Take home messages

• Soil texture is key to deep sowing (>10cm) success under marginal soil moisture conditions.
• On a loamy sand in the Mallee with stored soil moisture, deep-sown (10cm) treatments established 29–50% of total emergence before shallow treatments had germinated after the seasonal break on 30 May.
• On a sandy loam over clay on southern Eyre Peninsula, without available stored moisture, shallow treatments emerged before deep-sown treatments and established double the final plant stand of deep-sown treatments.
• Under marginal starting soil moisture in 2024, adequate soil moisture and soil strength above the seed were key to establishment success.

Background

Growers need adaptations to overcome the challenges of large-scale sowing programs, a highly variable seasonal break and increased emphasis on the importance of timely establishment for maximum yield. Adaptive strategies include combinations of novel genotypes (long coleoptile) and innovative crop management (deep sowing into sub-surface soil moisture). Sowing at depths exceeding 8–10cm can better utilise sowing opportunities after summer and early autumn rainfall to ensure earlier germination and establishment (Rich et al. 2021; Flohr et al. 2021). Alternative dwarfing genes have been identified with potential to reduce plant height and increase yields while increasing coleoptile length by 50–80% (for example, Rebetzke et al. 2022). Some of these dwarfing genes (for example, Rht8 and Rht18) have been used commercially overseas and are currently under evaluation nationally in the project ‘integrating long coleoptile into Australian farming systems through genetics, management and environment interactions’ (GRDC Project code. CSP2212-007RTX).

Previous research has shown the combination of long-coleoptile, deep-sowing strategies for wheat have substantial promise, but careful consideration of soil factors is required to ensure success and to improve on-farm decision-making regarding its use (Flohr et al. 2021, Stummer et al. 2023). Here we evaluate long coleoptile genotypes sown at multiple depths, soil types and soil strengths in South Australia, and share with growers what we have learnt about this system in the driest autumn on record.

Method

Two sister experiments were sown in Wynarka in the Mallee and Lipson on the Eyre Peninsula. The experiments evaluated six genotypes that contained either the alternative dwarfing gene (Rht13, Rht18) or the coleoptile length reducing Rht2 dwarfing gene, three press wheel weight treatments that varied soil strength above the seed, and two sowing depths (Table 1). Press wheel treatments were achieved by either removing press wheels (light), standard press wheel (standard, 22kg) and an additional 26kg added to the press wheel (heavy). Each experiment was sown ~2 weeks before the seasonal break in 2024. The experiments had two aims:

• provide understanding of genotype response to sowing depth and environmental conditions at sowing and during early growth
• determine the thresholds of soil strength required for long coleoptile wheat to establish under different depths of sowing.

Table 1. 2024 experimental details.

Results and discussion

The median seasonal break for the South Australian Mallee is ~10 May and 24 May on the Eyre Peninsula, and the seasonal break has reportedly shifted 7–14 days later in recent decades in south-eastern Australia (Flohr et al. 2021). In both environments, the seasonal break didn’t occur until after 30 May in 2024. The seasonal break itself was marginal, with 10mm in Wynarka and 13mm in Lipson, and March–May rainfall at both sites was lowest on record.

Figure 1. Cumulative average rainfall and 2024 rainfall at A) Wynarka (Mesonet 124) and B) Lipson (stn 18091).

Wynarka

In Wynarka, surface soils were very dry at sowing (Figure 2B), but below ~10cm, there was 116mm soil water in the soil profile 10–100cm that accumulated during the 2023–2024 fallow period (data not shown). Growing season rainfall was decile 1 in 2024 (107mm vs average 263mm Figure 1A). Standard and heavy press wheel treatments resulted in higher soil strength and moisture at ~10cm depth after sowing (Figure 2).

Figure 2. Seed bed soil strength measured with handheld penetrometer post-sowing, and gravimetric water post-sowing in response to the different press wheel weights, measured 15 May 2024 in Wynarka, SA. Broken vertical line is crop lower limit (CLL).

On a loamy sand, the mean establishment was 76 plants/m². While below target (120 plants/m²), this can be explained by the very marginal starting soil water conditions (Figure 2B), where only soil below ~8cm had soil moisture above the CLL (broken vertical line in Figure 2B). Planting depth for deep-sown treatments ranged from 104mm for heavy and standard, to 116mm for light press wheel, and 36mm for shallow-sown treatments (statistically significant effect of depth*pressure interaction (Lsd = 7mm at P<0.05)).

The effect of sowing depth x press wheel pressure was significant at all emergence assessment dates. On 5 June, deep-sown with standard press wheel plots had almost double the plant number than other deep-sown treatments (Figure 3). Deep-sown plants started emerging ~15 days earlier than shallow-sown (Figure 3).

Figure 3. Accumulated plant emergence (plants/m²) for sowing depth*press wheel pressure treatments at Wynarka.

Final plant emergence numbers show emergence was greatest for longer coleoptile Mace18, Calibre and Magenta (no significant interaction with depth and press wheel treatments, Figure 4). The significant effect of sowing depth and press wheel pressure on establishment suggest that on loamy sand, sowing depth and soil strength were larger drivers of plant establishment than genotype.

Figure 4. Final depth*press wheel pressure plant emergence recorded on 26 June 2024 (p-value <0.001, Lsd = 9), and final emergence for each cultivar 26 June 2024 (p-value 0.001, Lsd = 9) at Wynarka.

The results confirmed that at depth, long coleoptile, Rht13 and Rht18 dwarfing gene NILs (Mace18 and Magenta13) had significantly longer coleoptiles than genotypes containing the Rht2 gene (data not shown), and that when sown at depth, coleoptiles were shorter under heavy and standard press wheel pressure compared to light pressure (Table 2).

Table 2. Coleoptile length (mm) in response to press wheel pressure and sowing depth (mm) at Wynarka, SA.

Lipson

Summer fallow rainfall (Nov 23–Mar 24) at the experimental site was 82mm (with 58mm falling in January). Although this resulted in some deep soil moisture, only ~13mm of the rain fell between the end of January and the seasonal break, resulting in very low soil moisture levels at seeding, particularly in the 0–6cm layer (<6% moisture by weight) (Figure 1A).

Figure 5. Seed bed soil strength measured with handheld penetrometer, and gravimetric water post-sowing under the different soil strengths after sowing on 14 May 2024 at Lipson.

Mean establishment for the core experiment was 58 plants/m². There was a significant penalty in plant number (p<0.001, Lsd = 3.09) with deep sowing, and shallow-sown plots had more than double the plant density (an average of 84 plants/m² compared to 32 plants/m²). This was well below the target (120 plants/m²) and can be explained by the marginal soil water moisture at seeding (Figure 1A), where only soil below ~8 cm had soil moisture above CLL (broken vertical line in Figure 5B). There was also a significant relationship (p<0.001, Lsd = 5.6) between plant density and seeding soil press wheel pressure.

By 17 June (34 DAS), shallow-sown, standard and heavy press wheel treatments had 75–80% of their total plant numbers at establishment (with around 58% on the shallow-sown, light press wheel treatment). There was also some emergence from deep-sown treatments, with only 13–25% of the total plant numbers at establishment recorded by this date. Final plant numbers were 34–44 plants/m² for deep-sown, standard and heavy press wheel pressure, with only 24 plants/m² for the deep-sown light press wheel pressure treatment. The establishment benefit afforded by using heavier press wheel pressures did not outweigh the penalty from sowing deep at this site in 2024 (p-value 0.077). The shallow-sown standard and heavy press wheel pressures produced around 90 plants/m², with the shallow-sown light press wheel pressures producing fewer plants, at 71 plants/m².

Figure 6. Accumulated plant emergence for depth*seeding pressure treatments at Lipson (P<0.001, Lsd = 5.6).

When shallow-sown, there was no difference in mean plant numbers between genotypes. When sown at depth, shorter coleoptile Magenta and Scepter had the lowest plant numbers (p-value 0.017, Lsd = 5.5) and long coleoptile Magenta13 and Mace18 had the highest plant numbers, though numbers were well below shallow-sown treatments at this site in 2024 (Figure 7). While long coleoptile cultivars had greater plant numbers relative to short coleoptile cultivars when sown at 10cm (~30%), the marginal soil moisture at seeding in 2024 made sowing depth and press wheel pressure key factors in plant establishment success.

Figure 7. Final depth*press wheel pressure plant emergence recorded on 27 June 2024 (48 DAS, 25 DAR) (p<0.001, Lsd = 5.6), and final emergence for each cultivar*depth 27 June 2024 (p-value 0.017, Lsd = 5.5) at Lipson.

Yield and farming system implications

Crop establishment timing aligns crop cycle with environment and thus, date of emergence is often more important than crop plant density in determining yield. While a high plant population is often desirable, poor establishment will not necessarily always reduce yield. At low plant densities and particularly from early establishment dates, cereal yield per plant increases and therefore, shows little variation over a range of plant densities. We have focused on emergence timing and establishment here and less on yield. Reduced establishment and/or lower plant densities also has other implications, such as weed competition, and a wider range in time to emergence in deeper sown and/or dry sown shallow crops, can also make post-emergence herbicide management more difficult as there may be plants at different growth stages.

Conclusion

Under extremely dry starting conditions, crops established earlier in response to sowing deeper (10cm) on soil moisture above CLL on a sand in the Mallee, but not on a heavier loam on the EP lacking available stored moisture. Greater plant establishment occurred on the loam on the EP with shallow sowing. While the Rht13 and Rht18 genotypes expressed long coleoptiles and greater plant numbers on the heavier soil type, and coleoptiles were shortened with press wheel pressure (greater pressure resulted in shorter coleoptiles), it was more important to sow into adequate soil moisture and apply adequate press wheel pressure to maintain seed contact with moist soil for germination. Experiments will be repeated in 2025.

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. We thank Andrew Thomas for hosting the Wynarka experiment and Brenton and Tammy Stratford for hosting the Lipson experiment. We also thank our excellent technical teams from CSIRO and EPAG Research.

References

Flohr BM, Ouzman J, McBeath TM, Rebetzke GJ, Kirkegaard JA, Llewellyn RS (2021) Redefining the link between rainfall and crop establishment in dryland cropping systems. Agricultural Systems 190, p.103105. https://www.sciencedirect.com/science/article/pii/S0308521X21000585

Rich S, Oliver Y, Richetti J, Lawes R (2021) Chasing water: deep sowing can increase sowing opportunities across the grain growing regions of Western Australia. Proceedings GIWA Perth Updates  https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2021/02/chasing-water-deep-sowing-can-increase-sowing-opportunity-across-the-grain-growing-regions-of-western-australia

Rebetzke GJ, Rattey AR, Bovill WD, Richards RA, Brooks BJ, Ellis M (2022) Agronomic assessment of the durum Rht18 dwarfing gene in bread wheat. Crop and Pasture Science 73(4), 325–336.

https://www.publish.csiro.au/cp/pdf/CP21645

Stummer BE, Flohr BM, Rebetzke GJ, Meiklejohn R, Ware A, Haskins B, Whitworth R, McBeath T (2023) Long coleoptile genotype and soil texture interactions determine establishment success and early growth parameters of wheat sown at depth. Environmental Research Communications 5,055015.

Contact details

Bonnie Flohr
CSIRO, Waite Campus
Gate 4 Waite Road, Urrbrae SA 5064
0475 982 678
bonnie.flohr@csiro.au