Genetic improvement of canola establishment

Take home messages

  • A nationwide survey of canola growers and agronomists identified marginal soil moisture and variable seeding depth as two key factors leading to poor canola establishment.
  • Our project is pursuing genetic solutions to improve establishment of canola targeting early vigour and longer hypocotyls to enable deeper sowing to access soil moisture.
  • We identified several overseas varieties with enhanced vigour and/or longer hypocotyls, which emerged significantly better from 50mm sowing depth in the field than all five current Australian canola variety controls.
  • We have developed rapid and accurate screening methods for breeders to accelerate the development of canola varieties with improved establishment potential.

Background

Successful establishment is the foundational phase of any crop lifecycle and necessary to achieve high yield potential and compete effectively with weeds (Kirkegaard et al. 2020). Poor establishment of canola (Brassica napus L.) is a widespread problem in Australia and Canada with on average 50-60% of germinable seeds successfully establishing (Harker et al. 2012; Harries et al. 2017; McMaster et al. 2019). The success or failure of canola establishment is influenced by a range of factors including agronomic practices, environmental conditions, seed quality and genetics (Nelson et al. 2022).

Australian growers are increasingly seeking to sow their canola crops early to maximise yield potential. This increased yield potential comes with a trade-off of increased risk of poor crop establishment due to false breaks, poor soil moisture and high soil temperature (Fletcher et al. 2016). Canola has small, oil-rich seeds that typically do not emerge well when sown deep (below 30mm) (Brill et al. 2016) or in soils prone to crusting, so the target seeding depth tends to be shallow (typically around 20mm). Shallow sowing results in a greater likelihood of false breaks and inability to reach stored moisture deeper in the soil profile. Growers are faced with a trade-off when choosing the target sowing depth, which is further complicated by the common use of imprecise seeding equipment leading to variable seed depths. Studies have shown that larger canola seeds emerge better than smaller seeds from deeper sowing (Harries et al. 2017; McMaster et al. 2019; Riethmuller et al. 2003) and hybrids tend to establish better than open-pollinated varieties independent of seed size (Brill et al. 2016).

Most research into canola establishment to date has targeted improved agronomy, understanding environmental conditions and seed quality/size aspects. Little is known about the genetic determinants that influence canola establishment and what little is known comes from studies in Europe and Canada. In 2019, CSIRO and GRDC began a 4-year project to identify international varieties with better genetic potential for establishment, and to develop selection and screening tools to help breeders develop Australian canola varietes with improved establishment potential.

Method

In order to learn from grower experiences of canola establishment, we first conducted a survey of Australian growers and agronomists comprising 18 questions designed to understand grower practices, their experiences, and their views on causes and solutions of poor canola establishment. Participants were invited via Twitter and through grower group email lists to participate anonymously in a SurveyMonkey questionnaire, which remained open from January 30 to February 17, 2020. We used these responses, along with a comprehensive review of scientific literature, to build our understanding of the crop traits that must be improved to achieve higher and more consistent establishment rates of canola.

Our genetics approach was as follows:

  • For any breeding activity, it is essential to have genetic variation of the target traits and so we assembled a highly diverse set of 100 open-pollinated varieties of Brassica napus comprising mainly oilseed canola but also including vegetable and fodder types. These varieties originated from 18 countries representing the canola growing regions of Europe, Canada and China and included 23 historic Australian varieties. In order to achieve uniform seed quality and limit maternal effects on seedling performance, seed was multiplied in the same field production site at the Ginninderra Experimental Station, Canberra in 2019.
  • Efficient and accurate lab-based screening methods were developed to measure seedling vigour at three stages (germination, cotyledon and 4-leaf stages), and hypocotyl length and cross-sectional area. These methods were used to screen the 100 international varieties alongside 28 current Australian varieties (26 hybrids, 2 open pollinated) contributed by breeding companies.
  • A subset of 20 international varieties was selected from the lab screens to include the extremes for vigour and hypocotyl traits along with five current Australian varieties were evaluated. These 25 entries were evaluated for establishment from depth (50mm and 20mm) at four field locations in NSW (Boorowa and Griffith) and WA (Bejoording and East Kokeby) in 2021. Fifteen seeds were sown precisely by hand with 6cm separation in 1.2-1.5m rows at 20mm and 50mm sowing depths, with four replicates in a randomised, split-plot design. Field sites were visited on average 10 times each, with photographs taken of each plot which were used to track emergence counts at each sampling date. When the 20mm depth canola reached the 4-leaf stage, the rows were individually harvested, and biomass measured and expressed as dry weight per plant. This was the first year of a 2-year study on field establishment.

Results and discussion

Canola establishment survey

Canola seeding practices

The survey drew 63 responses (47 growers, 15 agronomists and 1 seed company technical lead), from the main canola growing states and representating a wide range of average annual rainfall (280 – 950mm). The 5-year average proportion of canola in their cropping programs was 27%, with the median canola sowing date being 15 April. Of respondents, 41% used only hybrids, 24% used only open-pollinated (OP) varieties and 35% used both. Most respondents using OP varieties (35 out of 37) graded their own seeds, typically using a minimum seed diameter of 1.7mm. Growers predominantly used tyne seeders (82.5%) and, to a lesser extent, disc seeders (11%) to sow canola, while the remaining 6% used both tyne and disc seeders, or other seeders. The typical seeding rates ranged widely from 1.0 to 5.0kg/ha but most (73%) were in the range 1.6-3.0kg/ha, and 73% of respondents sow into cereal stubble. Of the 47 respondents who provided target plant densities, 83% were in the range 21-50 plants/m2. Most respondents (71%) sow dry ‘usually’ or ‘always’ if the break of season has not already occurred.

Causes of poor canola establishment and operational solutions

The survey then asked what the growers and agronomists considered to be the most important factors limiting canola establishment. The two most frequent responses related to marginal soil moisture (76% of respondents) and sowing depth (65%) (Figure 1). Other common responses were poor soil structure/crusting (29%), seed treatments (22%), sowing too fast (17%) and poor seed quality/vigour (16%). Commonly cited solutions to improve canola establishment were largely operational or agronomic in nature: achieve more precise seeding depth (26%), slow down seeding (23%), increase seeding rate (13%), careful choice of wheel pressing/rolling (11%), and use of soil wetting agents (9%).

Figure 1. The views of 63 survey respondents about the factors that limit canola establishment in their experience.

Figure 1. The views of 63 survey respondents about the factors that limit canola establishment in their experience.

Insights to improve establishment potential in canola

When combined with a comprehensive review of the scientific literature, these survey responses provided a clear vision of the genetic improvements required by canola varieties to achieve better and more consistent establishment in Australia:

  • Canola varieties capable of emerging from deeper sowing. This would help canola access stored soil moisture and reduce the likelihood of responding to a false seasonal break in the season. This is especially important with the trend towards earlier sowing (Hunt et al. 2019). We identified increased germination vigour and longer hypocotyls as the key target traits to support successful establishment from deeper sowing.
  • Rapid post-emergent growth to maximise competitiveness with weeds and to rapidly access moisture deeper in the soil profile. We identified two key stages in seedling development to target: the cotyledon stage, which is sustained primarily by seed-stored energy; and the 4-leaf stage when the seedling has transitioned from reliance on stored energy to sustaining growth by photosynthesis.

Developing selection tools to improve canola establishment

Developing efficient selection tools

Our new lab-based screening methods to assess early vigour (at germination, cotyledon and 4-leaf stages) and hypocotyl (length and diameter) traits were very robust with repeatability ranging from 0.71 to 0.96 across the sets of international and current Australian varieties. This indicates these methods can be used with the confidence that they will provide consistently good measures of performance in controlled lab environments. For the three vigour traits, the best international OP varieties matched the best current Australian hybrid varieties, which had the advantage of heterosis (hybrid vigour). This is encouraging for breeders to make further improvements in the additive genetic value of their hybrid parents and even greater vigour in new hybrid varieties.

The standout trait, however, was hypocotyl length. The three best international varieties had significantly (p<0.05) longer hypocotyls than the 28 current Australian varieties tested (Figure 2). There is clearly scope for substantial increases in hypocotyl length in current Australian varieties.

Figure 2. The three best international varieties (yellow bars) had significantly longer hypocotyls than the 28 current Australian varieties (blue bars) tested. lsd = 24.9mm

Figure 2. The three best international varieties (yellow bars) had significantly longer hypocotyls than the 28 current Australian varieties (blue bars) tested. lsd = 24.9mm

Ground-truthing lab-based findings in the field

Having identified international varieties with improved vigour and hypocotyl traits in controlled lab conditions, we set out to determine if these traits reflected performance under field conditions. To this end, we conducted four field experiments in WA and NSW in 2021 of which the most insightful was the Boorowa trial. At the 50mm sowing depth, the three international varieties (from 20) with the highest emergence rates had a significantly higher emergence rate than the best Australian variety (from five) (Figure 3A). These three international varieties were identified in lab-based screens as having long hypocotyls or high vigour. Similarly, the three international varieties with the most biomass per plant had significantly higher biomass than the best Australian variety tested (Figure 3B). All three international accessions were identified as having high seedling vigour in lab-based screens. Thus, we had reasonable grounds to expect that the rapid lab-based screening methods are effective to identify varieties with improved establishment potential. However, it should be stressed that this is from one year of field experimental data and the best performing trial. These experiments will be conducted again in 2022 to confirm the repeatability of these field-based results.

Figure 3. Field performance of the best three (of 20) international varieties (yellow bars) which had significantly higher emergence rates (A) and/or biomass per plant (B) than the best Australian variety (of 5; blue bars) when sown at 50mm depth. Note that the anonymised varieties shown are not necessarily the same between both charts.

Figure 3. Field performance of the best three (of 20) international varieties (yellow bars) which had significantly higher emergence rates (A) and/or biomass per plant (B) than the best Australian variety (of 5; blue bars) when sown at 50mm depth. Note that the anonymised varieties shown are not necessarily the same between both charts.

Conclusion

Guided by a survey of Australian canola growers/agronomists and a comprehensive literature review, we identified early vigour and long hypocotyls as target traits to improve establishment of canola when it is sown at depth. We developed rapid, lab-based screening tools to identify international varieties with enhanced vigour and hypocotyl lengths. The best international varieties clearly outperformed the best Australian varieties in lab-based screening, and this appears to hold up in the first year of field experiments. These results show great promise to enable canola breeders to rapidly develop new canola varieties that can establish better from deeper sowing.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through the support of the GRDC, the authors would like to thank them for their continued support. We also thank the 63 anonymous survey respondents for their time and insights into canola establishment. We thank Shannon Dillon, Chris Helliwell, Alex Boyer, Emmett Leyne and colleagues on the CSIRO/GRDC canola phenology project (CSP1901-002RTX) for providing seeds for the international varieties. We also thank the canola breeding companies for providing seeds of current varieties. We thank Kalyx and the Boorowa Agricultural Research Station staff for assisting in setting up and/or monitoring field experiments.

References

Brill RD, Jenkins ML, Gardner MJ, Lilley JM, Orchard BA (2016) Optimising canola establishment and yield in south-eastern Australia with hybrids and large seed. Crop and Pasture Science 67(4), 409-418.

Fletcher A, Lawes R, Weeks C (2016) Crop area increases drive earlier and dry sowing in Western Australia: implications for farming systems. Crop and Pasture Science 67(12), 1268-1280.

Harker KN, O'Donovan JT, Blackshaw RE, Johnson EN, Lafond GP, May WE (2012) Seeding depth and seeding speed effects on no-till canola emergence, maturity, yield and seed quality. Canadian Journal of Plant Science 92(4), 795-802.

Harries M, Seymour M, Bucat J, French B, Sprigg S (2017) Agronomy for early sown canola. Proceedings GRDC Grains Research Update, Perth, February 2017, p. 62.

Hunt JR, Lilley JM, Trevaskis B, Flohr BM, Peake A, Fletcher A, Zwart AB, Gobbett D, Kirkegaard JA (2019) Early sowing systems can boost Australian wheat yields despite recent climate change. Nature Climate Change 9:244-247

Kirkegaard J, Lilley J, Berry P, Rondanini D (2020) Canola. In: Sadras V, Calderini D (eds) Crop Physiology Case Histories for Major Crops. Academic Press

McMaster C, Menz I, Stevenson A (2019) Canola establishment across central NSW. Proceedings GRDC Grains Research Update, Wagga Wagga, February 2019, pp. 229-234.

Nelson MN, Nesi N, Barrero JM, Fletcher AL, Greaves IK, Hughes T, Laperche A, Snowdon R, Rebetzke GJ, Kirkegaard JA (2022) Strategies to improve field establishment of canola: a review. Advances in Agronomy (in press).

Riethmuller GP, Carmody PC, Walton GH (2003) Improved canola establishment, yield and oil with large seed on sandplain soil in Western Australia. “Proceedings of the 13th Australian Research Assembly on Brassicas’. Tamworth, NSW”, 8–12 September 2003, pp. 55-59.

Contact details

Matthew Nelson
CSIRO Agriculture and Food
Floreat Laboratories
147 Underwood Avenue, Floreat, Perth WA 6014
0490 139 509
matthew.nelson@csiro.au
@Matthew67636066

GRDC Project Code: CSP1907-001RTX,