Oilseed rape (OSR) establishment has had its mix of luck over recent years, but its early drilling slot has allowed for some well-established crops as we enter the spring. Added to this the sharp frosts we experienced in the 2022/23 winter didn’t happen this year, so crops haven’t suffered the exacerbated levels of cabbage stem flea beetle (CSFB) larval damage, nor the localised rape winter stem weevil issues we saw in some regions last season.
The other good news is that ADAS research conducted over the last three years suggests that our spring management of crops can be approached in a similar way to what we did before CSFB arrived. ADAS is about to release a paper that shows tried-and-tested canopy management principles using nitrogen and the PGR Caryx® are still important for those crops with medium CSFB damage, (i.e. those with main stem development in the spring). Caryx® has been shown to help to offset the increased lodging risk caused by CSFB tunnelling. In terms of timings, the target threshold of GAI (Green Area Index) of 0.8 (about 50% ground cover) at the end of February still applies.
With CSFB being such a ‚‘make-or-break‘ issue in OSR, the ADAS research into this topic should come as good news to growers. The three years of trials have enabled ADAS researchers to develop a deeper, updated understanding of the relationships between PGRs and CSFB larvae damage, nitrogen applications and the risk of OSR lodging.
“We sought the answer to two main questions that haven’t been explored in detail before,” notes Pete Berry, ADAS Head of Crop Physiology. “To what extent do CSFB larvae weaken stems and make OSR plants more prone to lodging? And to what extent can a PGR counteract the increased lodging risk created by CSFB?”
Caryx® reduces lodging risk
Replicated trials were conducted at two locations, close to ADAS sites in North Yorkshire and near King’s Lynn in Norfolk. Both OSR crops had moderate to high level of CSFB larvae in the stems.
In order to calculate the lodging risk, the ADAS team used a model developed in conjunction with wind engineers from Birmingham University. It takes into account stem strength, stem diameter and plant height.
“We found that CSFB larvae reduce stem strength and stem diameter. Height may be slightly reduced but not enough to compensate for the decreased stem strength; the overall wind resistance is lower and the crop is more likely to lodge,” explains Dr Berry.
“The lodging risk of a typical undamaged crop was 33%, which rose to 60% where there was CSFB damage. The risk dropped to 20% with the use of Caryx®,” he says.
Caryx® had a positive effect on the wind resistance mainly by reducing plant height.
“Typically we see height reductions of 10cm with Caryx® – that will reduce lodging risk substantially. We’ve also seen reductions of up to 20cm, how much you get, depends on the growing conditions at time of application but Caryx® is probably the most effective PGR for reducing height,” says Dr Berry.
Impact of nitrogen regimes on lodging risk
ADAS has explored the relative impacts of reduced nitrogen applications, in conjunction with Caryx®.
“The forward crop had a high CSFB pressure of 14 larvae/plant in the March, but was otherwise exhibiting normal development,” observes Dr Berry.
The standard nitrogen regime (in accordance with RB209 guidelines) saw a total of 140kg N/ha applied to the crop; 40kgN/ha was applied at the end of February, 60kgN/ha at green bud and 40kgN/ha at yellow bud. The experimental reduced nitrogen regime omitted the first dose.
“In both trial cases, Caryx® significantly increased wind resistance and therefore reduced lodging risk.
“Despite previously finding reducing nitrogen can lower lodging risk, in this trial it did not. This may have been caused by the dry March and April, which may have affected uptake. Yield was reduced as expected,” summarises Dr Berry.
“While it’s true that CSFB larvae impact on lodging risk is mitigated by the fact that they also reduce the height of plants, it doesn’t outweigh the negative impact on stem strength. The net effect of CSFB larvae is increased lodging risk,” says Dr Berry.
Where there is a high lodging risk – whether due to canopy size, CSFB larvae damage or a combination of both – growers have two fertiliser options. The first split can be delayed which, Dr Berry says is particularly useful for those crops with a large canopy (a GAI of 2 or more) in February. A more drastic option is to reduce the overall rate of nitrogen but this may also reduce yield and can only really be justified where the lodging risk is very high and/or fertiliser prices make it a more viable option.
“Ultimately, the most consistent and effective way to reduce lodging risk comes from using a PGR such as Caryx®,” he says.
Managing lodging risk this spring
BASF agronomy manager, Matt Keane is positive about this year’s OSR crops, while there is some variation, a combination of factors encouraged good germination and establishment.
According to Mr Keane, any crops with a GAI above 0.8 at the end of February, just before stem extension, warrants an application of Caryx®. “Usually, we recommend application rates of 0.7L/ha to 1L/ha but where crops are very large, you can go up to 1.4L/ha,” he says.
Optimum timing of applications is green bud to yellow bud, when crops are actively growing. Mr Keane advises growers to aim for green bud rather than yellow when facing a larger canopy. “Also look at the nutrient status of that crop. Where the land is very fertile, consider upping the rate of Caryx®.
“Lodging at flowering reduces yield by 20% – when lodged to 450 – and an ADAS survey showed the average lodged area is 31%.”
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