Longer season wheat varieties what are the opportunities

Background

Longer season wheat varieties have been grown in central NSW over many years, but their area has declined in recent decades. The millennial drought, less frequent early autumn breaks and a succession of dry springs favouring quicker varieties have contributed to this change. Over the same period Australian wheat breeders focussed on cultivars with increasingly rapid development through selection of insensitive photoperiod and vernalisation alleles. There were few longer season varieties released, particularly mid-developing winter wheat cultivars (similar to EGA Wedgetail, Wylah, Whistler), following closure of the Temora breeding program in 2002.

A swing to mainly quicker varieties contracts the sowing window even as many growers are increasing their cropping programs, and there is evidence that April-May rainfall is declining. Late establishment can expose the crop to the increasing risk of drought and high temperatures in late spring. Summer and early autumn rainfall has not declined over the same period, and an opportunity exists to remove reliance on April-May rain for establishing crops by conserving soil water and using it to establish crops earlier than is commonly practiced.

In eastern areas, moisture from summer and early autumn rain is now routinely conserved via improved fallow weed control. In more western areas, long (18 month) fallow is an important component of farming systems. Long fallowing and early sowing are complementary practices, as the fallow reduces weeds and diseases which can be difficult to control in early sown crops, and early sowing with slow developing cultivars allows the crop to better use soil water that is stored during the fallow. Stored soil water also helps to establish early sown crops when there is minimal autumn rainfall. Sowing machinery with greater tine breakout pressures and presswheels is now common, as is stubble retention which preserves soil moisture closer to the soil surface. These changes improve the likelihood of good establishment with early sowing. A shift in establishment date requires cultivars which are slower to develop so that they do not flower too early and risk damage by frost or reduced yields from insufficient dry matter accumulation.

The importance of flowering time

One of the main drivers of wheat yield and quality is flowering time. When selecting a cultivar and sowing time combination, the intention is to match plant development with seasonal pattern and most importantly get the crop to flower during the optimal period for yield. In south-central NSW the optimal flowering period varies from late August in the west to early October in the east (Table 1). This period is a trade-off between increasing drought and heat, and declining frost risk. There is no ‘perfect’ time to flower where these risks are nil, only an optimal time where they are minimised and yield on the balance of probabilities is maximised.

Table 1: Optimal flowering periods, peak of the mean of frost-heat adjusted APSIM yield and corresponding flowering date and sowing date range for a mid-fast cultivar for 51 years (1963-2013) for locations in NSW (Flohr et al., 2016; in review).

From Table 1 it is evident that in most locations the preferred sowing date for peak mean yield of a mid-fast variety occurs in May. Slower developing varieties are needed for earlier sowing and breeders can achieve this several ways. One is by increasing photoperiod sensitivity, which results in flowering being delayed by short days. Another is by introducing a vernalisation response, so that a period of low temperature (e.g. <10oC ) is required before the plant will switch from producing leaves to forming a spike. The strongest requirement for vernalisation is in ‘winter’ wheat varieties.

In the presence of ample stored soil water, winter and slow developing spring cultivars sown early often yield more than faster cultivars sown later (Tables 2 and 3). This is because the longer growing season available to early sown crops allows them to grow deeper roots and extract more water, reduce soil evaporation and produce more biomass. They may also tolerate winter water-logging better. However, if there is no stored soil water for growth around anthesis and grain filling, early sown crops can hay off and will yield the same or in some cases less than faster developing cultivars sown later. Lower seeding rates and delayed nitrogen application can be used to mitigate this risk.

Table 2: Grain yield for a range of cultivars of different development rates sown on two dates on long fallow at Rankins Springs in 2015.

Table 3: Grain yield (t/ha) and stem frost damage (% tillers frosted) for a range of cultivars of different development rates sown on two dates on long fallow at Rankins Springs in 2014. This trial suffered severe damage from stem frosts in July and August which reduced yield in slow developing spring cultivars sown early.

Winter cultivars (e.g. Wedgetail) have the greatest range of potential establishment dates, but have been of more value to mixed farmers who can graze these crops in the vegetative phase (typically for ~1000 dry sheep equivalent (dse) /ha grazing days). Because they take longer to reach stem elongation, winter cultivars are also less susceptible to stem frost than slow spring cultivars (Table 3). There is unlikely to be a yield advantage of sowing winter cultivars intended only for grain production before early April as the extra vegetative biomass production will not contribute to grain yield.

Australian wheat breeders have increased their emphasis on varieties for earlier sowing in recent years. Slower spring varieties have been released (e.g. Lancer, Kiora, Flanker). Winter cultivars will be more attractive once cultivars better adapted to western NSW become available. Wedgetail can be too slow to flower in this region even when sown early; varieties with a lower vernalisation requirement or faster development once vernalised should perform better. An example of the yield potential of the lines being developed can be seen from a trial at Temora in 2015 (Table 4). A faster winter line, RAC2341, was outstanding from a 17th April sowing. There were few significant differences among the range of commercial varieties sown at their recommended times.

Table 4: Grain yield of six wheat varieties of different maturity sown at their recommended times at Temora in 2015.

Conclusions

Longer season wheat varieties, of both winter and slow spring growth habit, have a valuable place in modern farming systems. Combined with effective fallow water storage, they have the ability to out-yield later sowings. By extending the sowing window, they can contribute to timeliness of the whole sowing program and improve farm average yields.

References

Flohr BM, Hunt JR, Kirkegaard JA, Evans JR 2016 Drought, radiation, frost and heat define the optimal flowering period for wheat in south-eastern Australia. Journal of Experimental Botany. In review

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 would also like to gratefully acknowledge the guidance of local farmers in this work, particularly Michael Pfitzner who hosted Rankins Springs trials in 2014 and 2015, and the input of the CWFS, CSIRO and Ag Grow Agronomy trial staff.

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