Fast wheats to beat the heat – the performance of elite 100-day wheats sown mid-winter

Take home messages

• The changing Australian climate is shifting rainfall later in the season.
• The development of a short-season, winter-sown commercial wheat would provide growers with more flexibility and options.
• Winter sown wheats have the potential to yield incredibly well in mild climatic years.
• Early vigour and phenologically fast wheats are expected to outperform slower growing and developing wheats when sown late.

Background

The Australian climate is shifting toward hotter autumns with less rainfall (Bureau of Meteorology 2022). With the majority of southern Australian wheat sown during this period, less autumn rainfall will present considerable challenges to growers looking to maintain or increase their productivity. Wheat is sown during this window to ensure that it has adequate time to build biomass before anthesis for eventual conversion into grain yield (Xieet al. 2016). Furthermore, sowing is timed to ensure that the plant’s requirements of vernalisation and photoperiod are fulfilled to time anthesis during the optimum flowering window. This window is determined by local climatic conditions and is defined as a period of decreasing frost risk and increasing heat and water stress (Flohret al. 2017). Flowering during this window therefore reduces the overall risk that sensitive floral parts may be impacted by adverse temperature and weather conditions. While the timing of the window varies year to year depending on the weather conditions experienced, timing anthesis during this critical window ensures the maximum amount of grain is produced (Harris et al. 2020).

Wheat breeding in Australia has focused on breeding wheat varieties with longer maturity to allow for earlier sowing in the season (Hunt et al. 2019). The crop then has the longest possible time in the growing season to access moisture and build biomass, often resulting in growers sowing into dry soil and relying on the autumn break to germinate seeds (Fletcher et al. 2016). However, the autumn break is becoming less reliable, with total rainfall decreasing and that which does fall, occurring later in the season (Bureau of Meteorology 2022).

With an increase in the prevalence of herbicide-resistant weed species, grower management of weeds has had to adapt (Nakkaet al. 2019). Commonly, growers are turning to strategies such as double-knock herbicide application or tillage to ensure weeds are sufficiently controlled before sowing. With an increase in average wheat farm size (Sheng and Chancellor 2019), these practices can consume a large amount of time at the beginning of the season. Currently, there are no commercial wheat options for sowing mid-winter. A late-sown, short-season wheat would enable growers to postpone sowing until there is greater certainty of adequate soil moisture. This would allow greater flexibility for remediation of weeds and, with elite cultivars developed specifically for winter sowing, it would ensure good germination without the current concerns of dry sowing.

Early vigour field experiment methods

Field experiments were sown in 2021 and 2022 at the NSW DPI Wagga Wagga field site. The aim was to investigate the impact of mid-winter sowing dates on a set of elite wheat genotype’s growth and development. It was hypothesised that wheat lines with greater early vigour and faster time to anthesis would outperform slower growing and developing lines. In 2021, 96 cereal lines provided by CSIRO, NSW DPI, and InterGrain were sown. In 2022, a subset of 44 lines were sown. They included commercial wheat varieties, international lines, breeding lines, and a commercial barley variety. Three times of sowing was investigated across the two years: two times of sowing in 2021 of June 7 and July 12, and one time of sowing in 2022 which was June 28. They are referred to in this paper as TOS1, TOS2, and TOS3 respectively. In both years, the cereal lines were sown as plots consisting of six rows spaced 25cm apart and 6m long. All measurements were obtained from the middle four rows and at least 50cm in from each end. Plots were sampled through the season for key growth measures such as leaf number, biomass, ground cover, height, tiller number and final grain yield. Their development was also recorded through the season via Zadok’s growth stage.

Early vigour field experiment results and discussion

Mean yield and development

The simplified method of Sadras and Angus (2006) was used to predict grain yield based on the crop’s growing season rainfall using the following equation (Table 1):

Grain yield (t/ha) = ((GSR – 60) * 22)/1000

Table 1: Mean and observed growing season rainfall (GSR) for the NSW DPI Wagga Wagga field site (35°02'44.4"S 147°19'51.3"E). Mean and actual GSR for time of sowing 1 (TOS1- 7/6/2021) and sowing 3 (TOS3- 28/6/2022) are from June to October in 2021 and 2022 respectively, time of sowing 2 (TOS2- 12/7/2021) is from July to October 2021. Letters in superscript in the grain yield column denote Tukey’s post-hoc test groups (HSD = 0.191–0.206t/ha)

The greater amount of mean rainfall in TOS1 resulted in a higher predicted grain yield compared to TOS2 (5.45t/ha vs 4.34t/ha). Once GSR was recorded for the year, TOS1 still had a greater predicted grain yield than TOS2 (4.67t/ha vs 3.01t/ha). With lower-than-average rainfall received, the predicted values were predictably smaller. In TOS3, greater than average rainfall was recorded and therefore, the predicted grain yield was much higher than expected (6.61t/ha vs 5.45t/ha). For reference, Flohr et al. (2017) estimated peak mean yield of a winter wheat sown in Temora (75km north east of Wagga Wagga) on April 12 and flowering on October 9 to be 4.51t/ha.

Actual yields were higher than predicted in 2021 but the expected trends remained the same. In 2021, sowing later in the season resulted in significantly (p < 0.001) reduced yields, from an average of 5.16t/ha in TOS1 to 4.46t/ha in TOS2. The average yield of plots sown in TOS3 was 5.5t/ha. Interestingly, both TOS1 and TOS2 exceeded their water limited yield predictions, likely due to stored moisture not taken into account in this prediction (Hunt and Kirkegaard 2012). TOS3’s actual average grain yield was lower than predicted, this may have been due to a variety of other constraints such as radiation, temperature, or nutrition. Growers should note the variabilities in yield and, while aiming for a maximum water limited yield potential, also account for all crop requirements.

From a developmental standpoint, the effect of adaptable phenology was clear. TOS1 took an average of 133 days to anthesis while TOS2 took only 107 days, so despite sowing 35 days apart, average days to anthesis only differed by 11 days. Even though TOS3 was sown in a different season, time to anthesis took an average of 116 days, which, like its sowing date, was between TOS1 and TOS2 (see supplementary table 1). From a grower standpoint, this shows that, while the relationship between sowing date and time to anthesis isn’t strictly linear and is strongly driven by temperature and daylength, there is some flexibility of sowing window due to the adaptability of wheat phenology (Harriset al. 2020).

Traits of interest

Of particular interest in this experiment was an investigation of traits that led to higher yields across genotypes in late-sown environments. It was hypothesised that early vigour, the ability to produce greater early biomass, would provide a grain yield advantage when sown mid-winter. However, in this experiment, there were no strong correlations between early vigour, as measured by dry biomass present at Z31 (first node) and final grain yield (Figure 1).

Figure 1. Two years of field experiments using elite high vigour wheat genotypes display no correlation between early (Z31- first node) biomass and final grain yield, regardless of the year or time of sowing.

Likewise, at anthesis, there were no strong correlations between biomass at anthesis and grain yield. Another hypothesis was that a wheat sown mid-winter would need to reach anthesis by the optimum flowering date of around 12 October in Wagga (Flohr et al. 2017), estimated at days to anthesis of 127, 92, and 106 for the TOS1, TOS2, and TOS3, respectively. However, average days to anthesis were 133, 107, and 116 for the three TOS, respectively. On average, each time of sowing flowered at least 6 days late, and up to 15 days after the optimum flowering date. As shown in Figure 2, there was no linear association between days to anthesis and yield for any of the three sowing dates, so in these two years of experimentation, there was no observable penalty for late flowering. However, the genotypes that did manage to flower within the window may present a yield advantage over those that are phenologically slower, and that may be damaged by heat or drought at flowering in other years.

Figure 2. Two years of field experiments using elite high vigour wheat genotypes reveal no correlation between days to anthesis and final grain yield, regardless of the year or time of sowing.

Interestingly, 2021 and 2022 weren’t extraordinary years for temperature accumulation. Temperature was measured by the number of growing degree days (GDD). For example, TOS2 (the July sowing of 2021), which flowered on average 107 days after sowing, would, in an average year, undergo 1163 GDD. In 2021, it experienced 1176. Similarly in 2022, the June-sown TOS3 would in an average year have experienced 1202 GDD in its 116 days to anthesis, and in 2022, it experienced 1242 GDD (see supplementary table 1).

When analysing the mean minimum and maximum temperatures, neither year was exceptional. Average flowering date spanned from 17 October (TOS1) to 26 October (TOS2), with TOS3 in the middle on 21 October. The long term average minimum temperature in Wagga Wagga in October is 7.8°C and the maximum 21.7°C. In 2021, the average minimum was 7.3°C and the average maximum 21°C. In 2022, the average minimum was 10°C and maximum was 20.2°C. Therefore, there were no extreme changes in mean temperatures in the lead up to flowering, except in 2022, which had considerably warmer minimum temperatures.

In terms of temperature extremes, it becomes clearer why the slower developing and growing genotypes were not penalised in these two years. In September and October in 2021, there was only 1 day below 0 (-0.4°C on September 22) and in 2022, there was not a single day below 0°C in either month. In October and November, there was not a single day in either year above 30°C. The importance of pre-anthesis temperatures was originally highlighted by Fischer (1979) and then by Xie et al. (2016). Mild conditions in the buildup to anthesis allows the maximum number of fertile florets and then grains to be set. Similarly, mild conditions post-anthesis allow those grains to then fill and mature (Slaferet al. 2015; Drecceret al. 2018; Ababaei and Chenu 2020).

Conclusions

Elite early vigour cultivars sown mid-winter have the potential to yield exceptionally well in the right conditions. These experiments sown in Wagga Wagga in 2021 and 2022 demonstrate remarkably high average yields from June sowings (5.16t/ha and 5.5t/ha, respectively) and 4.46t/ha in July. Contrary to expectations, the genotypes with the greatest early vigour and quickest flowering didn’t necessarily perform the best when sown late compared to slower developing genotypes (Figures 1 and 2). As discussed, this was likely due to the mild growing conditions experienced in the pre- and post-anthesis periods, with no extreme cold or heat occurring in the month either side of anthesis. It is still hypothesised that, in a typical year, genotypic interactions would play a greater role in determining yield. Currently, there are no commercial wheat varieties that have been tailored to a later sowing window, as evidenced by the average flowering dates occurring past the optimum date. Further conclusions produced from this broader project will aid in the development of a short-season, 100-day wheat, to provide growers with an alternative option to combat herbicide tolerant weed species and shifting rainfall patterns. In isolation, this experiment has demonstrated the yield and growth potential of elite early vigour wheat lines when sown mid-winter, and the opportunities for their introgression into future commercial wheat varieties.

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.

Supplementary figures and tables

Supplementary table 1: Observed mean days to anthesis and growing degree days for the NSW DPI Wagga Wagga field site. Numbers in brackets in the mean days to anthesis denote the number of days mean anthesis occurred after the estimated optimal flowering date of October 12.

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Contact details

Timothy Green
tigreen@csu.edu.au