Discussion paper: Busting the big yield constraints – where to next?

Take home messages

• Yield increases are necessary to keep cost of production for Australian growers competitive in international markets.
• Recent yield increases associated with improved genetics and agronomy (25kg/ha/year) are struggling to counteract the yield decline due to climate change (-24 kg/ha/year), and in coming decades it is likely that yield gains need to double to maintain profits.
• Yield increases are rarely the result of a single practice or technology but occur when new and old technologies and practices combine to form improved systems that overcome a constraint to production.
• Removing nitrogen (N) limitation, allowing crop establishment in the absence of breaking rainfall and genetic tolerance to heat/drought or frost are currently major constraints to yield in Australia, and require multidisciplinary systems research to overcome them.

Why increase yield?

Cost of production ($/t) is an important factor influencing the ability of Australian grain growers to compete in export markets. One of the main ways in which Australian grain growers have been able to maintain relatively low costs of production despite declining terms of trade has been by increasing crop yield with relatively small additional overhead and input costs. While ways of reducing overhead, input and transport costs can be found (for example; through economies of scale), yield increases are still an important way in which cost of production will be kept at an internationally competitive value in the future. Yield increases are also necessary to meet the goals of sustainable intensification, whereby the additional food required for a growing global population is produced on the same area of land that is currently farmed, without the further destruction of natural ecosystems. This paper will take a brief look at where historic yield increases in Australian crop (particularly wheat) production have come from, and where we believe future gains can be made. It is based on a chapter written for the book ‘Australian Agriculture in 2020’, recently published by the Australian Society of Agronomy (Huntet al. 2019a), details of which can be found in the useful resources section within this paper.

Yield, yield and yield

The concept of potential yield (PY), Figure 1, and yield gaps is crucial when looking at ways to improve yield and we follow the nomenclature of Fischer (2015). The most important definition for dryland crop production in Australia is water limited potential yield (PY_w), defined as the yield of the best cultivar under optimum management with no manageable constraints (for example; nutrient deficiency, weeds, disease) except for water supply (Figure 1). Farm yield (FY) is yield achieved by growers in their fields. The difference between FY and PY_w is termed the yield gap. Economic yield (EY) is the yield attained by growers when economically optimal practices and levels of inputs have been adopted while facing all the vagaries of weather (Figure 1). Economic or attainable yield is typically 75-85 % of PY_w (van Ittersum et al. 2013). The difference between EY and FY is the exploitable yield gap. Hochmanet al. (2017) describes the proportion that FY comprises of PY_w as relative yield.

The yield gap of an individual farmer is dependent on management skill and level of investment in inputs, but also incentives and capacity to achieve higher yields. Management skill means the ability of a farmer to use management and inputs to reduce the biotic (weeds, pests and diseases) and abiotic stresses (water, nutrient and temperature stress) placed on crops. The different points in Figure 1 describe different situations under which growers may or may not be achieving potential yield. The first point describes a farmer with a high level of management skill, but who under-invests in inputs and is therefore not achieving economic yield. The second point describes a farmer with a high level of skill and appropriate investment in inputs who is achieving economic yield. This farmer has closed the exploitable yield gap.

The third point describes a farmer with a high level of skill who is over investing in inputs and while exceeding economic yield, is not as profitable (due to higher costs of production) or is more exposed to risk than the second farmer. The fourth point describes a farmer who is investing enough inputs to achieve economic yield, but due to a lack of management skill has an exploitable yield gap. This farmer will obviously not be as profitable as the second farmer. To close yield gaps, the first farmer needs to invest more inputs while maintaining current level of management skill. The fourth farmer needs to improve their management while maintaining current levels of inputs. The third farmer has closed the yield gap but can afford to invest less in inputs while maintaining their management skill, thereby increasing profits.

Figure 1. A graphical representation of potential yield, water limited potential yield, economic yield and farm yield. The numbered dots represent  growers with different yield gaps and different reasons for those yield gaps.

Where do yield increases come from?

Yield of crops is determined by the interaction between genotype (G = species, cultivar), environment (E = soil and climate) and management (M = rotational position, fallow management, tillage system, sowing date, fertiliser, control of weeds, pests and disease etc.) which is referred to as G x E x M. The case of wheat (and likely other grain crops) in Australia makes an interesting case study, because the climate has deteriorated (rainfall decreased and temperature increased) and reduced water limited potential yield by 27% during the period 1990 to 2015 (Hochmanet al. 2017), which is equivalent to 24kg/ha/year (Ababaei and Chenu 2020). However, growers have maintained yields by adopting improved genotypes and management practices and increased farm yield relative to water limited potential yield (closed the yield gap) at a rate of 25kg/ha/year (Hochmanet al. 2017). In other words, climate change has effectively robbed the industry of the yield gains it needs to stay competitive. Of course, national averages can be deceptive; many leading growers have increased yields despite climate change by increasing water-use efficiency, and therefore, remain globally competitive. For others, there is some room to move in terms of yield gap closure. Australian wheat growers are currently achieving 55% of water limited potential yield (Hochman et al. 2017), meaning that for many growers there still exists a substantial yield gap, and yield could be further increased through adoption of best practice. Leading growers have closed the exploitable yield gap, and increased yield requires an increase in water limited potential yield (van Reeset al. 2014).

Past yield increases

Throughout history, increases in crop water productivity have rarely been attributable to an individual innovation in technology or farming practice. Increases have occurred when new and old technologies and practices combine to form improved systems that overcome a constraint to production (Kirkegaard 2019). In the example of Australian wheat production, the yield gap closure of the last 30 years has been due to many disparate technologies combining to form improved systems. The advent of non-selective knockdown herbicides (mainly glyphosate) and grass selectives drove the rapid adoption of no-till (Llewellyn et al. 2012) which improved soil water conservation and allowed earlier sowing (Stephens and Lyons 1998; Flohret al. 2018c). Wheat was increasingly grown in rotation with broadleaf break crops (canola and pulses) rather than other cereals or weedy pastures which enhanced disease and weed management, and in the case of cereals following pulses, reduced fertiliser N requirements. Summer fallow weed control further increased soil water conservation, N accumulation and reduced root disease burdens (Huntet al. 2013). Meanwhile breeders consistently achieved genetic yield progress of 0.5% per annum (Siddiqueet al. 1990; Sadras and Lawson 2011; Fischeret al. 2014; Kitonyoet al. 2017; Flohr et al. 2018b) and overcame significant biotic and abiotic constraints to production which interact with management (cereal cyst nematode, stripe rust, acidity, salinity, boron). Early sown, disease free crops responded profitably to increasing N fertiliser, applications of which tripled over the last 30 years (Angus and Grace 2017).

Future yield increases

Yield increases comparable to or exceeding those of the last 30 years are necessary to keep Australian growers competitive and to meet the goals of sustainable intensification. Fischer and Connor (2018) estimate that crop yields must increase at around 1.1% per year globally to ensure adequate food supply. While Australian growers have been able to close the yield gap by 25kg/ha/year (equivalent to 1.2% per year increase in relative yield, Hochman et al. 2017), declining rainfall and increasing temperatures have reduced water limited potential yield.

Significant yield gains in the coming decades requires a transformational change in the way we do research, development and extension. We argue that focussing research effort on developing synergistic systems that overcome current and future production constraints, combined with effective extension and adoption, will accelerate increases in yield. This will require a coordinated effort from multi-disciplinary teams, and in Hunt et al. (2019a), we describe a process of ‘transformational agronomy’ to achieve this. Briefly, agronomic researchers must work closely with growers and advisers to accurately define and quantify constraints to production. Solutions can then be sought and evaluated from diverse sources. Multidisciplinary teams with leadership from agronomists and close cooperation with growers and advisers will be required to achieve this. Once solutions have been evaluated and tested using a combination of crop simulation, small plot experiments and paddock-scale experiments in growers’ fields, research teams need to work closely with growers and advisers to build and integrate improved, robust and adoptable farming systems that overcome the intended constraint.

Three constraints follow that we believe could be overcome with the multi-disciplinary research approach that is embodied in transformational agronomy. Indeed, if these could be achieved, we believe it would lead to transformational changes in production and profit for Australian growers. These are complex problems and will not be overcome cheaply or easily, but the pay-off from doing so would justify the investment.

Removal of N limitation

Nitrogen deficiency remains the single biggest factor contributing to the sizeable exploitable yield gap in Australian wheat production (Hochman and Horan 2018) and likely other non-legume crops (barley, oats, canola) as well. Even leading growers struggle with N management in favourable seasons (van Reeset al. 2014). At first this appears somewhat paradoxical; N management in grain crops should be extremely simple – crop requirement is well related to yield as described by the simple rule of thumb taught to all budding agronomists: 40kg/ha N per tonne of anticipated wheat yield. The supplies of N to the crop are also readily quantified – mineral N in the soil prior to sowing can be cheaply and easily measured from intact soil cores. Mineralisation is more difficult to estimate but it is possible and is self-correcting in that spring rain which leads to higher yield potential, also promotes more N mineralisation. The complexity comes in reliably estimating anticipated yields. This requires no less capability than the accurate prediction of weather several months in advance. But the difficulty arises from Australia’s extremely variable rainfall. For instance, in southern NSW when growers need to make decisions regarding post-emergent N applications (typically in July-August), possible yields range from 0t/ha to 7t/ha in seasons with no stored soil water prior to sowing, and yield and N demand all depend on September and October rainfall. In addition, over-fertilisation with N can reduce both yield and grain quality through haying-off (van Herwaarden et al. 1998). N fertiliser is also a costly input and, mindful of environmental losses (Turner et al. 2012; Schwenke et al. 2014), many growers tend to err on the conservative side in their applications.

There have been consistent attempts to improve prediction of yields and to make N management more precise. This has included the use of forecast systems (Assenget al. 2012) and decision support systems that integrate soil resources and management variables, and present likely response to N inputs in probabilistic terms (Hochmanet al. 2009). While seasonal forecasts are likely to improve, they will never be perfect. Given the substantial nature of the problem, a fresh approach is required. One such solution that may work in environments with low N losses (for example; low rainfall areas with high soil water holding capacity) is the use of N fertiliser to maintain a base level of soil fertility (‘N bank’) sufficient to achieve water limited potential yields in the majority of growing seasons (as is currently done for phosphorus). Implementation of this strategy would need to consider the amount of mineral N in the soil profile and to adjust inputs for carry-over of previously applied N fertiliser not used by the crop. If applied appropriately at the time of rapid crop uptake, environmental losses from the ‘N bank’ would be low in farming systems where stubble is retained, and the majority of applied N is either taken up by the crop or immobilised into organic forms. Losses could be further reduced through use of higher efficiency N application strategies (e.g. deep and mid-row banding). Once the N banks are built, the cost of N fertiliser for growers is deferred into the season following rather with the season of high yields; this could have substantial economic value through improved cash flow and tax benefits. It may also reverse the mining of soil N that has occurred under Australian crop production since the decline in area of legume-based pastures (Angus and Grace 2017).

A multidisciplinary team is essential to test this potential solution. It requires accurate measurement of N losses and N cycling within the soil, and this requires discipline-specific expertise from within the field of soil science. In addition, economic assessment will be critical, and an investigation of management techniques to minimise possible negative effects on yield and quality from high levels of soil mineral N is required. Pre- and post-experimental crop simulation would be essential to test assumptions, identify locations and treatments that would be promising to test in the field, and extend field results over multiple sites and seasons. If found to be successful, geographic information system tools (e.g. yield and protein mapping) would allow even greater efficiencies through mapping of N removal in grain.

The ‘N bank’ concept has been tested using simulations at different rainfall locations in southern NSW, and were found to increase yields with minimal environmental impact (Smith et al. 2019). The first field experiment designed to test this was funded by La Trobe University and established by BCG at Curyo in 2018. The first two years of results indicate that ‘N bank’ strategies are equally profitable as attempting to match N inputs to seasonal yield potential using Yield Prophet® (Figure 2). More research is required to evaluate the approach across environments and to closely measure N losses.

Figure 2. Mean annual N fertiliser application and mean annual gross margin 2018-2019 for different N management systems (N bank vs. Yield Prophet®) being tested in an experiment at Curyo in north west Victoria. The number following the Yield Prophet® (YP) treatments is the probability of different yield outcomes occurring at time of top-dressing in July (e.g. YP 75% targets each year the yield at which there is a 75% chance of exceeding). The numbers in the N banks treatments represent the total N supply (soil mineral N + fertiliser) that each treatment is topped-up to with N fertiliser (e.g. in the N bank 125 treatment, if 75kg/ha of soil mineral N is measured prior to sowing, it is topped up to 125kg/ha total N supply with 50kg/ha fertiliser N).

Crop establishment in the absence of autumn rainfall

From the early breeding work of Farrer, much of the agricultural research conducted in Australia has aimed to coincide critical periods of yield determination in crops with climatically optimal conditions for growth. The cool, wet winters during which crops are grown in Southern and Western Australia often transition rapidly into hot, dry conditions with supra-optimal temperatures and limited soil water. When combined with spring frosts, this creates a reasonably narrow period during which crops must undergo their critical development phases (e.g. flowering) for yields to be maximised (Dreccer et al. 2018). While the concept of the optimum flowering window has long been known (Andersonet al. 1996), it has been the advent of computer simulation that has allowed them to be quantified for multiple locations across many seasons, for wheat (Flohret al. 2017) and canola (Lilleyet al. 2019) and barley (Liuet al. 2020). Shifting crop development closer toward optimal flowering periods has been the major mechanism behind many of the transformational changes in Australian crop production. This includes iconic advances such as the release of Federation wheat with its faster development pattern (Pugsley 1983), the rise of no-till which allowed much earlier sowing (Stephens and Lyons 1998), and more recent shifts to dry and early sowing (Fletcher et al. 2016; Huntet al. 2019b).

Recent quantification of optimal flowering periods has revealed that leading growers are now coinciding critical periods with seasonal optima (Flohret al. 2018c). The only times they do not achieve timely flowering is when they have been unable to do so due following dry autumns with insufficient soil moisture to allow seeds to germinate and emerge. Somewhat ironically, this new understanding of optimal sowing times has coincided with declining autumn rainfall (Pook et al. 2009; Caiet al. 2012) making it harder than ever for growers to achieve optimal flowering periods. This defines our second opportunity to overcome a major constraint to crop production – achieving crop establishment in the absence of favourable autumn rain. Once again, an integrated solution to this constraint demands multidisciplinary expertise led by a generalist with appreciation of the G x E x M context. Input is required from disciplines of agricultural engineering, plant physiology, genetics and soil physics.

Knowledge of the regulation of seed germination has developed greatly in recent times, yet our understanding of the mechanisms causing variation of plant establishment in the field remains limited. This is probably because most seed biology experiments are performed in laboratories  under optimal conditions, whereas seeds in the field are subject to a complicated soil matrix where they experience a variety of different stresses (Finch-Savage and Bassel 2015). Domestication and breeding have provided incremental improvements in the ability of crops to germinate and emerge under sub-optimal conditions, but here we discuss ways in which agronomically directed research could be applied to transform seed performance when surface soil is dry.

Soil water potential is a major factor in determining seed germination and plant establishment. Many species can germinate at soil water potentials well below those that maximise plant growth (Wuest and Lutcher 2013). Distinguishing between adequate and marginal water to enable germination can be difficult for growers – there are no well-defined criteria for determining if a soil contains a high enough water content to germinate different crop species. At water potentials above -1.1MPa, germination rates are rapid (Wuest and Lutcher 2013). Water potentials below this slow the speed of germination, and below -1.6MPa, germination ceases. Pawloski and Shaykewich (1972) showed that these effects were similar between soils, even when soils differ in hydraulic conductivity.

Crop establishment could be enhanced by the ability of seeds to germinate at lower water potentials. This could be achieved by genetic or other means. Singhet al. (2013) found differences between wheat cultivars in the ability to germinate at low water potentials. Genetic variation for rates of seed water uptake (which initiates germination and is the first stage in the malting process) exists in barley, and it has been suggested that this could be exploited by breeders for the benefit of the malting and brewing industries (Cu et al. 2016). The same principles and expertise could be applied to field germination at low water potential. An obvious trade-off that may arise with the genetic ability to germinate at low water potentials is susceptibility to pre-harvest sprouting (Rodríguezet al. 2015). Expertise from plant physiologists concerned with the regulation of dormancy would be essential to harness this opportunity.

Beyond genetic means, strategies for manipulating germination processes used in horticulture crops and rice could be evaluated. Seed priming techniques limit the availability of water to the seed so that imbibition and seed metabolism commences, but germination is not completed (Halmer 2004). Seed priming has potential to reduce the lag time between imbibition and emergence, and to synchronise seedling emergence. It can improve emergence of wheat under low temperatures (Farooqet al. 2008), but not necessarily under low water potentials (Giri and Schillinger 2003). The inclusion of plant growth regulators, hormones or micronutrients during priming can also improve germination and emergence (Jisha et al. 2013; Aliet al. 2018). It is clear from the literature there are many potential solutions that could improve seed germination and establishment at low water potentials. Extensive field appraisal of these techniques is required.

Inadequate moisture at the ideal sowing depth has led to growers sowing deeper to seek moist soil and to make use of residual moisture stored from summer rains or the previous growing season. Their ability to do this is currently restricted by the availability of sowing equipment capable of placing seeds into moist soil at depth, and the ability of plants to emerge from depth. Coleoptile length is an important trait determining the success of emergence from depth, but there are also other genetic factors involved (Mohanet al. 2013). Modern Australian semi-dwarf wheat and barley cultivars show poor emergence when sown deep (greater than 8cm) due to short coleoptiles (Rebetzke et al. 2007). Warmer soils in the future may further exacerbate poor establishment and especially with deep sowing.

Pre-experimental modelling indicates substantial benefits for crop yield in southern Australia if machinery and genotypes could be developed that allowed placement and emergence of seed at depth (Kirkegaard and Hunt 2010; Flohr et al. 2018a). Establishment of crops in this way is routine in the drylands of the Pacific North West USA, where annual rainfall in some regions is as low as 160mm. Seeds of winter wheat and other crops are sown deep using deep furrow drills into moisture remaining from 13-month fallow periods and can emerge with 10cm to 15cm of soil covering them (Schillinger and Papendick 2008). Rebetzke et al. (2016) have argued the case for Australian breeders to use novel dwarfing genes that do not suppress coleoptile length. Larger seed size is also known to improve deep-sown crop establishment. Large-seeded canola improved the timeliness of establishment and subsequent grain yield when rainfall for crop establishment was marginal but there was moisture available deeper in the seedbed (Brill et al. 2016).

Frost, drought and heat

While optimisation of flowering times allows the combined stresses of drought, frost and heat to be minimised, these abiotic stresses still take a large toll on crops every year, and will continue to do so even if establishment in the absence of autumn rain could be achieved (see preceding discussion within this paper). Most avenues minimising the risks of frost, drought and heat have been explored, the only remaining means to increase yields in the face of these cardinal abiotic stresses is through crop tolerance. It is our opinion that this will most likely be achieved via genetic solutions, but these must be considered in an appropriate G x E x M context.

Frost, drought and heat risks are inextricably linked. Frost risk declines as flowering is deayed later into the spring, while the risk of drought and heat increases. This means that tolerances to all three stresses are not necessary to improve yields, and if tolerance can be found to either frost on the one hand, or drought and heat on the other, then the optimal flowering period will shift accordingly to reduce the likelihood of occurrence of the opposing stress. That is, if we can minimise frost stress then we can reduce the effects of drought and heat stress by flowering earlier, and vice versa. The value of this approach has been demonstrated by economic analyses of potential frost tolerance, where the benefit of shifting flowering time earlier to avoid drought and heat has also been quantified (An-Vo et al. 2018). Therefore, the important question is which of these stresses will be cheapest and easiest to solve?

Drought and heat are perhaps easier targets compared with frost in that they are reasonably easy to screen for within a breeding program, and some genetic regions associated with combined drought and heat tolerance have been identified (Trickeret al. 2018). Conversely, frost is virtually impossible to recreate under controlled conditions and tolerance is extremely difficult to identify. Heat and drought often interact. Heat tolerance in the absence of drought is associated with stomatal opening and rapid water-use that depresses canopy temperatures relative to the atmosphere (Reynoldset al. 1994). For heat tolerance to be useful in the Australian context, it must be effective under limited water supply (Hunt et al. 2018; Tricker et al. 2018).

While there may be some promise in selecting morphological traits known to confer both heat and drought tolerance, the greatest and most cost-effective progress may be made by breeders selecting for high yield at late flowering times where crops would be routinely exposed to concurrent drought and heat stress. However, this is where the wider crop physiology and management context becomes important. It would be crucial that late flowering be achieved with slow developing cultivars sown early and thus exploit a full growing season rather than by late sowing of faster developing cultivars where yield potential would be limited by shallow rooting depth and low biomass accumulation (Kirkegaardet al. 2015; Lilley and Kirkegaard 2016).

Conclusion

Yield increases are necessary to keep Australian growers competitive in international markets and to feed a growing population without increasing the area of land devoted to farming. Yield increases can be achieved by closing yield gaps, or increasing water limited potential yield. Climate change has reduced water limited potential yield and the industry needs to increase its efforts if real yield gains are to be realised. Historically this has happened when new and old practices and technologies synergise in improved farming systems. We argue that research should focus on developing new systems to overcome current constraints to production and we identify three opportunities for future yield gains - minimising N limitations, establishment of crops in dry or marginal soil moisture, and combined drought and heat tolerance. To solve these problems will require multi-disciplinary teams working closely with growers and advisers in appropriate farming systems context.

Acknowledgements

The N bank experiment described in Figure 2 is funded by La Trobe University Research Focus Area ‘Securing Food, Water and the Environment’ and conducted in collaboration with BCG and CSIRO.

Useful resources

This GRDC update paper is based on the following book chapter:

Hunt, JR, Kirkegaard, JA, Celestina, C, Porker, K (2019) Transformational Agronomy: restoring the role of agronomy in modern agricultural research. In 'Australian Agriculture in 2020: From Conservation to Automation.' (Eds J Pratley, JA Kirkegaard.) pp. 373-388. (Agronomy Australia and Charles Sturt University: Wagga Wagga). Agronomy_Australia Proceedings_special-publications

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Contact details

Assoc. Prof. James Hunt
Department of Animal, Plant and Soil Sciences
La Trobe University, Bundoora VIC 3086
0428 636 391
j.hunt@latrobe.edu.au
@agronomeiste