Take home messages

• Match optimal flowering period to local growing environment to managing environmental stress on crops is critical to maximise grain yield potential
• Vapour Pressure Deficit provides a useful predictor as to which periods are best to target for flowering and grain fill
• Understanding a varieties phenological development rate is key to determining optimal sowing dates to target optimal flowering period in each environment.
• Confidence in predicting flowering date for each variety is key to targeting optimal flowering periods.

Background

Since 2015, various wheat phenology/agronomy research work has been conducted in Emerald, Central Queensland (CQ). The 2015-16 data was collected as part of the final two years of the New South Wales Department of Primary Industries (NSW DPI) led Varietal Specific Agronomy Package project (VSAP). These trials had three components: varietal response to sowing date, row spacing and nutrition. The 2017–19 data was collected as part of a detailed phenology project (DAN00213) led by Dr Felicity Harris, through the Grains Agronomy and Pathology Partnership involving NSW DPI and GRDC.  Each year a number of ‘core’ and ‘non-core’ experimental sites were located across the northern grains region, including a core site in Emerald and non-core site in southern Queensland.

The 2017-2019 sowing dates were aligned (within 2 or 3 days) across all sites, allowing for cross site and cross year comparisons of the trials. The target sowing dates were April 5 (core sites only), April 20, May 5, and May 20. Varietal selections in the trials targeted a diversity of genetic material. So, while the variety list is not exhaustive of all commercially available genotypes on the market, it was hoped to have a representative spread of the spring and winter wheats available in the market at the time.

Stress

Management of frost risk (rightly or wrongly) always ranks highly in a grower’s decision-making process. Most will use tools like the NVT sowing guides to identify suggested windows for when varieties should be grown for certain regions, with the primary aim for most to have choose a variety that flowers after the potential frost risk period. Theoretically, anthers are most sensitive to damage which can occur at temperatures up to 2^oC at head height. However, factors such as duration of the cold period, humidity and crop stress can all influence the extent of damage sustained at a given minimum temperature.  For Emerald, the highest risk period by data since 1900 would historically be from late June to early-August (Figure 1), with a risk level of less than one every ten years. Looking at the same data set for the past 30 years (since 1990), the risk is almost negligible, but the heat stress period has tightened up.

Figure 1. Charts generated in the CliMate app, assessing the likelihood of experiencing minimum temperatures below 2^oC or above 30^oC based on Emerald weather data dating back to 1900. The highlighted period in green shows that during that period for that location, the probability of experiencing outside the defined limits is less than 10%.

Crop stress due to extreme heat and dry conditions can have just as much, if not a more significant effect on a crop yield and grain quality than frost. The risk of heat stress inducing conditions (temperatures above 30^oC) historically spikes rapidly in CQ (Figure 1 – 1990 to present) at the end of August. However, as recent experiences have shown (Figure 2), August can be susceptible to either stress type of events, with sub 2^oC degree events into late August, or temperatures reaching 30 ^oC in the first week of August.

Figure 2. Observed daily minimum and maximum temperatures (^oC) at the Emerald Research Facility (ERF) from 2016 to 2020. The blue dotted line indicates the theoretical heat stress temperature for wheat of 30^oC while the red dotted line shows when theoretical temperature cold stress/frost was likely to begin to occur (particularly during flowering).

High-risk times

The period from head emergence to the end of grain fill is a fraught time for winter cereal crops in CQ. The average in crop (April – September) rainfall for the Emerald site of approx. 70 mm for the past 5 years, the majority of which typically falls in the April to June period. By late July/August, typically you have a larger plant, trying to pull water from heavy clay vertosol soils at depths of 30 to 60 cm or deeper and grain fill is occurring as temperatures are highly variable spiking to 30^oC and drop to 0^oC.

But it is not just temperature the plants are trying to manage; it is also humidity (or the lack there of). In the same way that high delta T values (high temp/low humidity) can compromise spray applications, so to does this combination exacerbate crop stress. A succinct and long-established way to assess and manage this ratio (particularly in greenhouses or horticulture) is by measuring the Vapour Pressure Deficit (VPD) (kPa), or the difference between how much moisture is in the air, relative to how much moisture it can hold when fully saturated.

The greater the ’deficit‘or difference between these two values, the harder the plant (which can be up to 95% water) must work to extract more water out of the ground to replace what is lost through transpiration, but also maintain important processes like maximising yield potential. As temperatures increase and humidity drops, the plant will try and close the stomata on the leaves to minimise water loss, putting the plant into a state of stress if maintained for a prolonged period.

We know from the phenology research conducted in Emerald, that the average period for most common spring wheats grown in CQ, to go from 50 % flowering to maturity is typically around 40 days (Figure 3). We also know that when that variety is sown can have a considerable influence on when the clock starts ticking on that 40-day flowering period. The question in CQ environments is, how significant is the VPD values over the period from flowering to maturity and what effects does this have on yield potential?

Figure 3. Average duration (days) from 50% flowering (GS65) to maturity (GS90) from Emerald Research Facility trial data from 2017 - 2019 for a spread of spring wheat maturities. Error bars indicate average, longest, and quickest periods across the four sowing dates over the 3-year period of the trial.
(All varieties in this figure are protected under the Plant Breeders Rights Act 1994)

In a greenhouse or a temperature-controlled environment, for a range of plants, most industry recommendations for VPD (kPa) is a value between 0.8 – 1. While dryland broadacre growers have no such control over the environment crops grow in, they can make informed decisions on sowing dates for varieties to target periods which may be less harmful and maximise yield potential at known critical growth stages. Table 1. shows what average monthly VPD (kPa) observations at the Emerald Research Facility have been for the past 7 years and the average value for each month across that period.

The green squares show months where average VPD were more optimal for minimising plant stress, while values in the red show periods which would/could have induced significant stress on the crop, particularly if plant available water (PAW) was limited. Based on the VPD values alone, you would assume that the optimum period to be putting the greatest load (flowering and grain fill) would be in June and July.

Table 1. Monthly Average Vapour Pressure Deficit (VPD) (kPa) observed at the Emerald Research Facility from 2015 – 2021.

Days to 50% flowering

Nominating a fixed period for a cereal crop genotype to flower for any given sowing date at any individual location is problematic at best. With so many variables at play including genetics, climate, and agronomic management, even when efforts are made to minimise variables such as planting date, population and starting PAW, a year-to-year variance of as much as 10 days is still possible. The days to flowering data (GS65) from the 2017 to 2019 data set provides the average days to flowering for the 3 years of data across all four sowing dates (Figure 4).

Figure 4. Average days to 50 % flowering across all four sowing dates 5 April, 20 April, 5 May and 20 May at the Emerald site from 2017 to 2019. Note. Blue dots indicate flowering date lengthened the later it was sown, green dots were with 2 days across all sowing dates and orange dots shortened days to flowering, the later the sowing date. Error bars indicate min and max average days to flowering for all varieties across the 4 sowing dates.
(All the varieties in this figure (except H45, RGT Zanzibar and Janz are protected under the Plant Breeders Rights Act 1994)

The graph has been sorted on stability and variance rather than just average time to GS65.  The varieties have been sorted into three important groups. Varieties from H45 through to EGA Gregory (marked in blue), which tend to slow or extend the time from sowing to 50% flowering the later they are sown. How much average time to flowering slows is shown by the error bars, early sowing dates are faster, later sowing dates are slower, with a variety like Sunprime being as quick as 65 days planted early and out to 75 days if planted in late May.

Moving from left to right, the further to the right of the list of blue varieties you move, the more stable they are across sowing dates in these trials. The green dots (Beckham, LongReach Lancer & Catapult) show the three most stable varieties across all sowing dates in this trial, with LongReach Lancer showing up on average as the most stable of them all, with only 1-day average variance across the 4 sowing dates.

The varieties from LongReach NightHawk to Sunlamb (marked in orange), are ones which speed up, the later you plant them within a sowing window in CQ. They are generally longer season varieties and time to flowering variation increases, the further to the right you look across the list. Varieties like EGA Eaglehawk, Sunmax, RTG Zanzibar or Longreach Nighthawk are varieties which are performing poorly in CQ conditions even for early sowing dates as they take more than 100 days to reach GS65, and take advantage of the optimum flowering window.

Doing the math (case study)

Grower of legend ‘Bazza’s preferred variety is EGA Gregory. The first week of May is his preferred sowing date because of frost concerns. Based on the data discussed, what type of conditions can he expect in Emerald between flowering and grain fill?

• Based on Figure 4, we know EGA Gregory will take around 92 days to get to 50% flowering.
• Planted around 5 May, would put flowering roughly in the first week of August.
• We know from Figure 3 the grain fill will take about 40 days from flowering, which will mean harvest should be around the 2^nd week in September.

We know from this information, that monthly VPD would have been on the rise by the time this crop was ready to flower and then start trying to fill grain. Figure 5 shows what did happen in 2018 and 2019 based on the 4 sowing dates used in the phenology research. 2018 was a particularly dry and tough year, while 2019 was much milder with more in crop rainfall.

Just for comparison, I have also added a much faster spring variety, Sunprime, to compare what difference a quick maturity line sown at the same time would have made. This variety has a wider, days to flowering range, (Figure 3) depending on sowing date. So early sowing dates would have flowered in under 65 days, while later sown dates flowered closer to 75 days. For an early May plant, you would expect to be flowering around 70 days, putting flowering at around the second week in July, and maturity 45 days later or around the end of August.

Figure 5. The figure presents the predicted genotype performance in each environment included in the multi-environment trial (MET) analysis of the core wheat phenology field experiments conducted between 2017 and 2020 in Emerald, for the selected subset of genotypes and environments. Note:The vertical and horizontal error bars denote the 95% confidence interval.

The quicker maturity line would have flowered and been grain filling in the lowest stress period of the year, with average monthly VPD levels of 0.81 for July and 1.09 for August. Even with PAW diminishing close to the surface, the crop still develops in the lower stress period giving the crop the best opportunity to maximise yield. Conversely, the longer maturity EGA Gregory, starts flowering with a VDP of 1.09 kPa and by September and the later stage of grain fill, VDP levels are above 1.44 kPa, well into stress inducing territory.

The bigger picture

Stepping back and looking at a wide range of maturities, genotypes, sowing dates and flowering dates, the correlation between genetics, environment and management becomes clearer. Below I have superimposed onto the heat and cold stress graph (Figure 1), the yield response to flowering dates, relative to respective years trial mean yield levels, and finally the monthly average VPD levels. There appears to be quite strong correlation between higher yields and lower VPD levels. (Figure 6)

Figure 6. Yield response variance from each year’s trial mean yield plotted relative to flowering date. This chart has then been super imposed over the frost and heat stress risk chart for the region.  Each dot represents the average yield of a particular variety at a particular sowing date. The orange dotted line represents each year’s average trial yield, Table 2 lists the value of the orange line for each year of data. Finally the monthly average VPD levels for the Emerald Research facility has been placed across the top (Heavy blue line), with values indicated on the secondary verticle axis.

Table 2. Mean yield for each trial represented in Figure 6.

While there is a lot going on in Figure 6, at a macro level, as climate induced crop stress (VPD) starts to increase to levels beyond 1 kPA, any wheat crops trying to flower or fill grain well into August and beyond in CQ’s current climate will suffer significant yield loss relative to those crops which flowered earlier in the year.

Call to action

The biggest gains growers can make in maximising wheat production in CQ is to re-evaluate their sowing dates to ensure that flowering and grain fill take place during the lowest stress period (low VPD levels) possible for the conditions to match their local climate.

By bringing the flowering date back into these new optimum flowering period (OFP) for your region, it has the potential to lift yields between 20 – 40%, all other variables being equal. Flowering in this period will not only maximise yield potential, but also improve grain size and reduce screenings. It is also within the OFP that genetic advantages/differences between varieties are most able to express themselves, allowing growers to take full advantage of any improvement in yield one variety may have over another.

From a practical standpoint, changing flowering dates can be done in one of two ways - adjust sowing date of your current varieties to target the optimum flowering period, or change variety if you are reluctant to plant any earlier than you do currently. From a systems standpoint, early sown wheat tends to have a better chance of getting follow-up rain to get secondary roots down and will compensate much better if establishment is sub optimal (especially if deep sown), particularly mid to longer season varieties like Mitch, LongReach Lancer or Coolah.

It is important to note that while our data clearly indicates that for lower frost risk regions in the Central Highlands, the OFP is in the June-July period. This may not be the case for all regions and as such, growers must assess frost risk on a paddock-by-paddock basis. If you believe the chance of getting a frost in late June is 1 in 2 / 3/5 or 7 years instead of 1 in 10, then do not target a flowering date that will put you at that elevated risk of frost damage.

Frost and heat stress management for winter cereals in CQ is, and has always been, a fine balance between risk and reward. Our data collated over the past 7 years across a range of projects, clearly indicates there is some significant upside to be had when you get the balance of genetics (maturity), agronomy (population, nutrition) and environment (plant available water, temperature & humidity) right. The challenge for growers is to assess their risk profile (possibly on a field-by-field or even a bay-by-bay basis) and choose a combination of all these factors which will maximise production while minimising risk (be it frost or heat).

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.

I would like to acknowledge Dr. Felicity Harris from NSW DPI as lead of the “Optimising Grain Yield Potential in the Northern Grains Region” project: 2017 – 2019 (DAN00213) (Grains Agronomy and Pathology Partnership, GRDC and NSW DPI) and the statistical support from SAGI and particularly Michael Mumford and Clayton Forknall.

I would also like to recognise Ellie McCosker, Jane Auer and the rest of the QDAF Regional Research Agronomy team based in Emerald for their contribution to this work.

Contact details

Darren Aisthorpe
Department of Agriculture and Fisheries, QLD
99 Hospital Rd, Emerald QLD
Ph: 07 4991 0808
Email: darren.aisthorpe@daf.qld.gov.au

Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994

GRDC Project Code: DAN00213,