Take Home Messages
• Marginal rainfall events result in false germination and patchy establishment.
• In marginal moisture conditions it is better to sow shallow rather than deep.
• Hybrid varieties are less sensitive to sowing depth than open pollinated.
• Sowing too deep (>30mm) reduced yield
Background
Dry sowing is a common occurrence in Mallee and Wimmera cropping systems however, without a reliable forecast this can be a gamble for good establishment, particularly on a small seeded crop like canola.
2021 saw a lot of dry sowing in the Mallee and Wimmera and with little summer rain and unreliable forecasts some growers made the decision to drop canola out of the rotation for the season. Where it remained in rotation, establishment quality around the region was varied.
A responsive trial was established at Watchupga to investigate the impacts of different factors on canola establishment under marginal moisture conditions.
Aim
To investigate the effect of variety selection, sowing rate and sowing depth on canola establishment in marginal soil moisture conditions in the Southern Mallee.
Paddock details
Location: Watchupga
Crop year rainfall (Nov-Oct): 234mm
GSR (Apr-Oct): 172mm
Soil type: Sandy Clay
Paddock history: Vetch hay
Trial details
Crop type: Canola
Treatments: Refer to table 1
Target plant density: Refer to table 1
Seeding equipment: Knife points, press wheels, 30cm row spacing
Sowing date: 19 April
Replicates: Four
Harvest date: 6 December
Trial average yield: 1.4t/ha
Trial inputs
Fertiliser: Granulock®Z @ 60kg/ha at sowing (below seed), Urea @100kg/ha July 9 and Urea @ 80kg/ha August 11
Herbicide: Managed as per best practice
Insecticide: Alpha-cypermethrin @ 400mL/ha October 6, Sulfoxaflor @ 50g/ha and Alpha-cypermethrin @ 400mL/ha October 26
Fungicide: Prosaro® @ 450mL/ha July 21, Aviator Xpro® @ 400mL/ha 6 October
Seed treatment: Jockey® @ 2000mL/100kg and imidacloprid @ 400mL/100kg
Method
A replicated field trial was established as a split plot design with sowing depth as the whole plot and variety and sowing rate as subplots. Assessments included establishment rate counts, final establishment counts, seedling depth assessment, flowering biomass, grain yield and quality assessments.
Table 1. Trial treatment outline.
From November 2020 through to March 2021 the site received 44mm of rain. There was very little stored moisture. In April, there was only one rainfall event of 1.2mm received.
Results and interpretation
Seedling depth
Prior to sowing, three different depths were set for the seeder, targeting a shallow, normal and deep sowing for canola. Measurements were taken following emergence and found no statistical difference in the depth of shallow and normal seedlings however, deep sown treatments were significantly deeper (Table 2). While the measurements suggest limited differences between treatments, it is important to remember that measurements were taken on established seedlings and there may have been seeds at different depths (potentially very close to the surface) that did not establish.
Table 2. Average seedling depth (mm) achieved at different depth targets across the trial.
Rainfall and establishment
Emergence did not begin to occur until a month following sowing, after the receival of a 4.6mm rainfall event. Three previous rainfall events of 1–1.4mm did not trigger germination. Not all treatments began to emerge at this time, however. The majority of plants emerging following this event were Hyola 350TT at the shallow and normal depth. Emergence rate data found that the hybrid was able to establish higher numbers at a faster rate on less rainfall and is less sensitive to sowing depth than the open pollinated, small seeded ATR Stingray (Figure 1).
Figure 1. a) ATR Stingray establishment (%) at 50 plants/m2 sowing rate and b) Hyola350TT establishment (%) at 50 plants/m2 sowing rate against cumulative rainfall (mm). Data collected between 29 and 79 days following sowing.
Final establishment was somewhat patchy. It was not affected by sowing rate but was reduced from sowing deep (Figure 2).
Figure 2. Average establishment (plants/m2) across sowing depths. P<0.001, LSD= 5 plants, CV= 25.2%
Hyola 350TT established more plants than ATR stingray however neither reached full targeted establishment (Table 3).
Table. 3 Average establishment (plants/m2 and %) of varieties.
Biomass
Hyola 350TT had higher biomass at flowering than ATR Stingray with an average of 2.2 t/ha and 1.0 t/ha respectively (P<0.001). In both varieties flowering biomass was reduced by placing seed too deep which was reflected in final yield (Figure 3).
Figure 3. Average flowering biomass (t/ha) and yield (t/ha) for different sowing depths. Biomass: P<0.001, LSD= 0.47t/ha, CV% 37.8 Yield: P=0.005, LSD= 0.12t/ha, CV% 7.4
Yield
Depth sowing rate and variety both had an impact on yield. There was an interaction between variety and sowing depth. Hyola 350TT yielded higher than ATR Stingray however at depth yields were lower for both varieties (Table 4).
Table 4. Average yield (t/ha) of varieties at different sowing depths.
A trend of lower yield with deeper sowing was seen however when comparing the two varieties, Hyola 350TT yield was less sensitive to depth (Figure 4). The effect of sowing depth reflects the influence of depth on established plant numbers (Figure 5).
Figure 4. Yield (t/ha) for different seedling depths (cm) for ATR Stingray (left) and Hyola 350TT (right).
Figure 5. Yield (t/ha) for number of established plants (plants/m2) at different sowing depths for ATR Stingray (left) and Hyola 350TT (right).
Commercial practice and on-farm profitability
Canola can be successfully sown into dry soil and establish well. A few key factors need to come into play for this success. The soil must be dry enough to avoid premature germination and sufficient germinating rains, ~10mm of cumulative rainfall within a week depending on soil type and variety, must fall for even establishment to occur.
Research undertaken as part of the broader plant establishment project investigated how soil moisture can affect canola germination over time. Seed germination and emergence is sensitive to the availability of soil moisture. At low soil moisture, germination rates decline and the time to establishment is delayed. An example is given in Figure 6 for a sandy loam soil. Seed of hybrid canola was sown at 15mm depth in soil at different moisture contents. Emergence after three weeks fell from
100% in soil near field capacity (10–12% moisture content) to 20% in soil at 7% moisture content. There was no establishment in soil at 5% moisture content.
Figure 6. Changes in emergence time of canola sown into soil at moisture contents between
5% and 12%.
The effect of moisture content on final emergence in canola and wheat after three weeks is shown in Figure 7 and shows how quickly emergence can decline in dry soil. In canola emergence declined from approximately 100% to 70% by a 2% decline in the soil moisture and a further decline of 1–2% in soil moisture reduced emergence to approximately 30%. Wheat showed a greater tolerance and its emergence did not start to decline until about 7%. Similar trends will be seen in other soils although the actual soil moisture values will be different depending on soil texture.
Figure 7. The effect of different soil moisture contents on the emergence of hybrid canola and wheat. The vertical dotted lines represent the soil water content at field capacity (black) and permanent wilting (grey).
Patchy establishment was found in this trial with small rainfall events following sowing resulting in established plant numbers significantly below targets in some plots, particularly deep sown ATR Stingray. Variety selection, with factors such as large seed size and hybrid vigour contributed to faster and better plant establishment on small rainfall events. Selecting a hybrid variety over an open pollinated will provide a stronger choice for establishment due to the significantly greater early vigour of these varieties.
Canola has the ability to branch and compensate for low plant numbers to some degree. If a crop has achieved patchy establishment it is important to assess established plant numbers and consider the potential of lower plant numbers compared to a better established but later sown crop. Research by BCG in the last few years as part of this project has shown that yield can be optimised in canola with plant numbers as low as 23 plants/m2 (Clarke and McDonald 2020).
Traditionally, the recommended sowing depth for canola is between 1.5cm and 3cm however in some seasons seed can successfully be placed deeper (5cm) and successfully establish with access to moisture further down the soil profile (GRDC 2018). Results from this research and others consistently suggest that the risk of low emergence when sowing at a depth greater than 30mm is high and targeting a depth <30mm is recommended (GRDC 2019). Unless you are sowing into moist soil, targeting a deep sowing depth with canola will result in no advantage. Targeting a shallower depth will take advantage of smaller rain events for germination and will be more successful across most seasons in the Mallee environment. Sowing deeper, particularly on heavier soils when dry, can bring up clods and reduced seed to soil contact required for good establishment. Soil type should be considered when selecting sowing depth.
References
Clarke G, McDonald G, 2020, 2020 BCG Season Research Results, ‘Sowing for Optimised Establishment’
pp99–107. <https://www.bcg.org.au/sowing-for-optimised-establishment/>
Grains Research and Development Corporation (GRDC), 2018, GRDC Grownotes; Canola, ‘Planting’ pp 1–14. Available online at <https://grdc.com.au/__data/assets/pdf_file/0028/369316/GrowNoteCanola-South-3-Planting.pdf>.
Grains Research and Development Corporation (GRDC), 2019, Maximising Crop Potential in a Drying Environment, Available online at <https://grdc.com.au/__data/assets/pdf_file/0030/376761/Maximising-crop-potential-in-a-drying-environment-western.pdf>
Acknowledgements
This research was funded by the GRDC as part of the Optimising plant establishment, density and spacings to maximise crop yield and profit in the southern and western regions project (9176134) in collaboration with the University of Adelaide.
