reduced ARG seedbank populations to negligible levels.

Table 13. The average Annual Ryegrass seedbank population for the three management strategies for both disc and tine seeders, between 2014 and 2018.

Incorporate AHRI’s ‘The Big 6’

1. Rotate crops and pasture - use double break crops, fallow and pasture phases to drive the
   weed seedbank down over consecutive years.
2. Double knock to preserve glyphosate - Follow glyphosate with a high rate of paraquat to control survivors in a fallow or pre-sowing situation.
3. Mix and rotate herbicides – rotate between herbicide groups, use different groups within the same herbicide mix and ALWAYS use full rates.
4. Stop weed seed set! Crop top canola, pulses and feed barley. Consider options such as brown manuring hay cutting or long fallow, and spray top/spray fallow pasture prior to the cropping phase.
5. Increase crop competition. Adopt at least one competitive strategy (but two is better), including reduced row spacing, higher seeding rates, east-west sowing and competitive varieties.
6. Harvest weed seed control - Capture weed seed survivors at harvest using chaff lining, chaff tramlining, chaff carts, narrow windrow burning or integrated weed seed destructors

The integration of all six tactics will ensure weed seed set is neglible to low and increase profitability in stubble retained farming systems. More information on the ‘Big 6’ can be found on the AHRI website or at Kirkegaard et al (2017)b.

Conclusion

This paper has outlined several ways to flexibly manage stubble retained farming systems that ensure profitability and sustainability. It is extremely important for growers to NOT compromise managing weeds, disease, pests or being able to sow their crop due to excessive stubble loads. Growers need to be pro-active in managing their stubble which should have commenced before harvest and continued until sowing the following year to ensure their stubble management will suit their seeding system.

It has been shown that by diversifying a crop rotation (increasing the number of pulse crops and barley), deep banding nitrogen or adding 5kgN/t cereal stubble at sowing, managing stubble by mulching, baling, incorporating + nutrients or grazing, that it is easier to manage stubble without the need to burn. However, if the stubble load remains too large or the potential weed/disease/pest burden remains too high, then a one off strategic late burn can be used to ‘re-set’ the system.

Growers should think about stubble as a source of C for the microbes, not N, and if possible, finds ways of converting that energy source into humus. Above all, it is also important to incorporate AHRI’s ‘Big 6’ strategies to stay in front of weed issues and keep the farming system profitable and sustainable.

Useful resources

Maintaining profitable farming systems with retained stubble page on FarmLink website

References

Bond and Willis (1970) Soil Science Society American Proceedings, Vol 34.

Dalgliesh (2014) presentation based on data from Freebairn, D.M., Hancock, N.H. and Lott, S.C. (1987), Soil evaporation studies using shallow weighing lysimeters: Techniques and preliminary results, Trans. Mech. Engn., Inst. Engn. Australia, Vol. ME12. pp 67-72.

GRDC GROUNDCOVER Supplement Issue 135: July-August.

Hunt et al (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 & Pasture Science, 2013, 64, 922-934

Hunt JR et al (2016)a Sheep grazing on crop residues increase soil mineral N and grain N uptake in subsequent wheat crops.

Hunt JR et al. (2016)b Sheep grazing on crop residues do not reduce crop yields in no-till, controlled traffic farming systems in an equi-seasonal rainfall environment. Field Crops Research 196, 22-32

Kirkegaard et al (2017) Opportunities and challenges for continuous cropping systems. GRDC Updates in Adelaide, Wagga and Bendigo.

Kirkegaard et al (2018). The effect of stubble on nitrogen tie-up and supply. GRDC Updates Wagga, Dubbo, Campell Town, Walpeup & West Wyalong.

B Scott (2013): Stubble retention in Southern Australia, EH Graham Centre Monograph No 1

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

Swan AD et al. (2017)b. The effect of grazing and burning stubbles on wheat yield and soil mineral nitrogen in a canola-wheat-wheat crop sequence in SNSW

Swan et al (2017)b A flexible approach to managing stubble profitably in the Riverina and Southwest Slopes of NSW. GRDC Update Grenfell.

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.

Verburg K and Whish J (2016) Drivers of fallow efficiency: Effect of soil properties and rainfall patterns on evaporation and the effectiveness of stubble cover. GRDC Update paper

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC — the authors would like to thank them for their continued support.

I would also like to acknowledge all the collaborating scientists, technicians, staff, growers and consultants from all of the organisations and groups who have contributed significantly in terms of their time and research capability to each farming systems group to ensure the project in their region is producing the highest quality of work.

Contact details

Tony Swan
CSIRO Agriculture and Food, Clunies Ross St Acton, Canberra ACT 2601
0428145085
tony.swan@csiro.au