Pushing a tropical crop into the arctic zone? Sowing sorghum into cold soil temperatures ‐ risks and rewards

Take home messages

• Plant establishment was significantly impacted by the super early (early August) and early (late August) sowing times when soil temperatures were between 8.4 and 12.2°C.
• Plant populations from the early August (super early) sowing, particularly at the Mallawa and Gurley sites, were approximately half the population established from the standard sowing time (October) when soil temperatures were consistent with or greater than current recommendations of 16-18°C.
• Viable plant stands were established at sub optimal temperatures and produced higher yields than the standard sowing time at Mallawa and Breeza. Sowing sorghum into cool soils has the potential to shift the flowering window and reduce the risk of heat and moisture stress during flowering and grain fill and increase the likelihood of double cropping opportunities.
• However, this is dependent on being able to establish the crop in a timely manner. At the Breeza site for example, the time taken for emergence was significantly lengthened for the early August and late August sowing. As a result there was little impact on flowering date achieved by varying sowing time compared to the standard window in late September.
• There is a need to further understand the effect of cool and freezing temperatures on germination, emergence, canopy and growing point development before the practice of “super early” or “early” sowing could be extended commercially across the northern grains region.

Background

Grain sorghum is the most important summer cereal crop in NSW, providing important rotational, logistical and cash flow benefits for the northern grains region.

Many in the industry would agree that climatic variability in the past 10 years has seen an increasing trend of crop yield reduction, and sometimes failure, as a result of sorghum crops flowering and filling grain in periods of extreme heat and moisture stress.

NSW DPI and GRDC have partnered in research to evaluate options for sorghum sowing and agronomic management that challenge our current practices. This includes our accepted views on ideal sowing time and hybrid selection by comparing alternative practices which could be readily adopted by growers using current genetics and technology.

Seasonal overview

Three sites were sown in northern NSW in the 2017-18 season; Gurley (south east of Moree), Mallawa (west of Moree) and Breeza on the Liverpool Plains. At each of these sites three treatments were included:

1. Varying time of sowing, based on soil temperature at 8am EST; super early (~ 10°C), early (14°C) and a standard (16-18°C) in an attempt to bring flowering and grain fill forward.
2. Varying sowing depth; standard (3-4 cm) and deep (7-8 cm) seeding depth to chase warmer soil temperatures.
3. Comparing cold tolerance of 9 commercial hybrids: MR Buster, MR Apollo, Cracka, Tiger, HGS114, HGS102, Archer, Agitator, G33.

The 2017-18 season produced three distinct environments as outlined in Table 1.

1. Breeza which experienced cool spring temperatures; 21 days with temperatures <0°C for the early August sowing; and warm summer temperatures 35 days >36°C;
2. Gurley which had mild and wet late spring conditions (109 mm in Oct), warmer flowering and grain fill conditions compared to Breeza;
3. Mallawa which had cool conditions for August (12 days <0°C) and then extreme heat (60 days >36°C) and dry summer conditions for December – January.

Table 1. Summary of weather conditions for sorghum trials sown during the 2017-18 season. Soil temperature is at 8 AM across seven days after sowing.

Statistical methods

A split-split plot design was employed at each of the 3 sites. The data was analysed using the REML procedure in ASReml-R and the level of significance for least significant difference (LSD) testing was set at 5%.

Results

Plant establishment

The early August (super early) sowing times established only half the number of plants compared to the standard sowing time (Table 2) at Gurley and Mallawa. Establishment improved for the late August (early) sowing but was still less than the standard.

In addition to the lower plant populations established, the time taken for these plants to emerge was substantially longer. For example there were no plants present until 3 weeks post sowing at Breeza for the two early sowings and plants were still emerging up to 6 weeks post sowing.

There was no difference in the establishment of hybrids except at the Mallawa site where Agitator had significantly lower establishment (data not shown).

Table 2. Established plant populations at sorghum trials sites - averaged across treatments

Days to flowering

There was a large reduction in the time taken to reach 50% flowering at the Mallawa site when moving from the super early (120-136 days) and early (105 -116 days) sowing times (Figure 1, i). The spread of flowering times between hybrids also became smaller with the later planting, from 16 down to 11 days.

Similarly, at Gurley the days to 50% flowering was reduced between each of the sowing times, super early (116 – 132 days), early (101-116 days) and standard (66-69 days) planting dates, as was the spread between hybrids (Figure 3,ii).

This equates to flowering in late November – mid December for the super early panting times and mid-December for the early sowing time at Gurley and Mallawa. The standard sowing time flowered between Christmas and New Year and so no dates were recorded. There was a significant TOS by hybrid interaction effect at both sites. Sowing depth had no significant impact on flowering date at either site.

At Breeza, there was very little impact of sowing time on days to 50% flowering with all three flowering similarly around early to mid-December. This is likely due to emergence being spread over an extended period at this site for the super early and early sowing times.

There was a significant hybrid effect with MR Buster and Agitator flowering much earlier than the other hybrids with MR Apollo being the slowest to reach flowering. This was a similar trend at the other two sites.

At the Breeza site only, the shallow sowing depth was quicker to reach 50% flowering than the deep sowing.

Figure 1. Days to 50% flowering at i) Mallawa, ii) Gurley iii) Breeza

Grain yield

Grain yields ranged from a low of 1.3 t/ha on average at Mallawa to 3.2 t/ha at Gurley and 3.4 t/ha at Breeza averaged across treatments at 0% moisture content.

At Mallawa the early August (super early) and late August (early) sowing time produced higher yields than the standard sowing time despite having highly reduced plant establishment (Figure 2). There was no difference in grain yield associated with varying the sowing depth (data not shown).

At Gurley, the standard October sowing performed generally better than the early August (super early) and late August (early) treatments. MR Apollo showed no significant response to time of sowing which was similar to the Mallawa site (Figure 2). MR Buster, Agitator and Archer showed the same relationship with the standard sowing time out yielding the super early and early sowing times. G33 and HGS102 showed similar yield performance for the super early and early sowing times.

For HGS102 there was a significant interaction between hybrid and seeding depth, with the shallow seeding depth increasing yield whilst other hybrids had no significant response to seedling depth.

At Breeza, yields were generally higher from the early August (super early) and late August (early) sowing times compared to the September sowing even though established plant populations were only two thirds of the plant stands achieved with the standard sowing time in September (Figure 2).

There was also a significant TOS by seedling depth interaction effect with the early TOS having a significantly higher yield than the standard TOS for the shallow but not the deep seedling depth. Archer, Cracka and MR Buster performed relatively well in terms of yield across all three times of sowing at Breeza. G33 and HGS114 performed well in the super early and early sowing times but were disappointing for the standard sowing time. Agitator did not perform well across the three sowing times at this site in 2017/18.

Figure 2. Grain yield at 0% moisture at i) Mallawa, ii) Gurley iii) Breeza

Conclusions

Growers currently have access to a range of tools to vary the time to flowering and the conditions experienced by their sorghum crops during grain fill. These tools are, however accompanied by an increased level of risk.

In this single year of research across three sites, benefits were minor from varying sowing depth to seek warmer soils for early sowing conditions. While differences in soil temperatures between the two depths shallow and deep were detected this did not equate to improvements in plant establishment or grain yield at two of the three sites.

Sowing in early and late August at all three sites, showed that sorghum can be established at sub optimum temperatures and handle some cold (<0°C) conditions. However, this early sowing time came at a significant establishment cost. Further, evaluation of the impacts of severe frosting on plant growth and survival are needed.

At Mallawa and Gurley, establishment was less than half that which occurred in the standard sowing time. Therefore a lot of seed never contributed to grain yields but an input cost had been incurred. The impact of drying soil conditions at these two sites needs to be considered also.

In contrast, at Breeza where soil moisture was controlled through irrigation, establishment losses from the early and late August sowing were still significant but not as great as the other two sites, when compared to the standard sowing time.

At all three sites viable plant stands were established. At Mallawa and Breeza, the two early sowing times resulted in superior yields compared to the standard sowing time. At Gurley it was the opposite, most likely due to the timing of in-crop rainfall.

Flowering data has shown that it is possible to move the flowering and grain fill window to earlier in the season, provided that the time taken for crop establishment is not excessively prolonged by cool early growing conditions such as occurred at Breeza. At Breeza there was little difference in days to flowering between all three sowing times even though sowing time varied from 10^th august to 21^st September.

It was not possible to detect many differences between hybrids with respect to cold tolerance in this year’s field trials, due to confounding background genetics, seed production and quality attributes. A pot trial has been conducted to try and further separate some of these confounding factors.

As expected, these results should be considered preliminary as they are the results of three experimental sites in one season. It is hoped that this research can be continued into the future to further validate these preliminary findings.

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 and NSW DPI, the author would like to thank them for their continued support.

In particular thanks to our trial co-operators; Mark & Wendy Manchee, Gurley; Jason & Geoff Hunt, Mallawa and Scott Goodworth NSW DPI Breeza for their assistance with the sites.

Thanks to Delphi Ramsden, Alice Bowler and Simon Tydd for technical assistance.

Contact details

Loretta Serafin
NSW Department of Primary Industries
4 Marsden Park Road, Calala, NSW
Ph: 0427 311 819 or 02 6763 1100
Email: Loretta.serafin@dpi.nsw.gov.au