Take Home Messages
- The trial site at Wallup experienced minimal frost damage in 2023.
- Mechanical defoliation successfully delayed flowering but also caused a significant yield penalty.
- Selected chemical agronomic interventions delayed flowering but incurred a yield penalty as well
Aim
To investigate agronomic interventions of mechanical or chemical defoliation, plant growth regulators and time of sowing and their effect on flowering time and frost risk in canola.
Background
Frost can be a significant issue in cropping regions such as the Wimmera. With growers aware of the consequences of dry finishes and the benefits of establishing canola crops early in the sowing window, along with the likelihood of increased frost risk, growers are interested to know if there are tools to mitigate the effects of frost on crops. In particular, a deeper understanding is required for management of crops that may be in sensitive growth stages at risky times of the season. It is known that grazing can delay phenology (GRDC Frost Tips and Tactics 2016) however, not everyone has livestock, or enough stock to crash graze crops in a vulnerable growth stage. There is also the potential of yield penalties. Frost damage avoidance could be achieved through the manipulation of crop phenology using novel agronomic practices, including the application of plant growth regulators or mechanical defoliation. The effectiveness of these approaches however has not been tested in the field across a range of environments. This research aims to assess the value of opportunistic and novel in season practices for potential frost avoidance in canola and the trade-offs potentially associated.
The agronomic interventions investigated include time of sowing, cultivar selection, mechanical and chemical defoliation measures as well as the use of plant growth regulators to effectively create a reset strategy in canola through delaying flowering and exposure to significant frost risk periods.
Some of the herbicides used in this trial are not registered for use in certain crops and were tested for experimental purposes only. Always read the label and adhere to directions when using herbicides.
Paddock Details
Location: Wallup
Crop year rainfall (Nov–Oct): 311mm
GSR (Apr–Oct): 177mm
Soil type: Clay
Paddock history: Wheat
Trial Details
Crop type/s: Canola – HyTTec® Velocity, HyTTec® Trophy, HyTTec® Trifecta and InVigor® T 6010
Treatments: Refer to Table 1 and Table 2
Target plant density: 40 plants/m²
Seeding equipment: Knife points, press wheels, 30cm row spacing
Sowing date: Refer to Table 1 and Table 2
Replicates: Four
Harvest date: Chemical defoliation trial and TOS 1 & 2: 8 November,
TOS 3: 15 November
Trial average yield: 3.46t/ha
Trial Inputs
Nutrition, weeds, insects, and disease were managed as per best practice.
Method
Two frost trials were established at the Wallup site 25km southwest of Warracknabeal. one was a variety/time of sowing/mechanical defoliation trial and the other a chemical defoliation/PGR trial.
Variety, time of sowing and mechanical defoliation trial
This trial incorporates three sowing times, four varieties, and two defoliation intensities to investigate the impact on flowering times across treatments (refer to Table 1 for details). This trial was set up as a partial factorial design containing four replicates in a split plot layout, with the main plot being time of sowing, and sub plots being variety and defoliation combinations. Grazing treatments were strategically assigned to varieties and did not occur across all varieties in all sowing times. Assessments included regular temperature monitoring, plant height, NDVI, flowering date, biomass, harvest index, grain yield and quality measurements.
Chemical defoliation trial
A replicated trial was sown in a randomised block trial design, with one variety, HyTTec® Trophy. Treatment sprays were applied when the crop began stem elongation. Assessments included temperature monitoring, NDVI, regular height measurements, biomass cuts, flowering date, harvest index and grain yield and quality measurements.
Results & Interpretation
Time of sowing
The 2023 season began with a large rain event in early April that helped TOS1 establish well. However, small rain events and warm conditions in the next five weeks inhibited establishment of the second time of sowing. By planting of the third time of sowing, temperatures had cooled and conditions were again favourable for crop establishment, although growth was slower. Temperature loggers were installed to monitor for minimum temperature and recorded temperatures below 0°C 34 times between the start of July and the end of October. The majority, and most significant of these occurred before or early in the main flowering and early grain fill period at the site. These events either had minimal impact on crop yields (as they were early enough) or there was sufficient time to allow compensatory growth as damage was not detected in yield or grain quality parameters. A subset of the temperatures encountered is presented in Figure 1 alongside the yield data from some of the grazing treatments.
Time of sowing
Figure 1. Mechanical defoliation and its impact on delaying flowering times. p<001,LSD 4.3>.
Mechanical defoliation was a technique used to delay flowering. Two intensities were investigated: medium and heavy. In medium, the green bud was removed but most of the above biomass was retained, and in heavy only 4–8cm was retained above ground, removing approximately 90 per cent of the above ground biomass. The impact of mechanical defoliation on flowering is displayed in Figure 1. This figure plots the minimum temperatures as indicated by the bars and left-hand axis, to highlight frost events (temperature below 0° C) relative to flowering time. The data points plot the 50 per cent flowering date against yield. Up to 27 days’ delay in flowering was achieved where the heavy defoliation treatment was applied, suggesting the practice can help reset phenology. However, it comes with a substantial risk of yield reduction. There was a significant reduction in yield across all grazing treatments with more intense grazing resulting in later flowering and lower yield. Later sowing also resulted in lower yield which is a common occurrence and why growers are looking to sow canola earlier and set crops up to maximise yield.
Time of sowing
Interestingly, with the way the season eventuated, there were no significant differences between the times of sowing in regard to yield. The only variety that displayed a time of sowing response was HyTTec® Trophy with TOS 3 yielding significantly lower than TOS 1 and 2. In TOS 1, the quick-mid and mid varieties of HyTTec® Trophy and Trifecta yielded significantly higher than the quick maturing Velocity and the mid slow InVigor® T 6010.
Time of sowing
Three key chemicals were used to investigate their effect on flowering times: plant growth regulators Moddus® Evo and Kult® and the herbicidal defoliant Basta® (rates in Table 2). When compared to the control, the only treatments that significantly shifted the day of 50 per cent flowering were the two higher rates of Basta® (800mL/ha and 1200mL/ha), which shifted flowering by 16–20 days (data not shown). It should be noted it has proven difficult to predict a response when using Basta® as a defoliant to shift phenology over two seasons. The visual effects of this treatment are stark (Figure 2), and this was highlighted by the impact it had on yield, resulting in a yield penalty of about 0.6t/ha in the 800mL/ha and 1.6t/ha in the 1200mL/ha rate (Table 4). Interestingly, the low rate of 400mL/ha of Basta® was not significantly different to the control for yield but also did not successfully delay flowering. Moddus® Evo and Kult® did not significantly delay the 50% flowering date. The yields of the Moddus® Evo treatments across the three rates and the lower two rates of Kult®, were all statistically similar to the control. However, the higher rate of 600mL/ha was significantly lower. No significant differences in grain quality were recorded.
Commercial Practice and On Farm Profitability
Exploring the idea of novel practices to shift phenology to avoid frost where crops are more advanced is valid in theory, however frost events are unpredictable so it is difficult to employ practices to mitigate or avoid frost. Defoliation strategies may be beneficial in seasons where crop development has progressed quickly in the early part of the year, and a check is needed to slow things down.
The work carried out in 2023 highlighted that these practices are still highly experimental, particularly regarding chemical defoliation, or the use of PGRs. Under the growing conditions faced in 2023 they offer little to no benefit and add additional cost to the program, either through product application, or lost yield.
Growers are advised to stick to the big levers for managing frost risk by choosing appropriate varieties to match the region and sowing window, with the aim of positioning flowering as close to the optimum window as possible. Where livestock are involved, there may also be an opportunity for grazing and gaining some feed value from the crop. However, when frost conditions do not occur, undertaking this practice on spring cultivars, especially those sown early and with short maturity, imposes significant risk of lost yield potential. Under conditions where frost occurs, this outcome could be different, as the practice of grazing does have the potential to shift the flowering window. Nonetheless, it is a high-risk approach requiring further research under frosty conditions.
References
GRDC, 2016, Tips and Tactics, Managing Frost Risk [accessed 18 January 2024].
Acknowledgements
This research was funded by the GRDC project ‘Enhancing Frost Tolerance’ and led by Max Bloomfield of FAR Australia; we thank Alexander McCrae for hosting the site. Also thank you to Kate Burke of Think Agri for providing a technical review of this article.
