Take Home Messages
- Where possible consider the temperature forecast when sowing early – soil surface temperatures can be up to 20°C hotter than air temperature
- Successful canola establishment is crucial for aligning crop development with the environment and determining yield potential.
- Canola is a great “compensator”. Timing of establishment is generally more important than plant density.
Aim
This trial aims to investigate the effect of sowing depth, variety selection and seed size on canola establishment, and understand how temperature and moisture can affect the outcome.
Background
Establishing canola remains a challenge in Wimmera and Mallee farming systems because of the need to align crop development with the environment in which the crop is grown to maximise yield potential. Consequently, canola is sown at a time of year when availability of soil moisture can be variable. This combined with other factors – such as small seeds, stubble, heat and cloddy seed beds impacting seed soil contact – means that often we are only establishing about 50 per cent of germinable seeds sown. This can reduce yield, increase weed problems and, in the worst cases, require re-sowing with an estimated annual cost to the industry of $100 million to $200million.
Canola establishment, is considered successful when a crop develops a canopy and root system able to grow on its own without seed reserves. Emergence is noted when cotyledons appear, but establishment is achieved at the 3-4 true leaf stage. This involves coordinated processes of seed germination, hypocotyl extension and growth of leaves and roots.
This project looks at reducing the risks to canola establishment through an integrated understanding of genetics, management and environment. The trials conducted by BCG in 2023 are satellite sites which are part of a bigger project in which moisture and temperature thresholds for canola establishment are being investigated. The goal is to develop some rules of thumb that underpin successful canola establishment across agricultural regions.
Trial treatments were identified according to locally relevant challenges with varieties and seeding depth being carefully considered.
Paddock Details
Location: Kinnabulla Wallup
Crop year rainfall (Nov–Oct): 276mm 311mm
GSR (Apr–Oct): 187mm 177mm
Soil type: Sandy clay loam Clay
Paddock history: Lentil brown manure Wheat
Trial Details
Crop type/s: Canola – HyTTec® Trophy and ATR Bonito
Treatments: Refer to Table 1
Target plant density: 40 plants/m²
Seeding equipment: Knife points, press wheels, 30cm row spacing
Sowing date: Kinnabulla: 24 April 2023, Wallup: 26 April 2023
Replicates: Four
Harvest date: Kinnabulla: 7 November 2023, Wallup: 17 November 2023
Trial average yield: Kinnabulla: 2.4t/ha, Wallup: 3.3t/ha
Trial Inputs
Nutrition, weeds, insects and disease were managed as per best practice
Method
Two split plot trials were established at Kinnabulla and Wallup with four replications containing two varieties and three target depths (Table 1) and targeting a plant density of 40 plants/m2. Seeding depth was the main plot factor, with variety and seed size making up the sub plot factor. Rate of emergence, seedling depth, crop establishment, soil moisture and soil temperature were monitored over the establishment period with yield and grain quality data collected at harvest.
Results & Interpretation
Time of sowing
The Kinnabulla canola establishment trial was sown on 24 April, nine days after an 8.4mm rainfall event, into drying soil. Follow up rain greater than 1mm did not occur until late May (25 May) after sowing. The moisture percentage requirement for germination is lower for the sandy loam soil compared to Wallup. However, the lack of rainfall resulted in low moisture percentage (gravimetric) in the 0–5cm of around 5–8 per cent which would be considered marginal for establishment (Figure 1). This had a clear effect on establishment rate and final establishment numbers (see establishment section). The Wallup trial was sown into a higher moisture percentage on 26 April following three small (<3mm) rain events in the week after sowing, with an extra 6mm event occurring on 25 May. While greater soil water content is required for germination on this clay soil, numbers were likely adequate for germination (Figure 2). These more ideal seed bed conditions at Wallup, compared to Kinnabulla, resulted in establishment being faster and more even across treatments (see establishment section).
Sowing Depth
Achieving the desired sowing depths for this research varied across both trials. Both sites failed to deliver the 1cm:3cm:6cm depths of interest, and instead the correct sowing depth was determined by removing plants from plots and measuring the hypocotyl (Figure 2). The data showed Wallup was more in line with the intended sowing depths, whereas getting the Kinnabulla site deeper than 3cm was challenging. This is likely due to the soil type and reflective of the variation in soils that may occur when looking to dry sow into hard seedbeds. Due to the inability to achieve the intended depths, sowing depth is referred to by the actual depths achieved in future sections.
Temperature
Sowing depth plays an important role in moderating the soil temperatures the seed is exposed to. Studies focusing on canola and related brassicas have primarily investigated germination responses to low temperatures. High temperatures above 35°C–40°C can impair germination, however these were not recorded in these experiments. An important point of seeding depth is temperature. In April–May, surface soil temperatures at both sites were up to 20°C hotter than air temperatures. In contrast, temperatures at 2.5cm depth in the seedbed were commonly no more than 5°C higher than air temperatures (Figure 3). This trend was observed across both Kinnabulla and Wallup sites, with only Wallup reported.
Establishment rates and crop yield
Crop establishment was influenced by conditions experienced at sowing. The Kinnabulla establishment data, while not significant at 14 days post-sowing, did show significant differences 28 days after sowing, with the shallow sown treatments establishing significantly higher populations than other depths. This is likely a result of the drying seedbed in combination with only small rain events for an extended period in the critical establishment window. Despite the impact on establishment from sowing depth, canola plants were able to compensate or at least level out for lower numbers with no significant differences between treatments (results not shown).
Data from Wallup, which was sown at the intended depths with more success, showed significant differences between the treatments in terms of establishment at the two-week post-sowing assessment time, with the graded Bonito seed establishing more plants and a more even stand than the ungraded Bonito and HyTTec® Trophy (Table 3). While it makes sense that the larger Bonito seed establishes better than smaller Bonito seed, it is unclear why the hybrid HyTTec® Trophy established poorly. The only consideration is whether its seed size, which fell between the other two varieties, slowed growth. Sowing depth was also a significant influence on plant numbers at the two weeks post-sowing assessment, although not as substantial as at Kinnabulla. This effect was overcome by the five-week assessment time, suggesting sowing depth was responsible for a delay in establishment as opposed to final plant numbers established.
Yields differed significantly between both treatment and sowing depth but no interaction between the two (Table 4). HyTTec® Trophy yielded significantly higher than the open pollinated Bonito regardless of whether seed was graded or not, suggesting genetic drivers for yield were in play. Sowing depth had significant differences for yield with the 1cm depth yielding significantly higher than the deeper sowing depths. This highlights the importance of quick and effective establishment of crops, as there is ability to maximise yield under favourable conditions when crops are sown and establish on time. Even though the establishment scores for the graded Bonito were significantly higher than the ungraded Bonito and HyTTec® Trophy, there was no direct implication for yield. This was potentially aided by late spring rainfall that ensured a softer finish for the trials in this region and would have maximised grain filling opportunities compared to Kinnabulla. Grain quality was also assessed, with differences only being noted for variety effects rather than any of the treatments of interest (data not shown).
Commercial Practice and On Farm Profitability
When planning to sow canola, there are many contributors to the success of establishment and resultant yield. They includ soil moisture or pending rainfall, soil temperature, seed size, sowing depth, and many other aspects – some within a grower’s control and others not. These trials highlight the sensitivity of canola to depth and marginal moisture. Engineering solutions may be required to better achieve depth control. The next steps are to relate the percentage moisture to rainfall amounts on different soil types to give growers better confidence when planting canola under marginal conditions. The best thing a grower can do to minimise losses from canola establishment concerns is get the basics right. Sow on time to give the crop the best chance of success, and spend time setting up the seeder bar so sowing depth is managed to allow quick establishment in most seasons. There will be times when reliable rainfall in the sowing window is not delivered, but crops have the best chance of compensatory growth if the above principles are managed well. This data reinforces principles that – where possible – larger seed can help with speed and evenness of establishment. Where this is in the control of the grower (such as retained OP varieties), use grading to improve seed size and consistency. Research into identifying the supra-optimal moisture:temperature relations for canola establishment will continue in this project during 2024 with a view to developing better rules of thumb about conditions that maximise the chances of success.
References
GRDC, 2020, GRDC Research Update Papers ‘A survey of crop establishment in canola and lentil – what have we learnt?’ <https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2020/02/a-survey-of-crop-establishment-in-canola-and-lentil-what-have-we-learnt>
Acknowledgements
This research was made possible and funded by the GRDC, and led by Andrew Fletcher and Kenton Porker (CSIRO). We also thank Lincoln Lehmann and Alexanda McCrae for hosting the trials in 2023.
BCG thanks Kenton Porker for providing technical review of this article.
