Maximising growth and yield – canopy management is more important in seasons of better potential

Take home messages

• Light and temperature are more likely to set the boundary of yield potential than rainfall in higher productivity zones and seasons.
• Release of cultivars with high yield potential and sowing dates that better align crops to maximise light interception during the critical period has increased yield in the HRZ.
• Improved genetic resistance and disease management strategies were essential to achieve the yield potential and close the yield gap in the HRZ , especially in seasons like 2022.
• Additional nitrogen (N) supply and increased fungicide application were more important than cultivar and plant growth regulator (PGR) application to achieve high yields in 2022.
• Building more fertile farming systems with higher levels of organic N supply to supplement fertiliser applications will be necessary to support higher yield potentials.

Background

While most crops in Australia are water limited, an important realisation is the fact that, in high rainfall seasons, other factors such as the availability of the key resources – light, temperature, and nutrients (especially nitrogen) – are more likely than rainfall to set the boundary of yield potential. The 2022 April–November rainfall was well above average in most regions of south-eastern Australia, with enough water supply to achieve yields approaching 8–10t/ha, consistent with those regularly achieved in the high rainfall zones (Table 1). The question is, how do growers achieve high yields in seasons of better potential? The GRDC Hyper Yielding Crops Initiative (HYC) has shed light on the agronomic and environmental factors required to achieve high yields and can also be useful to guide growers in the low to medium rainfall zones in high rainfall seasons like 2022.

Principles of building and protecting high yields

Building high yielding crops in wet and mild environments (seasons) is primarily achieved by increasing grain number. Grain number is predominantly determined by the amount of growth (or dry matter accumulation) during late spike development in the period between emergence of the penultimate leaf (flag -1) and shortly after flowering. This period is known as the critical period for yield determination. Grain number, and thus yield, are very sensitive to stress during this time (Fischer 1985). Building yield relies upon adopting techniques that allow crops to intercept more radiation (sunlight) and transpire more water into productive biomass at the right time of the season.

Harvest indices (resulting from the conversion of biomass into yield) of greater than 50% should be possible with good management. Therefore, to achieve 8–10t/ha cereal grain yields, the final biomass needs to be greater than 16–20t/ha. Other constraints, such as disease, lodging, head loss and extreme weather shocks, will either chip away at the potential or reduce harvest index and need to be managed if possible.

Critical resources define yield potential

Water supply

The water-limited potential yield (PYW) for cereals and canola can be estimated using the well-known French and Schultz (1984) approach, which has been updated for cereals by Sadras and Angus (2006). In this approach, the water supply to the crop is estimated using readily available rainfall data (in-season rainfall + estimate of stored fallow rainfall – evaporation) and converted to yield using the recently published upper limits for transpiration efficiency in the best cereal crops of 25kg grain per ha for each mm of water supply (Sadras and McDonald 2012). The minimum evaporation in the best crops reported in the literature is generally 60mm. While there are other considerations and assumptions that can be taken into account, for simplicity this approach is informative for benchmarking.

Temperature and solar radiation

Yield formation is sensitive to the ratio of solar radiation to temperature (referred to as photothermal quotient or PTQ) experienced during the critical period. The relationship between PTQ and potential yield of wheat published by Rawson (1988) has been verified by a number of hyper yielding sites and high yields worldwide, for example, the world record wheat yield in NZ. The world record canola yield was achieved at Oberon NSW (7.2t/ha) assisted by temperatures being moderated by altitude (1000m asl) under bright conditions. A high PTQ equates to high yield potential and can be achieved by bright, sunny conditions or long days of solar radiation to drive growth (photosynthesis) combined with low temperatures that extend the critical period as long as possible. The PTQ derived yield potential was less at Millicent SA in 2022 because temperatures were average or slightly higher during the critical period (October), but solar radiation was lower, generating a lower PTQ, reduced photosynthesis and lower grain number (Figure 1). The relationship between PTQ and yield assumes no other major stresses in the critical period, such as extreme temperature, water or N stress and no pests or disease, and assumes ideal growing conditions outside of the critical period for conversion to yield.

Figure 1. Long term (- - - - ) and 2022 (―) monthly mean temperatures and solar radiation for Millicent, SA.

2022 yield potential

Using a water use efficiency of 25kg/ha/mm, estimated water limited yields were higher than the long term averages across a number of locations in 2022 (Table 1). Another defining feature of 2022 was that while temperatures were consistent or slightly warmer than long term trends, solar radiation was lower than average, and thus, low PTQs were limiting yield potential more often than water supply. This was especially the case at coastal sites (for example, Millicent, Kingscote), and where water supply exceeded 400mm. While 2022 was clearly a wetter than average year, it is worthwhile considering the frequency with which PTQ is likely to limit yield potential compared to water. At Millicent for example, PTQ has likely limited yield potential in 27 of the last 33 years, at Cummins SA and Giles Corner SA, only 6 of the last 33 years.

Table 1: Selected southern Australian sites and calculated water limited and photothermal quotient yield potentials for 2022. Shaded cells indicate sites where the PTQ was limiting yield potential and not water supply based on a 25kg/ha/mm transpiration efficiency.

Managing resources in the critical period – putting canopy management into practice

Interception of light is more important in seasons of better water limited potential, and therefore, crop agronomy needs to focus on strategies that keep green leaves photosynthesising in the critical period. Learnings from the GRDC HYC and NGN experiments for barley in 2022 present a case study as to how to achieve yield potential in low to medium rainfall zones using management strategies more common in high rainfall zones.

Barley experiments on canopy management for achieving yield potential

Materials and methods

Sowing dates were utilised to shift flowering time and critical periods. Nitrogen rates were adjusted in winter with topdressing to achieve yield potentials for a decile 3 and 8 water limited yield. Low and high intensity fungicide treatments aimed to protect green leaf area and maximise yield were applied to determine the effect of disease control on the yield. Additional treatments such as plant growth regulators and defoliation were utilised to determine the benefit of keeping crops standing. Cultivars of known differences in yield potential and lodging susceptibility were included for comparison.

Table 2: Average effect of in-crop canopy management interventions on yield (t/ha) across both sowing dates at Hart field site (SA), and Birchip field site (Nullawil Vic) in 2022. Shaded numbers are the highest yields for each cultivar.

¹Nitrogen Inputs

Starting soil N supply (0–60cm) = 77kg N at Nullawil, and 93kg N at Hart. Low Nitrogen = 60kg N (Nullawil) and 55kg N (Hart) applied in season to achieve decile 3 (N inputs for a 3.5t/ha and 3.6t/ha yield potential) High Nitrogen = 160kg N (Nullawil), and 135kg N (Hart) applied in season to achieve decile 8 (N inputs for a 6t/ha and 5.7t/ha yield potential)

²Fungicide Intensity Low = 1 x foliar fungicide unit - Prothioconazole/Tebuconazole (Prosaro® 300mL/ha) @GS31 High = 4 x fungicide units - Systiva® seed treatment, 3 x foliar fungicides including QoI (strobilurin) and SDHI combinations with DMIs)

³Plant growth regulation (PGR) (Moddus® Evo 200mL/ha @GS30 and Moddus Evo 200mL/ha @GS33-37).

⁴Defoliation = simulated grazing @GS16 (TOS 1 and 2) and GS30 (TOS 1 only)

Learning 1 - Protect upper crop canopy during critical period

Ensure a high proportion of the upper crop canopy leaves remain intercepting light (retain green leaf area via disease management) during the ‘critical period’ for grain number formation. Irrespective of whether it is a low, medium or high rainfall zone (M-HRZ), it is essential growers and advisers consider disease management as one of the most important components of growing high yielding cereal crops in seasons with high yield potential. For example, responses to fungicide timing and number of applications in 2022 across all rainfall zones saw yield increases greater than two- to three-fold. When N wasn’t limited yields of Planet barley increased from 5.7t/ha to 6.6t/ha at Hart, and from 4.8t/ha to 6.8t/ha at Nullawil under high fungicide intensity (Table 2). Note Millicent Planet yields in Table 3 are less than lower rainfall sites and below potential despite treatments for disease and canopy control. The disease pressure in the high rainfall zone has revealed that genetics with improved disease resistance are now likely required to improve yields.

Learning 2 - Optimise crop development

A fundamental principle that is relevant across all agroecological zones is to ensure crop development and flowering are matched to the environment. This is best optimised by sowing the right variety at the right time to ensure it flowers in the optimum window when the combined risk of heat, frost and drought is low, and when the critical period is best aligned with cool and sunny conditions. The following examples in 2022 highlight the reduction in Planet yield potential from flowering earlier than the optimum due to excessive PTQ (radiation and temperature). For example, at Birchip yield potential increased from 7.6t/ha when flowering on 28 August, to 9t/ha when flowering on the 13 September – achieved by delaying the sowing date.

Table 3: 2022 grain yields of Planet barley managed for disease and lodging across three diverse agroecological zones and sowing times compared to long term (1990–2022) and 2022 water limited (WYP), and photothermal quotient limited yield (PTQ) potential.

*dry sown effective sow date in brackets

Learning 3 – Keep the crop standing and improve harvest logistics

In high production conditions, excessive growth prior to stem elongation is unproductive and leads to lodging, shading and poorer light interception in the critical period. Poor canopy structure can extend beyond this phase by reducing conversion of biomass into yield post-flowering. Disease control and the combined application of PGRs and timely harvest ensures pre-harvest yield losses are reduced, particularly in barley. This reduced the incidence of lodging, brackling, and head loss in 2022, for example at Hart (Figure 2). While this did not translate to yield differences in small plot trials harvested on time, previous experience suggests PGRs and varieties with improved head retention do not lose yield as quickly when harvest is delayed.

Figure 2. Incidence of lodging and brackling in three barley varieties at Hart in 2022 across canopy interventions.

Learning 4- Ensure the farming system can support a higher N demand

Cereal crops need to be supplied with roughly 40kg N/ha for every tonne of potential yield. To achieve an 8t/ha cereal yield, the crop will need about 320kg N/ha from the soil and fertiliser. Supplying this amount of total N with fertiliser N alone will be very costly. Crop responses to added fertiliser has reached a plateau in the HYC research at many sites, even when the N requirement for the yield achieved was high, indicating a large proportion of N is being supplied by the soil. This is where a fertile farming system becomes important, supplying large amounts of N from soil organic matter, manures or legume residues during critical periods of growth and supporting high yields with manageable levels of fertiliser N. The data from Hart and BCG field sites suggest more N is required than current district practice to ensure yield potentials are met in seasons like 2022. While this was achieved with applied N, a more effective long-term approach would be to maintain soil fertility and organic matter using pasture or legume phases, crop residues and limited tillage. Addition of manures and composts may also support this system to ensure ‘mining’ of soil nutrients does not occur at higher yield levels.

Conclusion

Recent research has demonstrated that other than appropriate variety selection, maximising growth in the critical period through canopy management using fungicides, sowing time, N and plant growth regulation can generate yield responses ranging from 3t/ha to 8t/ha within similar genetics in cool and mild production environments when rainfall is not limiting. In barley, there may be more scope to close the yield gap in the short to medium term, with further improvements in disease management, head loss, brackling and lodging control, but this has not yet been realised.

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 author would like to thank them for their continued support. The authors also thanks FAR Australia and Brett Gilbertson of Millicent.

References

Fischer RA (1985) Number of kernels in wheat crops and the influence of solar radiation and temperature. The Journal of Agricultural Science 105, 447-461.

French RJ, Schultz JE (1984) Water use efficiency of wheat in a Mediterranean type environment. I. The relation between yield, water use and climate. Australian Journal of Agricultural Research 35, 743-764.

Rawson HM (1988) Constraints associated with rice–wheat rotations: effects of high temperatures on the development and yield of wheat and practices to reduce deleterious effects. In ‘Wheat production constraints in tropical environments’. A Proceedings of the International Conference, 19–23 January 1987, Chiang Mai, Thailand. (Ed. A.R. Klatt) pp. 44–62. (The International Maize and Wheat Improvement Center (CIMMYT): Mexico).

Sadras VO, Angus JF (2006) Benchmarking water-use efficiency of rainfed wheat in dry environments. Australian Journal of Agricultural Research 57, 847-856.

Sadras V, McDonald G (2012) ‘Water use efficiency of grain crops in Australia: principles, benchmarks and management.’ GRDC, SARDI, The University of Adelaide.

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Contact details

Kenton Porker
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